by Heather Alberro, RJD Intern

Phytoplankton, microscopic marine photosynthetic organisms, have a vastly significant role to play not only in the marine food web of which they’re part of, but also on a more global scale. Despite their infinitely small size in comparison to other marine organisms, these tiny creatures occupy an immensely important ecological niche: they are the foundation of the marine food web, and as primary producers, play key roles in supporting all other organisms in the marine environment, as well as in the regulation of the Earth’s climate through the sequestration of carbon, oxygen production, and other related processes. Phytoplankton account for roughly half of all global primary productivity; therefore, their significance extends far beyond the marine environment alone. There is an intriguing sense of irony in the realization that these tiny living beings that often live out their existence unnoticed and undetected by the rest of the world, have such a far-reaching impact on the lives of virtually all other living organisms on the planet, particularly on those in the marine environment. The world that we have become accustomed to has been and is continuously shaped by the workings of these miniscule yet vital “plants of the sea”.

Freshwater phytoplankton, mainly Diatoms and Dinoflagellates

The name “phytoplankton” can be divided into two meanings, “phyto” being the Greek word for “plant”, and “plankton” meaning “to wander or drift”; thus, phytoplankton are microscopic “drifting” plants that live in aquatic environments, and are not restricted to the oceans. However, phytoplankton are not merely one homogenous group of organisms, they represent a rich diversity of shapes, colors, and varieties, ranging from single-celled photosynthetic bacteria such as cyanobacteria, to plant-like diatoms and armor-plated cocolithophores[1]. As the aquatic counterparts to plants on land, the terrestrial primary producers, phytoplankton contain a green pigment known as chlorophyll, which captures sunlight and then, through the photosynthetic process, transforms it into chemical energy. Sunlight is crucial for phytoplankton productivity, as is the case for terrestrial plants. This photosynthetic ability is characteristic to all phytoplankton, and it is through this ability that they perform the crucial functions of absorbing carbon dioxide and releasing oxygen, processes that are essential for the continuation of life.

Coccolithophore – Single-celled marine phytoplankton

Cyanobacteria warrant a closer look, as they are by far the most ubiquitous and ancient group of phytoplankton on our planet, a point on which Paul Falkowski et al dwell in their article, Phytoplankton and Their Role in Primary, New, and Export Production. Cyanobacteria, also known as “blue-green algae” due to their color, are a class of prokaryotic[2] phytoplankton that evolved over 2.8 billion years ago, playing an essential role in shaping the Earth’s carbon, oxygen, and nitrogen cycles over sweeping expanses of time[3], and leading to the biogeochemical conditions of the present. In light of the vast numbers of cyanobacteria currently present in the biosphere, Falkowski et al note, “There are approximately ,10-24. cyanobacterial cells in the oceans”, a number that exceeds “all the stars in the sky”. These organisms are all-important and ever-present, yet remain virtually imperceptible to all other living beings.

The photosynthetic abilities of phytoplankton play a key role in the regulation of the Earth’s climate, largely through their impact on the carbon cycle. Just like terrestrial plants, phytoplankton, through the photosynthetic process, consume vast quantities of carbon dioxide. This carbon dioxide is stored within the phytoplankton. When phytoplankton are preyed upon by other organisms, some of the carbon makes its way back to near-surface waters, and some travels to the ocean depths. According to Rebecca Lindsey and Michon Scott’s article, What are Phytoplankton?, this ““biological carbon pump” transfers about 10 gigatonnes of carbon from the atmosphere to the deep ocean each year. Even small changes in the growth of phytoplankton may affect atmospheric carbon dioxide concentrations, which would feed back to global surface temperatures.” These miniscule beings, through a series of chemical processes, regulate key global activities in the biosphere such as the climate system, which affect all other living organisms in marine and terrestrial ecosystems alike.

Cyanobacteria under microscope

In The Functioning of Marine Ecosystems, Philippe Curry et al shed light on the significance of phytoplankton in the overall functioning of the marine food web, and how phytoplankton exert a sort of “bottom-up” control on the food web’s various components. As primary producers who provide some of the basic elements essential to life, such as oxygen, phytoplankton essentially regulate food web dynamics, as they form the very basis of its existence. Phytoplankton, the “plants of the sea”, form the foundation of the marine food web, supporting successive trophic levels such as zooplankton[4], organisms that feed on zooplankton such as fish, and then predators that feed on the fish such as seals, sea lions, sharks, and marine mammals. Therefore, even organisms at the very top of the food web, including apex predators such as sharks and orcas, ultimately depend on the ecological base that is formed by phytoplankton. Declines in phytoplankton populations, apart from its effects on the Earth’s climate, can result in subsequent dwindling zooplankton populations, which in turn affect secondary and tertiary-level consumers such as fish and sharks.

Phytoplankton, as photosynthetic primary producers, not only form the ecological foundation of aquatic environments, but also serve as key drivers of the Earth’s carbon and oxygen cycles. These vital ecosystem functions are crucial to life on Earth for all living organisms. It is rather remarkable that such infinitesimal creatures have played, and continue to play, such principal roles in shaping the Earth’s biogeochemical composition. Phytoplankton are the source of crucial processes such as photosynthesis which provide the elements necessary for nearly all other organisms to survive. As Science Daily illustrates, “Phytoplankton is the fuel on which marine ecosystems run. A decline of phytoplankton affects everything up the food chain, including humans.” Even the largest being in existence, and to have ever existed, the blue whale, ultimately relies on a viable population of phytoplankton in order to sustain itself. The very small in many ways control and support the very large.

REFERENCES

Falkowski, Paul G., et al. “Phytoplankton and their role in primary, new, and export production.” Ocean biogeochemistry. Springer Berlin Heidelberg, 2003. 99-121.

Lindsey, Rebecca, M. Scott, and R. Simmon. “What are phytoplankton.” NASA’s Earth Observatory. Available on http://earthobservatory. nasa. gov/Librar y/phytoplankton (2010).

Cury, Philippe, Lynne Shannon, and Yunne-Jai Shin. “The functioning of marine ecosystems: a fisheries perspective.” Responsible fisheries in the marine ecosystem (2003): 103-123.

Dalhousie University. “Marine phytoplankton declining: Striking global changes at the base of the marine food web linked to rising ocean temperatures.” ScienceDaily, 28 Jul. 2010. Web. 31 Oct. 2013.

by Hanover Matz, RJD Intern

While global climate change is often the environmental concern at the forefront of the discussion about greenhouse gas emissions, ocean acidification is a marine conservation issue just as closely tied to the amount of carbon dioxide (CO₂) humans have put into the atmosphere since the Industrial Revolution. It is understood that the oceans act as a sink for atmospheric CO₂: as humans increase the amount of carbon dioxide in the atmosphere by burning fossil fuels, more carbon dioxide diffuses from the atmosphere into the world’s oceans. This increase in the uptake of CO₂ affects the ocean by reducing the pH, or increasing the acidity, of seawater, an effect known as ocean acidification (Kleypas et al. 2006). Chemically, ocean acidification occurs through the following process: an increase in the concentration of CO₂ in the water leads to an increase in the concentration of two chemicals: bicarbonate (HCO₃^–) and hydrogen ions (H⁺). By increasing the concentration of H⁺, the pH of the water is lowered and becomes more acidic. This shift in equilibrium towards bicarbonate and hydrogen ions also causes a shift in the chemistry of calcium (Ca²⁺) and carbonate (CO₃^2-) ions. Hydrogen ions react with available carbonate ions to produce more bicarbonate, a process which reduces the formation of solid calcium carbonate (CaCO₃). Thus ocean acidification has two significant chemical effects on the marine environment: it lowers the pH and decreases the availability of carbonate (Hoegh-Guldberg et al. 2007)  

The chemical reactions involved in ocean acidification (Hoegh-Guldberg et al. 2007)

What does this mean for coral reefs? The hard coral species that make up reefs today belong to the order Scleractinia. These scleractinian corals are a colony of polyps that form a hard exoskeleton by secreting aragonite, a solid form of calcium carbonate. Increasing ocean acidification reduces the availability of carbonate in the water as well as the pH, so it is more difficult for the corals to form necessary hard skeletons. Many cellular and physiological responses have been observed in corals subjected to increased acidification, as shown in a 2012 study by Kaniewska et al. on Acropora millepora. The corals in the study were subjected to increasing levels of CO₂, and were shown to exhibit changes in metabolism, calcification, and cellular activity. Not only do high levels of CO₂ make it more difficult for corals to calcify, or form hard skeletons, due to the lack of carbonate, but they make the energy investment in calcification for the coral more costly. Corals rely on endosymbiotic algae in their cells known as Symbiodinium, or zooxanthellae, for energy from photosynthesis. Kaniewska et al. showed that increasing the level of CO₂ caused the coral branches to lose their symbiotic algae, a process normally caused by increasing ocean temperature known as bleaching. Those corals that retained their zooxanthellae exhibited a 60% reduction in net photosynthesis per cell. A reduction in photosynthesis means less available energy to coral polyps, which in turn reduces coral health and reproductive ability. The study also indicated an increase in internal cellular pH regulation by the corals due to changes in CO₂ levels. Increasing internal pH regulation may result in less energy being devoted to calcification. By decreasing calcification, not only does ocean acidification decrease coral growth, but it also decreases the accretion of the reef system as a whole.

Why do these physiological effects on corals matter to the reef ecosystem, or to human society? Corals constitute the primary three dimensional structures of most reef systems; any negative effect to their health will detrimentally affect the health of the reef. A study by Hoegh-Guldberg et al. published in 2007 demonstrated the effect increasing ocean acidification will have on coral reef ecosystems. The use of field studies and experimental simulations produced a model that showed as global ocean temperatures rise and pH levels fall due to increasing atmospheric CO₂, it is expected that coral dominated communities will be replaced by macroalgae and non-coral dominated communities. The basic cause behind this is decreased coral calcification: if it becomes harder for the corals to produce their calcium carbonate skeletons, their structures will become weaker, their growth decreases, they may be eroded or damaged, and they will be outcompeted by other species, specifically macroalgae. The stress induced by ocean acidification may also cause reduced coral reproduction, yet another factor leading to decreased coral dominated reefs. Without corals, the biodiversity of a reef system greatly decreases as there is no longer a viable habitat for many fish species. For humans, this means significant potential damage to both fishing and tourism industries that rely on coral reefs and the fish they support. Without tourism and fishing, many countries would not only lose a significant source of income, but a significant food source for their growing populations. Coral reefs also provide protection from wave action and storms, reducing coastal erosion. The study indicates that the model takes into account atmospheric CO₂ increases at the lower end of predictions for the coming century. The authors astutely note that it is “sobering” to realize these serious effects on coral reefs are based on the most optimistic outcomes of atmospheric CO₂ and global temperature changes.

Potential dominant reef communities at predicted levels of atmospheric CO2 and ocean temperature increases (Hoegh-Guldberg et al. 2007)

Is there any hope for coral reefs? Is it at all possible that they can adapt to the threat of ocean acidification? One study does indicate that some corals may have the ability to adjust to decreasing ocean pH. McCulloch et al. published a study in 2012 that focused on the ability of corals to up-regulate their internal pH levels. Corals precipitate new calcium carbonate in a fluid between the existing skeleton and part of the polyp known as the calicoblastic ectoderm. At this calicoblastic layer, corals are capable of increasing the pH relative to the pH of ambient seawater in order to facilitate calcification. The study results indicate that for some coral species, as the ambient seawater pH decreases due to acidification, the corals are capable of further up-regulating their internal pH in response in order to reduce the overall internal change in pH and to continue to calcify. This up-regulation of internal pH results in higher coral calcification rates compared to abiotic or chemical precipitation of calcium carbonate at the same seawater pH. The coral species demonstrate an ability to adjust their internal pH in order to continue calcifying in acidic conditions. Does this mean these coral species will be better able to survive increasing ocean acidification? Perhaps, but the study indicates that it is necessary for the corals to maintain their symbiotic relationship with zooxanthellae in order to produce the energy needed for calcification. The loss of zooxanthellae to stress or bleaching events would reduce the effectiveness of this ability. While some corals may exhibit less sensitivity to pH changes than others based on their ability to up-regulate internal pH, all coral species will likely have difficulty adapting to not only ocean acidification, but the combined effects of ocean acidification, changes in ocean temperature, and the impact of human pollution.

Seawater pH versus internal pH of calcifying fluid of coral species. Foraminifera (forams), another type of calcifying marine organism, do not exhibit this ability to up-regulate internal pH (McCulloch et al. 2012)

Ocean acidification is a significant threat to the health of coral reef systems. What can be done to prevent potential damage from acidification? In the face of this danger to reef ecosystems, there are possible conservation methods that can be taken to protect coral species. Coral reefs that are already in a healthy state are better prepared to handle changes in pH than those suffering from other environmental stressors. Reefs with stable levels of herbivorous grazers, such parrotfish or the sea urchin Diadema antillarum, are also more resilient to stress due to reduced competition with algae (Hoegh-Guldberg et al. 2007). Effective conservation management of coral reefs provides the best method for ensuring their survival. Continuing research to determine how to mitigate the effects of acidification is also necessary. As coral reefs are threatened by climate change, pollution, and other human induced stressors, ocean acidification will remain one serious part of the ongoing endeavor to protect coral reefs.

REFERENCES

1. Hoegh-Guldberg, Ove, et al. “Coral reefs under rapid climate change and ocean acidification.” Science 318.5857 (2007): 1737-1742.
2. Kaniewska, Paulina, et al. “Major cellular and physiological impacts of ocean acidification on a reef building coral.” PLOS ONE 7.4 (2012): e34659.
3. Kleypas, J.A., R.A. Feely, V.J. Fabry, C. Langdon, C.L. Sabine, and L.L. Robbins, 2006. Impacts of Ocean Acidification on Coral Reefs and Other Marine Calcifiers: A Guide for Future Research, report of a workshop held 18–20 April 2005, St. Petersburg, FL, sponsored by NSF, NOAA, and the U.S. Geological Survey, 88 pp.
4. McCulloch, Malcolm, et al. “Coral resilience to ocean acidification and global warming through pH up-regulation.” Nature Climate Change 2.8 (2012): 623-627.

by Pat Goebel, RJD Intern

In the South Pacific, periodically harvested fisheries closures are often implemented as a conservation and fisheries management tool. This is an important management tool because it allows resource users a greater say in the development and enforcement of rules, which in turn will lead to a successful fisheries management. Periodically harvested fisheries closures are areas of fishing grounds where fishing is normally prohibited, but is occasionally permitted for a short period. This is not to be confused with periodic closures, where areas of fishing grounds are normally open and occasionally closed. Previous studies have shown that periodically harvested closures can sustain higher fish biomass and larger individuals, particularly of targeted species. However, there is a lack of knowledge on whether periodically harvested closures can provide both social and ecological benefits.

A recent study conducted by Januchowski-Hartley and colleagues investigated the role of fish behavior, the effects of periodic harvest on fishery targeted families and total fish biomass in the Ngunao-Pele Marine Protected Area Network, North Efate, Vanuatu (Figure 1). A before-after-control-impact pair design, was used to quantify flight initiation distance (FID), and biomass of two fishery-target (Acanthuridae and Scaridae) and one non-target (Chaetodontidae) families in two periodically harvested closures, two no-take marine reserves, and two open fished areas, prior to and after harvest of the periodically harvested closures. Creel surveys were used to quantify catch per unit effort in open fishing grounds and during the periodic harvest.

Figure 1 from Januchowski-Hartley et al. 2013

Before harvest, FID of targeted families was higher in fished areas than periodically areas (Figure 2). Total Biomass was lower in fished areas than in no-take reserves and periodically harvested closures. As a result of lower FID and higher biomass, CPUE increased for fishing trips inside the periodically harvested closures than regular fishing activities. Also, fishes were generally larger in catches from periodically harvested closures.

Acanthuridae FID differed significantly between and pre- and post-harvest, while Scaridae did not differ pre- to post harvest. However, Scaridae FID in no-take reserves was significantly lower than in periodically harvested closures, which in turn was significantly lower than in fished areas (Figure 2).

Acanthuridae were significantly more abundant in the harvest than Scaridae. Before, harvest Acanthuridae had a mean FID below the maximum effective range of spear guns, while Scaridae had a mean FID at the maximum effective range of spear guns. Spear fisherman will target fish with a higher catachability or lower FID. This finding is an important tool for fisheries management as some fishery-target families are more susceptible to harvesting than other based on behavioral changes.

Figure 2 from Januchowski-Hartley et al. 2013

This study provides evidence that lightly harvested periodically harvested closures are an alternative tool that can maintain similar levels of biomass to marine protected areas, while increasing fishing efficiency when opened for harvesting. This increase in efficiency appears to arise primarily through changes in the behavior of fishery target reef fish. Before harvest, mean FID of Acanthuridae and Scaridae were lower in the periodically harvested closures than fished areas, while CPUE during harvest of the closures was almost double that of normal fishing activities. When fishes are protected temporarily from fishing, their cautiousness declines, which makes them more susceptible of being harvest when fishing is reinstated. Periodically harvest fisheries closures can maintain biomass and provide many of the benefits that are expected from permanent no-take areas. However, more research is needed to understand the sustainable limits of periodic harvested closures. In this study a low-intensity harvesting strategy was used and did not lead to a decline in biomass. A longer duration or more intense harvesting could possible lead to a decrease in biomass.

REFERENCE

Januchowski-Harlety F. A., Cinner J. E., Graham A.J. (2013) Fishery Benefits From Behavioral Modification of Fishes in Periodically Harvested Fisheries Closures. Aquatic Conservation: Marine and Freshwater Ecosystems.  DOI: 10.1002/aqc2388

by Jessica Wingar, RJD Intern

Sea otters, Enhydra lutris, are very playful and charismatic marine organisms. They can often be seen swimming on their backs, just floating in the ocean. In addition to the fact that they are extremely charming creature, they have very many distinct qualities that not many other marine animals possess. Sea otters can often be seen using tools. By this, it is meant that they use rocks and whatever other objects that they can find in the ocean. They use these tools to crack open the shells of crustaceans, which happen to be one of their main sources of food (“Sea Otter”). Not only are sea otters important because of their unique tool use, but they also play a critical role in the food chain of the ecosystem that they live in.

A Sea Otter floating on its back in seagrass.

Every ecosystem on the entire planet, has a system of what organism eats what and how these organisms affect each other. This cascade of organisms is often referred to as a trophic pyramid. In this pyramid there are different levels. Starting at the bottom, the levels go from the primary producers to the primary consumers to as many consumers as the system has, and then finally to the top consumers, which are the predators (Garrison, 2010). There are many ways in which these systems are controlled. Tropic cascades that are controlled top down are systems in which the consumers control the abundance or the lower levels, and systems that are controlled bottom up are systems in which the amount of consumers is limited by the availability of resources that are at the lower levels (Richardson, 2013). With increasing anthropogenic affects on ecosystems, what controls top up and bottom down is rapidly changing.

Sea otters and their interaction with other organisms in their environment, are often used as a classical example of a trophic cascade. As mentioned previously, sea otters like to feed on crabs. These crabs like to feed on seagrass in their marine community. Thus, sea otters control the abundance of crabs and crabs control the abundance of seagrass in their oceanic ecosystem. However, human affects have altered some top down and bottom up controls on this community. Some of the top down anthropogenic affects include hunting of the top predators. An example of this would be killing sea otters for their fur. Some bottom up controls include increasing the amount of nutrients in the water. The increase of nutrients comes from run off from the land. The question is, does the increase in sea otters, still cause the crab community to decrease and the seagrass community to increase?

It is critical to know what is happening with the seagrass community because of the number of sea otters in the ecosystem because seagrass has a very important ecological role. Seagrass serves as food for crabs, it controls secondary production, and it can also serve as a nursery habitat for many organisms. Since, sea otter decline has been happening for many years, when a sea otter community began to increase in numbers and nutrient run off increased in this area, it provided scientists with a perfect opportunity to study this trophic cascade.

The trophic cascade of the sea otter ecosystem.

There was a study conducted in Elkhorn Slough in California. One of the most staggering facts from this survey was that when sea otters recolonized the area in 1984, the eelgrass population increased by 600%, which definitely displays that sea otters have something to do with the population of seagrass. According to the trophic cascade, an increase in sea otters would lead to a decrease in the crab population. It was discovered that the crab population did decrease. It was concluded that sea otters were one of the main causes of this decrease because during this time, other predators in the ocean, such as sharks, were in decline due to overfishing. In addition, during this time crab harvesting had gradually decreased in this area because it became a Marine Protected Area in 2007. In addition to studying history and the current populations of sea otters, crabs, and eelgrass, these researchers did a computer model of the area, and they found the same results. Sea otters are controlling the amount of crabs and seagrass present in their communities, and are crucial to the well running of their habitats (Hughes et al, 2013).

REFERENCES

Garrison, T. (2009). Oceanography:an invitation to marine science. (7th ed.).

Hughes, B.B., Eby, R, Van Dyke, E, Tinker, T, Marks, C.I., Johnson, K.S., and Kerstin Wasson. “Recovery of a top predator mediates negative eutrophic effects on seagrass.” PNAS. 110.38 (2013): 15313-15318.

Sea otters. (2013). Retrieved from http://animals.nationalgeographic.com/animals/mammals/sea-otter/

Richardson, Jill. “Feeding Ecology and Behavior.” MSC 350. University of Miami, Coral Gables. Mar. 2013. Lecture.