Goals

On completing this module, students are expected to be able to:

• describe the physical, chemical and biological controls on the livelihood of coral reefs, calcareous plankton, dinoflagellates, and jellyfish;
• explain how human activity is causing ocean acidification, coral bleaching, red tides and blooms of jellyfish
• interpret evidence for the decline in reef health and its relationship with changing sea surface temperature

Learning Outcomes

After completing this module, students should be able to understand the following topics:

• projected change in pH in 2100 under SRES A2
• significance of pH, omega
• dissolution and precipitation reactions
• diversity of reef environments
• biology of corals, polyps, hard and soft corals, reproduction, role of zooxanthellae
• changes in calcification in the Great Barrier Reef
• causes and effects of bleaching
• coralline algae and their significance
• impact of acidification on plankton, pteropods, foraminifera, and coccolithophores
• causes of HABs
• life cycle of dinoflagellates and diatoms
• general impacts of toxins on fish and humans
• Pfiesteria controversy
• origin of CyanoHABs
• origin of giant jellyfish blooms
• impact of giant jellyfish blooms on coastal ecosystems

Below is an overview of your assignments for this module. The list is intended to prepare you for the module and help you to plan your time.

Assignment

1. Lab 7: Reef Ecology
2. Submit Module 7 Lab 7.
3. Take Module 7 Quiz.
4. Yellowdig Entry and Reply

Background

Increasing levels of CO₂ in the atmosphere are slowly causing the surface of the ocean to become more acidic. This is because the ocean absorbs some of the CO₂, forming a weak carbonic acid. At present, the ocean absorbs about a third of fossil fuel emissions, but this amount is likely to increase to 90% in the future. Over the last century, the average pH of the ocean has decreased, and there are hints that the current levels are beginning to impact organisms that make their shells out of the minerals aragonite and calcite (both composed of CaCO₃). Aragonite is more susceptible to dissolution in more acidifc waters than calcite. Coral reefs that are made of the mineral aragonite and are particularly vulnerable to ocean acidification. A recent study has found, for example, that the area of coral covering the Great Barrier Reef in Australia has been cut in half since 1985. However, coccolithophores and foraminifera, organisms that serve a vital role at the base of the marine food chain that are composed of calcite, are becoming increasingly susceptible. Moreover, the future appears to be even more bleak; some CO₂ projections suggest by the year 2100 there will be a 150% increase in the ocean’s acidity compared to preindustrial times. Here we review the chemical changes in seawater that result from increasing CO₂, and then we discuss the impact on reefs and planktonic organisms in the ocean. Finally, we discuss the evidence for acidification in ancient oceans and its impact on life in the past.

Changes in atmospheric CO2 and consequences on the oceans.

Click for a text description of the Ocean Acidification Timeline diagram.

A diagram illustrating the impact of ocean acidification over time, spanning from the Pleistocene era to a projected future in 2100. It shows a timeline with key periods: Pleistocene, Industrial Revolution, 1900, 2000, and 2100, created by David Fierstein in 2007 for MBA. The diagram depicts a marine environment with decreasing pH levels over time, from 8.2 in the Pleistocene to 7.8 by 2100, due to increasing CO2 absorption in the ocean, as shown by a chemical equation (CO2 + H2O → HCO3- + H+). Visual elements include a mammoth on land in the Pleistocene, factories emitting smoke during the Industrial Revolution, a ship with fish in 2000, and a diver in a degraded coral reef by 2100. Microscopic images of marine organisms highlight their decline due to acidification.

• Timeline Periods
  • Pleistocene
    • pH: Stable at ~8.2
    • Visual: Mammoth on land, mountains in the background
  • Industrial Revolution
    • pH: 8.2
    • Visual: Factories with smokestacks emitting smoke
  • 1900
    • pH: Not specified
    • Visual: Transition period, no specific element highlighted
  • 2000
    • pH: 8.1
    • Visual: Ship on the ocean surface, fish swimming below
  • 2100
    • pH: 7.8
    • Visual: Diver in a degraded coral reef, sparse marine life
• Ocean Environment
  • Surface: Blue water with waves
  • Underwater: Coral reefs and marine life
    • Pleistocene: Vibrant coral reefs, microscopic image of a healthy marine organism
    • 2000: Fish swimming, healthy marine environment
    • 2100: Degraded coral reefs, microscopic image of a damaged marine organism
• CO2 Impact
  • Label: CO2 absorption
  • Chemical Equation: CO2 + H2O → HCO3- + H+; CO3^2- + H+ → HCO3-
  • Position: Top right, with an arrow pointing to the ocean
  • CO2 Levels: 280 ppm (Pleistocene) to 377 ppm (2000)
• Visual Elements
  • Land: Mountains and factories in earlier periods
  • Ocean: Fish, coral reefs, and a diver in later periods
  • Microscopic Images: Healthy marine organism (Pleistocene), damaged organism (2100)

Credit: Used with permission from Monterey Bay Aquarium Research Institute (MBARI)

The following video provides a thorough overview of the potential impact of acidification on the oceans.

Video: Acid Test: The Global Challenge of Ocean Acidification (21:34)

Chemistry 101

The ocean contains a massive reservoir of dissolved CO₂, hundreds of times more than in the atmosphere, and, actually, by contrast, the amount derived from fossil fuel burning is relatively modest. Since the beginning of the industrial revolution, about 340 to 420 petagrams carbon (a petagram or Pg is 10¹⁵ grams) in the form of CO₂ has been emitted to the atmosphere, with about a third of that amount absorbed by the ocean, approximately 118 Pg. Seawater today may already contain more CO₂ than at any time in many millions of years.

As we discussed in Module 5 on the Carbon Cycle, the absorption of CO₂ in the ocean forms weak carbonic acid (H₂CO₃). Some of this acid dissociates in seawater releasing H⁺ ions, which make the water more acidic, as well as HCO₃- (bicarbonate ions) and CO₃^2- (carbonate ions). This reaction is as follows:

$$C O_2( a q ) + H_{2}O = H_{2}C O_3= H^{+}+ H C O_{3 - }= 2 H^{+}+ C O_3^{2 - }$$

Behavior of carbonate species in the ocean with the addition of CO2 from anthropogenic sources.

Click for a text description of the Ocean Acidification Process diagram.

A diagram illustrating the process of ocean acidification caused by increased atmospheric carbon dioxide, created by the National Research Council and the National Academies. It shows a coastal scene with land on the left, featuring a car, houses, and factories emitting CO2, and the ocean on the right with marine life like fish and coral. The diagram explains the chemical reactions: atmospheric CO2 dissolves into the ocean, combining with water (H2O) to form carbonic acid (H2CO3), which dissociates into bicarbonate ions (HCO3-) and hydrogen ions (H+). These hydrogen ions then react with carbonate ions (CO3^2-) to form more bicarbonate ions, reducing the availability of carbonate ions needed for calcium carbonate (CaCO3) formation in shells and coral skeletons.

• Left Section (Land)
  • Features: Car, houses, factories emitting CO2
  • Background: Hills, trees, and a sun in the sky
  • Color: Green hills, gray factories, blue sky
• Right Section (Ocean)
  • Features: Fish, coral, and other marine life
  • Color: Blue water with colorful marine organisms
• Chemical Process
  • Atmospheric Carbon Dioxide
    • Label: Atmospheric Carbon Dioxide (CO2)
    • Position: Top, emitted from factories
  • Reaction 1: CO2 Dissolves into Water
    • Equation: CO2 + H2O
    • Position: Center, above the ocean
  • Reaction 2: Formation of Carbonic Acid
    • Equation: CO2 + H2O → H2CO3
    • Label: Carbonic Acid (H2CO3)
    • Position: Center, within the ocean
  • Reaction 3: Dissociation into Ions
    • Equation: H2CO3 → HCO3- + H+
    • Labels: Bicarbonate Ions (HCO3-), Hydrogen Ions (H+)
    • Position: Center-right, within the ocean
  • Reaction 4: Hydrogen Ions React with Carbonate Ions
    • Equation: H+ + CO3^2- → HCO3-
    • Labels: Carbonate Ions (CO3^2-), Bicarbonate Ions (HCO3-)
    • Position: Right, within the ocean
• Impact on Marine Life
  • Calcium Carbonate (CaCO3)
    • Label: CaCO3 (shells, coral skeletons)
    • Description: Reduced availability due to fewer carbonate ions
    • Position: Center-right, near marine life
  • Calcium Ions
    • Label: Calcium Ions (Ca^2+)
    • Position: Center-right, part of CaCO3 formation
• Visual Elements
  • Arrows: Indicate the flow of reactions from CO2 to HCO3-
  • Marine Life: Fish, coral, and shells affected by acidification
  • Color Coding: Red arrows for chemical reactions, blue for ocean, green for land

Credit: National Research Council of the National Academies

Going back to your elementary chemistry course, you might remember that a pH of greater than 7 is regarded as alkaline whereas a pH of less than 7 is acidic. Surface ocean waters have a pH of between about 7.9 and 8.3, which means that they are, by definition, alkaline. Anthropogenic CO₂ is thought to have decreased the mean pH of the ocean by 0.1 unit since 1800. This may not sound like that much, but more ominous is the projection that if CO₂ levels continue to rise unabated (i.e., projections based on SRES A2 “business as usual”, pH levels will drop a further 0.3 by 2100. As we will see below, in parts of the ocean, these levels would be extremely damaging to organisms that build their skeletons out of CaCO₃, which is very sensitive to CO₂ addition.

CaCO₃ is the dominant material used by invertebrate organisms to build their skeletons. There are two different minerals made of CaCO₃, known as polymorphs: calcite and aragonite. These minerals have the same composition but different crystal lattice structure and thus their properties and behavior in seawater differ, including their ability to dissolve. To understand how CaCO₃ dissolves and precipitates, we need to introduce a term Ω that represents the saturation state of the water. Where waters are highly saturated with respect to CaCO₃ and Ω is high, calcite and aragonite are less likely to dissolve than where these waters are less saturated or even undersaturated and Ω is low. Likewise, calcite and aragonite are more likely to precipitate under higher Ω values. The dissolution and precipitation reactions are as follows:

Dissolution reaction: $\begin{array}{c}C a C O_3( \mathrm{solid} ) = C a^{2 + }+ C O_3^{2 - }\end{array}$

Precipitation reaction: $C a^{2 + }+ C O_3^{2 - }= C a C O_3( \mathrm{solid} )$

An increase in CO₂ from the atmosphere presents a double whammy for skeletons formed from CaCO₃, both aragonite and calcite. The H⁺ ions and carbonate ions (CO₃^2-) that derive from the dissociation of carbonic acid combine to form bicarbonate ions (HCO_3-). This rapid reduction in available carbonate ions decreases Ω and limits calcification by organisms with aragonite- and calcite-based skeletons. However, here we need to dispel two myths. The first myth is that the precipitation of CaCO₃ is directly controlled by pH. In fact, precipitation is affected principally by the decrease in CO₃², which is coincident with the addition of H⁺ ions, and reduction in pH. The second myth is that precipitation of CaCO₃ can occur in any water that is oversaturated with respect to the particular CaCO₃ mineral. In fact, both corals and coccolithophores have been shown to have difficulty calcifying in environments when waters were actually oversaturated. Different organisms can calcify at very different Ω values, but for most the decrease in saturation that results from decreasing CO₃^2- content is a direct threat to calcification. Fiinally, it is key to move that aragonite is more susceptible to dissolution than calcite. Thus, shells made of the CaCO₃ polymorph aragonite, including the corals, will be the first to dissolve, followed by those made of the polymorph calcite.

The following video explains the threat of ocean acidification to the calcareous plankton.

Video: The Other Carbon Dioxide Problem (3:57)

The saturation of CaCO₃ in the oceans is also a function of temperature and pressure. A delicate balance exists between the production of CaCO₃ via the formation of skeletons in the shallow part of the ocean and the dissolution of this aragonite and calcite in the colder and deeper realms of the ocean where waters are less saturated. In most parts of the ocean, undersaturation occurs far below the surface. However, recent increase in dissolved CO₂ is leading to a shoaling of the saturation horizon of CaCO₃, and, in the future, this will impact especially the organisms that live at depth or in colder waters as well as those that make their shells of the mineral aragonite, which is more soluble in seawater than calcite.

Coral Reefs

Introduction

Coral reefs have existed for hundreds of millions of years and provided a habitat for some of the richest diversity on the Earth’s surface. They are the marine version of tropical rainforests. Reefs harbor a slice of the marine food chain, all the way from tiny autotrophic protistans (autotrophs fix carbon through photosynthesis) to large, predatory fish. Hundreds of millions of humans live near reefs and receive important resources from them. Reefs host productive fisheries; they also provide protection to low-lying coastal areas from storms and are vital for a number of key habitats, including mangrove forests.

Examples of Reef Corals

Examples of Deep Water Corals

The organisms that have constructed reefs, largely corals, have evolved over time, and with that change so have the locations of reefs and the dynamics of the reef community changed. Over their long history, reefs have had several intervals of crisis; in particular, they almost ceased to exist at the Permian-Triassic boundary, where over 90% of marine species became extinct, and during the Cretaceous about 100 million years ago when giant clams took over these structures for several tens of millions of years. Both of these ancient times were potentially characterized by ocean acidification. However, reefs have been remarkably resilient over geologic time and generally have been able to adapt to environmental change. For example, as we will see in Module 10, they are able to grow fast enough to keep up with very rapid rates of sea level rise.

Impact of Ocean Acidification on coral reef ecosystems

Click for a text description of the Carbon Emissions on Ocean Chemistry diagram.

A diagram comparing the effects of high carbon emissions versus reduced carbon emissions on ocean chemistry, created by the Great Barrier Reef Foundation, Australia, in 2008. It features a cross-sectional view of a coastal environment with land on both sides and the ocean in the center. On the left, under "High carbon emissions," factories and vehicles emit carbon dioxide (CO2), which dissolves into the ocean, forming carbonic acid (H2CO3) that dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3-), reducing carbonate ions (CO3^2-) and damaging coral structures. On the right, under "Reduced carbon emissions," wind turbines and vegetation indicate lower CO2 levels, resulting in less carbonic acid formation, higher pH (8.2), and healthier coral growth. A magnified inset on each side shows the contrast in coral health.

• Left Section (High Carbon Emissions)
  • Label: High carbon emissions
  • Features: Factories, vehicles, and a person on land
  • Emissions: Carbon dioxide (CO2)
  • Ocean Process:
    • CO2 dissolves into water → Carbonic acid (H2CO3)
    • Dissociates into Hydrogen ions (H+) and Bicarbonate ions (HCO3-)
    • Reduces Carbonate ions (CO3^2-)
  • pH: Lower (not specified)
  • Visual: Damaged coral, sparse marine life
  • Inset: Magnified view of eroded coral with labels for carbonic acid, hydrogen, and carbonate
• Right Section (Reduced Carbon Emissions)
  • Label: Reduced carbon emissions
  • Features: Wind turbines, vegetation, and a car on land
  • Emissions: Reduced CO2
  • Ocean Process:
    • Less CO2 → Less carbonic acid formation
    • Higher Carbonate ions (CO3^2-) availability
  • pH: 8.2
  • Visual: Healthy coral, thriving marine life
  • Inset: Magnified view of healthy coral with labels for carbonic acid, hydrogen, and calcium carbonate
• Ocean Environment
  • Color: Blue water with green and brown shading for coral and land
  • Position: Central area between land sections
• Chemical Labels
  • Carbon dioxide (CO2)
  • Carbonic acid (H2CO3)
  • Hydrogen ions (H+)
  • Bicarbonate ions (HCO3-)
  • Carbonate ions (CO3^2-)
  • Calcium carbonate (CaCO3)
• Land Features
  • Left: Industrial landscape with buildings and vehicles
  • Right: Renewable energy landscape with turbines and trees

Credit: Great Barrier Reef Foundation, Australia

With this background, recent human activity has placed reefs in as precarious a position as at almost any time in their history. The last fifty years have witnessed an extremely dramatic decline in the health of many of the major reefs around the world, including reefs of the Caribbean, the Bahamas, and the Florida Keys as well as those in the Indian and Pacific Oceans, including the massive Great Barrier Reef of Australia. The outlook for these rich and complex ecosystems is about as bleak as any ecosystem on Earth. As it turns out, ocean acidification is one of several environmental threats to reefs, with warming, pollution, overfishing and physical destruction all exerting major threats to reefs in the future. As we will see, acidification is perhaps the greatest of all of these threats long term.

Before we start, let's consider how reefs grow. The main organism that constructs modern reefs, the coral, includes a number of species belonging to the Cnidaria, a phylum of organisms that uses stinging cells to capture their prey. Modern corals are colonial structures of millions of individual polyps that grow primarily in shallow and clear tropical and subtropical waters, restricted to these areas by light levels and temperatures as well as by nutrients.

Red and yellow soft coral from Fiji.

Credit: Flickr, Derek Keats (CC-BY 2.0)

Both types of coral reproduce both sexually and asexually. Asexual reproduction involves simple cell division, or budding, and takes place within the colony, whereas sexual reproduction involves the release of gametes into seawater. This is an amazing process that for many species happens once a year, timed by the lunar cycle. The fertilized egg forms a larval planula that settles before forming a new colony.

Video: Coral Reefs 101 National Geographic (3:54)

Large reef structures including fringing and barrier reefs, as well as atolls, represent the growth of these colonies over many thousands or millions of years.

Evolution of an Atoll (1:20)

Examples of modern corals living in symbiosis with dinoflagellate algae called Zooxanthellae

Branching coralline algae from California

Credit: Steve Lonhart/SIMoN

The algae live within the coral polyp and receive CO₂ from the polyp that it requires for photosynthesis. Through photosynthesis, the Zooxanthellae convert the CO₂ to O₂ and provide this vital gas to the coral as well as crucial nutrients. This is important because by removing CO₂, the Zooxanthellae drives up saturation which facilitates calcification in the coral skeleton. Reefs are made up of much more than corals and their algal symbionts. Other organisms, including coralline algae (algae that secrete high magnesium calcite and conduct photosynthesis on their own), are active framework builders in modern reefs. Today, sponges, sea anemones, sea urchins, a diverse array of fish, and many other organisms live within reefs, some playing a vital role in building reefs and keeping them healthy and others taking advantage of their decline.

Examples of Sea Urchins, Sponges, Sea Anemone and Coral Reef

Although, as we will see, ocean acidification is only just beginning to affect reef growth, there is already another very serious affliction in many reefs around the world---coral bleaching. One of the key changes in corals over the last few decades has been an increase in the frequency and severity of bleaching events. As we have discussed, shallow-water coral species live in symbiosis with algae called a dinoflagellate or Zooxanthellae whose colorful pigments give living corals such beautiful colors, purple, brown, green, yellow, and more. As discussed the symbiosis is key to the coral, the Zooxanthellae removed CO₂ from the water, boosts the saturation, and facilitates calcification of the coral. When temperatures become too hot, however, the coral become physically stressed and expels the Zooxanthellae leading to a bleached white color.

Coral in Florida invaded by blooms of red and brown algae.

Credit: eutrophication&hypoxia on Flickr (CC-BY-NC-SA 2.0)

Bleaching today is common when temperatures increase very slightly (by about 1°C) in summer months and thus is often an annual summer event. Bleaching is accompanied by slower growth and increased coral mortality. The response to bleaching differs considerably between different species. Some species are able to recover normal growth rates quite rapidly, whereas others are more vulnerable and unable to recover. Bleaching also changes the ecology of a reef, promoting the growth of algae that blanket corals, making the recovery of corals more difficult.

Impact of bleaching on coral livelihood Click here to see a text description.

Coral Bleaching - Have you ever wondered how a coral becomes bleached?

1. Healthy coral - coral and algae depend on each other to survive: Corals have a symbiotic relationship with microscopic algae called zooxanthellae that live in their tissues. These algae are the coral's primary food source and give them their color.
2. Stressed coral - if stressed, algae leaves the coral: When the symbiotic relationship becomes stressed due to increased ocean temperature or pollution the algae leave the coral's tissues
3. Bleached coral - coral is left bleached and vulnerable: without the algae, the coral loses its major source of food, turns white or very pale, and is more susceptible to disease

What causes coral bleaching?

• Change in ocean temperature: increased ocean temperature caused by climate change is the leading cause of coral bleaching
• Runoff and pollution: storm-generated precipitation can rapidly dilute ocean water and runoff can carry pollutants–these can bleach near-shore corals
• Overexposure to sunlight: when temperatures are high, solar irradiance contributes to bleaching in shallow-water corals
• Extreme low tides: exposure to the air during extreme low tides can cause bleaching in shallow corals

Credit: NOAA National Ocean Service

A major coral bleaching event took place in the Florida Keys in summer 2023 when water temperatures rose to 101 deg F and there was widespread bleaching. It is too soon to determine the long-term impacts of this bleaching on the reefs in the Keys but the images are dramatic and the future does not look promising.

Partially bleached table top coral from the Florida Keys, summer 2023, The white areas are bleached, the brown areas are not.

Source: Andrew Ibarra, NOAA

One of the most significant bleaching events took place in 2016 and 2017 in the Great Barrier Reef. This catastrophic event caused by warmer than average water led to 50% coral mortality in some areas. Bleaching was also extreme in 2020 and you can see from the maps below that it extended southward which in the southern hemisphere means it afflicted cooler waters. The extent of bleaching shocked ecologists and is clearly a window into the future. Now it seems every year, bleaching is a threat to the Great Barrier Reef, and with the warm El Nino conditions, the Austral summer of 2024-2025 may be the most severe bleaching event ever.  One scientist has remarked that the reef recently looked like it had been "carpet-bombed".

Mass bleaching events 2016-2020 (note bleaching in 2024 is more severe)

Credit: ARC Centre of Excellence for Coral Reef Studies (CC BY-ND)

Video: Australia’s Great Barrier Reef hit by mass coral bleaching (2:37)

The goal of this lab is to:

• observe and compare the health of reef by exploring different parts of the ocean.

Introduction

First, you will be watching videos from the Catlin Seaview Survey as well as photos to learn how to determine the health of reefs. Then you will be looking at before and after photos of reef bleaching events. Finally, you will be looking at the risk to reefs in the future.

Please watch the videos below. It is easiest to watch the videos in your browser (click button top right of Google Earth) in full screen mode.  To move around, insert your cursor in the videos to manipulate the camera and stop on particular items of interest or to change direction. The best way to move the camera is with your keyboard arrows (side to side) and cursor for up and down.

The goal of this part of the lab is for you to show you can identify different types of coral as well as overall health of the reef at different locations. Make sure you have read the material on reefs in the module before attempting to complete the lab. Below are the different types of coral for you to identify. In addition, we show pictures of algae that colonize reefs as well as reef damage from storms. Healthy corals show a variety of colors from the different algal symbionts. Unhealthy corals show fewer colors, more algal colonization, more breakage and often are bleached white. Remember, algae are some of the key markers of an unhealthy reef.

Photographs of different types of coral

Photographs of algal colonization

The following images show some of the range of morphologies and colors of colonizing algae

Photographs of reef damage from storms or people

Files to Download

ReefsAllFin.kmz

Prep and Instructions

Load the ReefsAllFin.kmz file. The locations of reefs of interest are shown with flags. The videos are shown with diver markers, the still photographs are shown with wave markers. The videos run best if you open them a browser such as Firefox (see Google Earth window, top right). Before submitting your lab, let’s begin with the practice. Make sure you do this part of the lab to get comfortable with the tasks you will be asked to do and to receive feedback about your answers. Watch the videos at the following locations and answer the questions below. Make sure you maneuver up and (especially) down, as well as side-to-side.

Practice Questions

Part A

In this section, you will be looking at the health of reefs using their color, the presence and abundance of algal overgrowth and the and the presence of bleaching. Go to the Belize reef (click on the flag in Google Earth, then open the address in your browser). Look around the reef and answer the following questions.

1. Is there living coral? (look for different colored coral) (Yes/No)
2. Is there brain coral? (Yes/No)
3. Which types of coral do you see?
   A. Table bottom
   B. Table bottom and staghorn
   C. Staghorn and brain
   D. Table bottom, staghorn, and brain
4. Is there any bleached coral? (Yes/No)
5. Is there any algal overgrowth? (Yes/No)
6. How would you describe the health of the reef?
   A. Very healthy
   B. Moderately healthy
   C. Not healthy

Part B

In this part of the lab, you will compare photographs from before and after major events that have impacted reefs. Answer the questions about the changes in the abundance of different types of coral, algal overgrowth, or percent bleaching.

Go to Lizard Island. Please look at the two pictures and answer the following questions.

7. Which picture has the healthiest reef? (upper/lower)
8. What is most diagnostic of the health of the upper photograph?
   A. The healthy reef has very tall coral
   B. The healthy reef has a lot of different colors
   C. The healthy reef has bleached corals
9. What is most diagnostic of the health of the lower photograph?
   A. The unhealthy reef is bleached
   B. The unhealthy reef is growing quickly
   C. The unhealthy reef is covered by algae

Introduction

Corals and coralline algae are not the only organisms highly susceptible to ocean acidification. Coccolithophores, foraminifera, pteropods, three very different groups of plankton (a term that refers to organisms that float passively in the upper ocean) are also threatened by increasing atmospheric CO₂ levels. Pteropods, often called sea butterflies, are tiny snails made of aragonite that thrive in shallow waters and play a particularly important role in polar ecosystems.

Pteropods

Pteropod on the right and a very small octopus on the left

Credit: Photo by Matt Wilson/Jay Clark, NOAA NMFS AFSC from NOAA Photo Library on Flickr (CC BY 2.0)

​​​​​​

Change in pteropod shells subjected to seawater with pH and saturation projected levels for 2100

Credit: NOAA PMEL Carbon Program; photo by David Liittschwager/National Geographic Stock.

Because of the susceptibility of aragonite to decreasing saturation levels combined with the effect of temperature (calcification requires more energy in cold water) on saturation, pteropods may cease to exist in polar latitudes by the middle of the 21st century. In some polar areas, these organisms account for over 60% of the zooplankton biomass, thus their extinction or migration to warmer regions could have major repercussions to organisms up the food chain.

Foraminifera and Coccolithophores

The coccolithophores and foraminifera are constructed of low-Mg calcite and therefore are more stable than pteropods under conditions of increasing CO₂.

As we have learned earlier, foraminifera are groups of zooplankton with habitats both near the surface of the ocean (planktonic foraminifera) and on the ocean bottom (benthic foraminifera).

Foraminifera

The planktonics are sensitive to changes in CO₂, have shown subtle changes in shell mass during the Pleistocene in response to CO₂ fluctuations, and have already begun to get lighter in response to recent CO₂ increase. This recent thinning of shells is likely a result of the effect of decreasing CO₃^2- on calcification by foraminifera. Interestingly, many planktonic species also harbor dinoflagellate symbionts that play a role in calcification. Changes in calcification appear to be a threat to the planktonic foraminifera, but we do not yet know how serious this threat is. However, we do know that the foraminifera are also threatened by the potential loss of their dominant source of food, the coccolithophores, as a direct result of CO₂ addition.

Living coccolithophorids of Emiliania huxleyi

Credit: Jeremy Young, the Resilient Earth

Coccolithophores

Coccolithophores are ubiquitous in the oceans, essentially serving as the dominant species of phytoplankton in vast regions of the open ocean that are characterized by lower nutrient levels. For this reason, considerable attention has been devoted to the potential effects of increasing CO₂ on the coccolithophores.

Coccolithophores are haptophyte or golden brown algae (similar to diatoms) that produce tiny calcite scales known as coccoliths during certain phases in their life cycle.

The plates are several microns in diameter and can remain attached to the cell covering the soft organelles with a protective shield or break off after they are fully grown. Coccolithophores reproduce asexually, and, given the right conditions have the potential to multiply rapidly. When conditions are suitable, coccolithophores can form blooms of millions of cells per liter of seawater. These organisms are consumed by foraminifera and copepods, and transported to the bottom of the ocean as marine snow.

Video: Marine Snow (00:33)

Because of their prolific production and global distribution, coccolithophores are a vital part of the carbon and carbonate cycle of the oceans. Modern coccolithophorids are dominated by the species Emiliania huxleyi, a species with very small (1-2 micron) and delicate coccoliths. Due to their size and ecology, coccolithophores are inherently more difficult to study in the natural environment than are corals. However, samples can be collected using filters and cores of sediments can be studied to determine the effects of past changes of CO₂ on coccolithophore morphology. Coccolithophores can also be cultured in the laboratory where CO₂, CO₃^2- and pH levels can be altered to observe the effect of these variables on calcification. Although the results of both field and lab studies are by no means simple, it appears that in the case of E. huxleyi, increasing CO₂ and decreasing CO₃^2- has the effect of causing thinner coccoliths with smaller masses. In addition, laboratory studies show that such conditions also lead to malformations in E. huxleyi coccoliths, which are a potential sign of difficulty in calcification. Such trends of decreasing mass are by no means universal, however. Some lab experiments and field collections in particular environments show that species other than E. huxleyi actually grow thicker in low CO₃^2- conditions and even E. huxleyi recently has increased in mass in parts of the ocean. More study is required to determine the precise outcome of the coccolithophores in a high CO₂ world. However, the current signs generally point to significant reduction in the rate of calcification, which could lead to significant changes both in marine ecosystems and in the carbon cycle.

Coccolithophores have a spectacular 220 million year fossil record. This record allows paleontologists to observe the effects of past climate change, including increasing CO₂, on the livelihood of this group of plankton. Ancient global warming events, including those at 120 million years before present and 55 million years before present, have been gleaned for evidence of ocean acidification in the morphology of coccolithophores. The event at 120 million years shows evidence for a decrease in coccolith size, but this change is not apparent in the 55 million year event. As the species existing during these ancient events was entirely different from those at the present time, it could be that ancient species responded in a different fashion from those living today. Alternatively, the changes in ancient plankton assemblages may be in response to environmental variables other than CO₃^2-, for example, temperature and nutrients. The coccolithophores, like the corals, have been able to survive intervals of great ecological upheaval in the past. However, given that the rates of modern environmental change are so rapid compared to ancient events, we cannot assume that these same groups will have the ability to adapt to the changes that are to come. Moreover, while the coccolithophores appear to be less endangered by increasing CO₂ than the corals, largely as a result of their mineralogy, the impact of the decline of these vital algae would likely be even more devastating to the oceans.

Red tides are common events in warm and polluted coastal oceans. They form when dinoflagellate algae explode to huge population levels. Because the dinoflagellates have red plastids, the waters literally turn red. Dinoflagellates take advantage of harsh environmental conditions that kill off other organisms. As you will find out in the pages to follow, these tides can be major public health hazards.

Red tide off the coast of California

Credit: Woods Hole Oceanographic Institution/NOAA

I remember that the grouper was delicious. Blackened, spicy, with an ear of corn and some slaw. On a white paper plate. I polished it off with a Heineken. It was a warm early January day, a seafood festival in Florida City. We went canoeing after lunch. I remember waking up the next morning with an intense thirst and extreme nausea. When I tried to get out of bed, I sprawled on the floor, my left side was completely paralyzed! I rolled to the sink, crawled up, and poured myself a glass of water, I gulped it down, and to my horror, it felt boiling hot! The neurological symptoms soon went away, but nausea and fatigue lasted weeks, I lost at least ten pounds. The doctors took that long to diagnose the problem---I had Ciguatera poisoning. That delicious grouper had ingested a lot of toxic dinoflagellates and transmitted the toxins to me. My first experience with harmful algae.

Red tides represent one of the most serious threats to coastal ecosystems today. Most red tides result from the input of an excessive amount of nutrients from fertilizers, sewage, and soils of nearby land areas to bays, estuaries, and shallow seas. These nutrients cause explosive growth of microscopic species of algae, a number of which carry toxins that are harmful or even lethal to other organisms. Red tides can be caused by major storms such as hurricanes, which cause excess runoff from the land and resuspension of the seed stages of the algae.

Red tides occur when dinoflagellates, and rarely diatoms, grow in massive quantities in surface waters. The photosynthetic organelles of these organisms, known as plastids or chloroplasts, are red, or golden brown in the case of diatoms, and the profusion of cells in surface waters imparts a red or brown color. Some of the culprit dinoflagellate and diatom species produce by-products that are highly toxic to many other organisms living in the coastal zone, all the way from fish to turtles to large mammals such as dolphins, manatees, and whales, as well as to humans. In some cases, shellfish or small fish such as sardines that consume the plankton are not harmed by the toxin but concentrate it for organisms that feed on them, a process known as bioaccumulation. Ingestion of the toxins can result in developmental, immunological, neurological, and reproductive damage of the host organism. For this reason, red tides are also known as harmful algal blooms (HABs). In fact, we will use the term HAB here because these events are not associated with ocean tides, because many HABs are not associated with a red color, and because blooms of dinoflagellates are often not harmful.

Examples of Dinoflagellates and Diatoms

Harmful algal blooms are a global phenomenon and have increased in frequency in the last thirty years. Part of the increase may result from awareness of the phenomenon, but increasing pollution is also considered responsible. There are a number of reasons why climate change may further increase the occurrence of HABs. For examples, the increase in the frequency of large storms such as hurricanes will lead to greater runoff and input of nutrients from land. In addition, changes in temperature, wind patterns, upwelling, and stratification will alter the distribution of species. Because the degradation of a large amount of cellular material produced in HABs consumes oxygen, HABs can result in hypoxic or anoxic conditions.

Dinoflagellates are a group of microscopic single-celled organisms or protists that are dominantly autotrophic (i.e., primary producers). Interestingly, many species are also mixotrophic, having the ability to ingest their prey as a source of energy. Some species are entirely heterotrophic, lacking chloroplasts or plastids, and have been termed carnivorous. In fact, there have been reports in the scientific literature that some species have the ability to consume fish after having paralyzed them with neurotoxins. These claims are controversial but have given dinoflagellates a near-mythical reputation among the oceanic plankton.

Simplified life cycle of a dinoflagellate

Click here to see a text description.

Life Cycle of a Single Algal Cell

1.  For certain red tide species, a resting cyst lays dormant on the ocean floor, buried in sediment. If undisturbed by physical or natural forces, it could stay in this state for weeks, months, even years.
2. Warm temperatures and increased light cause the cyst to germinate. It breaks open and a swimming cell emerges. The cell reproduces by simple division within a few days of "hatching."
3. If condition remains optimal, cells will continue to divide, reproducing exponentially 2 to 4 to 8 to 16. A single cell could produce 6000 to 8000 cells within one week.
4. (and 5.) When nutrients are gone, growth stops and gametes are formed. Two gametes join to form one cell, which develops into a zygote and then into a cyst. This falls to the ocean bottom, ready to germinate.

Credit: Woods Hole Oceanographic Institute/NOAA

Diatoms and dinoflagellates are most common in the coastal oceans but also have the ability to live in freshwater environments and in intermediate salinity environments where fresh and marine waters mix in estuaries. They are the most prolific group of primary producers in the ocean. Dinoflagellates have a highly complex life cycle that consists of an alternation between a motile stage and a resting or cyst stage. In short, dinoflagellates enter the resting stage via sexual reproduction when conditions in the surface ocean are not suitable for them to thrive. They can remain dormant for weeks, months or years before they “excyst,” when surface conditions improve, reproduce vegetatively, and populate the surface ocean. Excystment and repopulation are triggered by changes in temperature, light, or oxygen levels or even resuspension of cysts by storms. Dinoflagellates generally thrive when nutrient levels are elevated, and, under conditions of extremely high nutrient levels, cell division can be so rapid that extremely high cell counts (millions of cells per milliliter of seawater) are reached, resulting in red tides. The cyst stage acts as a very effective mechanism for seeding blooms.

Dinoflagellates

The following video summarizes the life cycle of the dinoflagellates.

Video: Life Cycle of the Dinoflagellates (1:27)

Dinoflagellates have a broad range of different ecologies. As we saw earlier in the module, they can be endosymbionts of corals, facilitating calcification in the host colony. They have this same role in foraminifera and radiolarian, a group of siliceous zooplankton.

Diatoms

Diatoms are autotrophic protists that produce a delicate, microscopic test of opaline silica.

Pseudo-nitzschia, a toxin-producing diatom.

Credit: Raphael Kudela, USCS Ocean Data Center

They are non-motile, and, for most of their life cycle, they reproduce asexually. Many nearshore diatom species also have a resting stage akin to the dinoflagellates, allowing them to exit the surface zone when conditions are unfavorable for their growth. This may occur in winter when surface waters are cold or at times when nutrients are depleted. The resting spore stage actually may resemble the vegetative stage of dinoflagellates. Like the dinoflagellates, diatoms are able to reproduce extremely rapidly by simple cell division, and this allows them to rapidly dominate the surface ocean when nutrients are readily available.

Video: Diatom Life Cycle (1:38)

Species that are harmful belong to the pennate diatoms that are long and thread-like and have the ability to attach to a host, although the relationship is not symbiotic. Both dinoflagellates and diatoms cysts can move around the oceans by currents, storms, dredging of the ocean bottom, and when cysts act as ballast on ships or even higher-level organisms. Toxins are not known in the cyst stage of either group.

Of some 60 or so species that cause red tides, only a handful is known to be toxic. Dominant dinoflagellate HAB genera include Alexandrium, Karenia, and Pfiesteria. The diatom genus most commonly associated with HABs is Pseudo-nitzschia. Each of these genera produces a different toxin and thus has a different role on organisms further up the food chain. Next, we discuss some of the HAB species in detail.

Scanning electron microscope view of cysts of the toxic alga Alexandrium tamarense

Credit: Woods Hole Oceanographic Institute/NOAA

Alexandrium spp. is the dominant taxon in coastal regions of New England and eastern Canada but it is also found from California to Alaska.

It is a heterotrophic dinoflagellate that produces a saxitoxin, one of the most powerful known types of neurotoxins. These toxins destroy the function of nerve cells and can thereby cause paralysis. Saxitoxins are most effectively concentrated by shellfish such as clams, quahogs, mussels, scallops and oysters that filter large volumes of seawater to acquire their nutrition. Although the saxitoxin does not harm these shellfish, even in small quantities, the toxin can be extremely dangerous for humans, resulting in a serious illness known as paralytic shellfish poisoning (PSP). The saxitoxin attacks the human nervous system within 30 minutes of ingestion with symptoms that may include numbness, tingling, weakness, partial paralysis, incoherent speech, and nausea. In severe cases, the toxin can lead to respiratory failure and death within a few hours. Alexandrium spp. toxins have also been harmful to whales, sea otters and birds.

The following videos describe the causes and impacts of red tides, as well as possible antidotes for shellfish poisoning.

Video: Florida's toxic red tide is a perfect storm for the Gulf Coast (7:47)

Video: Prized Science Episode 4: Taming the Red Tides (4:28)

Karenia is the dominant taxon causing red tides in Florida and Texas, but rarely species of Karenia have also been found up the east coast in North Carolina.

Examples of Karenia

It produces a toxin known as a brevetoxin (named after a species of Karenia, K. brevis). Like saxitoxins, brevetoxins damage nerve cells, leading to disruption of normal neurological processes and causing neurotoxic shellfish poisoning (NSP). In humans, gastrointestinal symptoms and a variety of neurological ailments result, but there are no known fatalities. However, in fish, the brevetoxins attack the central nervous system and cause respiratory failure. Karenia dinoflagellates are responsible for massive fish and bird kills in the Gulf of Mexico. The brevetoxins are colorless, odorless, and heat and acid stable, thus they survive food preparation.

Gambierdiscus toxicus a species that can produce ciguatera toxins

Credit: Smithsonian Institute

The genus Gambierdiscus lives in tropical waters, usually in reefs, and produces a toxin known as Ciguatoxin that causes gastrointestinal problems followed by mild neurological symptoms. This syndrome is known as Ciguatera fish poisoning. Because the toxin is fat soluble, it gets concentrated up the food chain by bioaccumulation from seaweed to smaller fish than to larger fish. The larger fish, which are the most dangerous to eat because they have the highest toxin concentrations, include commercially available seafood such as grouper, snapper, and barracuda. Ciguatera is responsible for more human illnesses—estimated between 10,000 to 50,000 cases annually—than any other HAB toxin.

Scanning electron microscope image of Pseudo-nitzschia australis

Credit: Woods Hole Oceanographic Institute

Pseudo-nitzschia and the species Nitzschia navis-varingica are common diatom genera in Californian red tides. These taxa produce domoic acid, which is concentrated by filter-feeding shellfish. This neurotoxin can also bioaccumulate in fish such as anchovies that feed directly on the diatoms. Domoic acid causes a variety of gastrointestinal ailments, memory loss and brain damage in humans and is hence referred to as amnesic shellfish poisoning. Rarely, the neurotoxin can be fatal. It can also affect marine mammals, causing seizures.

Scanning electron microscope image of Pfiesteria piscicida.

Credit: Virginia Institute of Marine Science

The species Pfiesteria piscicida and P. shumwayae have been the most common dinoflagellate species in red tides in estuaries and bays along the east coast of the US from Delaware to Florida. The species occur in environments where freshwater and saltwater mix and have not been reported from freshwater environments or the open ocean. Pfiesteria blooms are restricted to summer months. Species have been associated with massive kills of menhaden and other estuarine fish in the Chesapeake Bay and the Tar-Pamlico and Neuse River Estuaries in North Carolina. The fish in contact with Pfiesteria rapidly develop bleeding lesions and have skin actively flake off them, and it has been proposed that the presence of live fish stimulates the production of toxin in the dinoflagellate. Ultimately the open lesions may destroy gill function and lead to death. Nevertheless, the connection between fish mortality and Pfiesteria is still doubted by some scientists. In fact, the effects of Pfiesteria on fish and human health has been one of the largest and nastiest controversies in marine science over the last 25 years and it does not appear that the conflict is anywhere near over.

The following video explains some of the research on Pfiesteria.

Video: Pfiesteria Update: An Enduring Debate (4:57)

At least part of the debate has been fueled by the popular press, who have focused attention on the organism after studies suggested that it was carnivorous. These studies indicated that Pfiesteria species ingested the skin of fish after it flaked off. In fact, lab studies have shown that when fish were left in tanks with Pfiesteria, the fish died within hours. For this to occur, however, the dinoflagellate and fish must be in direct contact. Without contact, the same studies show that fish suffered no ill effects. Some scientists alternatively point to water molds or fungi including the species Aphanomyces invadans as the pathogen cause of the ulcerative lesions, skin loss, and damage to gills. A. invadans and other fungi are universally present in fish with ulcers and skin loss. Also weakening the case for Pfiesteria, this genus is still known to exist in North Carolina estuaries, but fish kills have become less frequent recently. Moreover, where lesions on the fish menhaden were observed, nearby fish including catfish, perch, and carp were unaffected. These disparities have cast some doubt on whether Pfiesteria is harmful to fish at all. In fact, great differences exist among public health professionals, and warnings from different state agencies are in conflict.

Examples of Pfiesteria

The health impact of Pfiesteria on humans is also uncertain, as is the method of transmission of the potential toxin. Scientists working with Pfiesteria in the laboratory have suffered from long-term neurological symptoms, such as memory loss, fatigue, and dermatological problems, and fisherman in contact with Pfiesteria-related fish kills have also suffered from similar ailments. However, other groups of fishermen who have come in close contact with lesion-covered fish have not reported adverse effects. There are reports that the hypothetical Pfiesteria toxin is transmitted via aerosols.

Until recently, the missing link in the Pfiesteria conundrum is that the toxin produced by this organism has been elusive. However, in 2007 scientists in a government lab claimed the first positive identification of a toxin associated with Pfiesteria. Even with this identification, questions remain; for one, the toxin is unstable in the natural environment, and second, it is not been proven to have adverse health effects. Thus, the controversy about Pfiesteria is far from over. In all reality, a number of factors may result in the fish kills; in particular, the fish in estuaries may have already been under great stress from other biological agents (bacteria, viruses, fungi, parasites), exposure to chemicals (pollutants, toxins), suboptimal water quality, and rapid water temperature change, that have the potential to cause lesions to form.

Far less controversial than the relationship of Pfiesteria with fish kills are studies that directly relate fish kills to low oxygen levels caused by algal blooms. In a number of estuaries along the eastern US, warmer waters in summer combined with increased production by algae, as a result of increased runoff and eutrophication, lead to severely decreased oxygen levels and major fish kills without the involvement of a toxin.

Finally, HABs are not always produced by dinoflagellates and diatoms. Cyanobacteria, or blue-green algae, another group of single-celled organisms, but one that is prokaryotic rather than eukaryotic, is also known to produce extremely potent toxins that can cause illness in fish, birds, and mammals including humans. Because of their potential to be harmful, this group is known as CyanoHABs. Cyanobacteria are some of the oldest species on Earth and are known to tolerate very tough conditions including hot, cold, salty waters and darkness. They live under ice sheets, near hydrothermal vents, and were some of the first organisms to colonize the ocedans after the massive asteroid that killed the dinosaurs.

Samples of CyanoHABs

Although the full scale of health effects of CyanoHABs on humans is not yet determined, the toxins may have gastrointestinal, respiratory, allergic, and neurological responses, and potentially lead to liver damage. In addition, prolific growth of cyanobacteria can block sunlight, which can harm other organisms, and use up oxygen which can lead to hypoxia and anoxia. As in the case of the HAB dinoflagellates, the growth of CyanoHABs may be stimulated by nitrogen loading from agricultural industrial runoff, as well as sewage disposal.

As we have seen, there is a great variety in the biology and ecology of HAB species. However, they share one major thing in common: all of them have the ability to wreak havoc on coastal fisheries. Since the growth of most if not all of the species directly responds to nitrogen loading, limiting the harm on fisheries will require significant changes in agricultural practices combined with modifications of drainage in coastal regions. Such changes will be difficult, if not impossible, to accomplish in the near term. Thus, the best strategies to deal with HABs focus on the integration of highly detailed algal sampling programs, ecological forecasts, and resource management. For example, if HABs can be predicted, then warnings can be issued and areas placed off-limits to fisheries. Such strategies are being employed in areas where HABs are common, including the Gulf of Maine, the Gulf of Mexico, and the Pacific Northwest.

HAB forecasts integrate ecological models, based on the physics, chemistry, and biology of nearshore and offshore regions, with satellite data and in situ measurements of cell counts and toxin levels. For example, models can be used to predict the development and movement of a HAB in the region of interest. Models can be used to identify HAB triggers (i.e., nutrients or temperature), likely areas of cyst seedbeds, likely bloom toxicity based on cell density, and progression of the toxin through the food chain, as well as the ultimate decline of the HAB.

A new threat to fisheries around the world has developed over the last decade---a surge in the number of jellyfish in coastal waters. The most dramatic of these outbreaks is in Japanese waters, where the giant Nomura’s jellyfish has increased significantly, wreaking havoc with fisheries in the Sea of Japan.

Jellyfish populations are normally held in check by fish, mostly because these two groups compete for the same food sources. However, overfishing in many parts of the ocean has led to increasing jellyfish populations. Jellyfish may also be aided by warming ocean temperatures, which favors their development, and by the destruction of habitats of other natural predators such as turtles.

The massive Nomura’s jellyfish is a great threat to Chinese, Japanese and Korean fisheries. These creatures can grow to two meters diameter (the size of large refrigerators) with a weight of 200 kg.

Examples of Giant Jellyfish

Because of their size, they consume massive amounts of zooplankton, depleting this vital part of the food chain for other organisms. The key threat of the Nomura’s derives from the fact that this jellyfish reproduces extremely rapidly. A mature jellyfish has the ability to produce billions of eggs at a time, and they can do this when they are attacked. Once fertilized, these eggs develop into a resting polyp stage that also has the ability to multiply rapidly, effectively carpeting areas of the seafloor. When conditions are suitable, the polyp reproduces asexually, developing into the medusa stage, which grows into the mature jellyfish.

Video: Jellyfish Life Cycle (1:41)

Once ideal conditions develop, either by increasing nutrients or warming of the surface ocean, Nomura’s jellyfish populations literally explode and render fishing virtually impossible because nets become filled with jellyfish. These jellyfish can also continue to reduce the number of fish in the oceans by feeding on their eggs. Moreover, there is evidence that jellyfish can tolerate conditions, like hypoxia, that fish cannot.

The following video summarizes the impact of giant jellyfish on Japanese fisheries.

Video: Monster Jellyfish (2:35)

Blooms of other jellyfish species are being reported in many other parts of the ocean. In the Gulf of Mexico, for example, the last thirty years populations of two species of jellyfish, the sea nettle, and the moon jellyfish, have exploded especially in dead zones as these are one of the few organisms that can tolerate hypoxia. Jellyfish in the Gulf now swarm over hundreds and perhaps even thousands of square miles each summer.

A swarm of sea nettles (Chrysaora quinquecirrha) in the Gulf of Mexico.

Credit: Lyn Gateleyn(CC BY-NC-SA 2.0)

Here also, invasive species of jellyfish, including the Australian jellyfish, have been reported. Several other factors besides hypoxia have caused the increase in Gulf of Mexico jellyfish. As in the Sea of Japan, overfishing has reduced one of the main jellyfish competitors. In addition, drilling platforms have provided habitats in which jellyfish polyps can multiply. As in the Sea of Japan, jellyfish in the Gulf of Mexico are impacting the fishing industry.

Jellyfish swarms, too, have plagued other regions; they include northern Australia where the highly venomous box jellyfish has expanded its range, the Black Sea, and the Bering Sea off Alaska. Worldwide, jellyfish are one of the few organisms that can thrive in dead zones. With the spread of such dead zones in the oceans as a consequence of marine pollution and climate change, we could be entering the age of the jellyfish.

End of Module Recap:

In this module, you should have learned the following concepts:

• factors that cause the pH of the oceans to change and how it impacts saturation of the minerals calcite and aragonite;
• the biology and ecology of reef environments and how they are likely to change in the future with acidification and rising temperature;
• the potential impact of acidification on calcareous plankton groups and implications for the marine food chain;
• the biology of harmful algae including dinoflagellates and diatoms;
• factors that cause blooms of harmful algae, why they are likely to increase in the future, how they can be forecasted, and how they impact human health;
• about Cyanobacteria and the factors that cause them to bloom;
• about large jellyfish and factors that have led to major increases in their populations.

Assignments

You should have read the contents of this module carefully, completed and submitted any labs, the Yellowdig Entry and Reply, and taken the Module Quiz. If you have not done so already, please do so before moving on to the next module. Incomplete assignments will negatively impact your final grade.

Labs

• Lab 7: Reef Ecology