changing oceans:effect on biodiversity

Biodiversity and Climate Change 2000

Changing Oceans

Effects on biodiversity

PlanktonPhytoplankton are regarded as the most important biomass producers in the oceans. They are responsible for removing carbon dioxide from the atmosphere and transferring the carbon to other trophic levels. Phytoplankton also provide a vital link to deep ocean organisms when they die and organic material falls to the seabed. They are found in the top layer (the euphotic zone) where they receive enough solar radiation for their photosynthetic requirements as well as nutrients that are transported by the upwelling process.

Our knowledge of the impacts of temperature change on phytoplankton populations is poor and reliant on predictions of changing physical processes. Two such changes that could result from an increase in sea surface temperature are altered wind patterns and increased stratification of the water column. These effects could significantly reduce nutrients reaching the euphotic zone in certain parts of the world. This would lead to a decrease in primary production which would mean less overall productivity as well as less carbon dioxide removed from the atmosphere. The deep sea ecosystem would also be affected by a reduced carbon input.

Alterations in nutrient levels and temperature may also effect the species composition of phytoplankton populations in different regions, which would have serious repercussions for the rest of the marine environment. Not all types are readily eaten by zooplankton, some are even toxic, and different species also absorb carbon dioxide to varying degrees. A shift in species in any one area could lead to less carbon dioxide being absorbed from the atmosphere.

While phytoplankton are responsible for carbon fixation, bacterioplankton play a vital role in the mineralisation of nutrients and provide a trophic link to higher organisms. The breakdown products of organic matter are taken up by bacterioplankton and an indirect result of climate change could be a reduction in nutrients available to these microorganisms, impeding their function in the carbon flux of aquatic ecosystems.

The reduction in numbers or change in species composition of phytoplankton could lead to a reduction in zooplankton abundance. Since the 1950s scientists have recorded a decrease in zooplankton numbers in the Californian current. Whether this steady decline is attributable to a decrease in primary production or an increase in predation on the zooplankton, is not certain.

Plankton are not only affected by temperature changes but the increase in ultraviolet (UV) radiation, as a result of ozone depletion, also poses a threat to their survival. Studies have shown that UV radiation actually inhibits photosynthesis in phytoplankton but it is difficult to quantify these effects in the open ocean due to variation caused by factors such as vertical mixing and cloud cover. It is thought that rapid vertical mixing in the water column further inhibits photosynthesis whereas a greater cloud cover reduces the harmful effects of UV radiation. In addition UV rays affect growth and reproduction as well as the functioning of enzymes and cellular proteins in phytoplankton. Damage to phytoplankton has been demonstrated at the molecular, cellular, population and community levels but the effects of increased UV radiation at the ecosystem level remain uncertain.

Bacterioplankton seem to lack UV-screening pigments and radiation affects the enzymes responsible for breaking down their food. When bacterioplankton move to deeper waters, however, the photorepair process carried out by UV-A (blue light) allows the take up of organic breakdown products. More research is required to determine the true impact of UV radiation on these minute but important organisms.

It has also been demonstrated that zooplankton are damaged by UV radiation and even small increases in UV exposure could result in significant reductions in the consumer community.

Fish

A decrease in plankton production, which could result from a changing climate, would mean less food for fish populations. In addition, temperature increases are expected to shift many species polewards where the climate is cooler and hence metabolic rates could be kept low. Warm-water species may increase in abundance with more favourable conditions at lower latitudes. This situation could have a significant impact on commercial fisheries such as that of Atlantic cod as this species feeds on temperature-sensitive ones like mackerel and herring. Research into the decline of important salmon stocks on the west coast of Canada and Alaska has suggested that these temperature changes are already having an impact.

Natural climate variability experienced during the El Niño Southern Oscillation (ENSO) event provides further warning of the potential impacts of climate change. In the past ENSO events have led to a mass die-off of fish species such as Peruvian anchovies and sardines. Further large-scale mortalities could result if the unusual temperatures experienced during an ENSO period become more commonplace with climate change.

The eggs and larvae of many fish species are sensitive to ultraviolet (UV) radiation. The naturally high mortality of fish larvae make it difficult to accurately assess the direct effects of ozone depletion when the primary causes of population decline are predation, poor food supply for larvae, overfishing of adults, water temperature, pollution and disease. Nonetheless, UV radiation appears to be harmful at both primary and secondary production levels. The number of variables involved make predictions of impacts on a global scale and further along the food web a complex task.

Seabirds

Many of the seabirds that prey on fish and plankton are likely to have their food supply reduced or relocated if the predicted effects of climate change on lower trophic levels occur. The natural climate variability of El Niño Southern Oscillation (ENSO) events has already given us an insight into what we may expect with more permanent climate change. Starvation was unmistakeably the cause of a large decline in Alaskan shearwaters and common murres during the 1997-98 El Niño Southern Oscillation and cormorants and pelicans experienced mass mortalities during the 1982-1983 event. This is not, however, a simple cause and effect relationship as other species, such as Leach's storm petrel in the Californian current, have increased in number during warmer periods and may benefit from such changes in climate. Some species have shown an ability to shift their food sources when preferred prey are unavailable.

A lack of food also affects the reproductive success of seabirds with a reduction in numbers of eggs produced, those successfully hatching and the number of breeding pairs. Shifts in food supplies associated with ENSO events have been blamed for reproductive failure in some seabirds. Population changes experienced by species off California, where the frequency of warm sea surface temperatures has been increasing since 1977, have been well-documented. There is also evidence that some species of seabirds are changing their breeding season during the periods of warmer sea temperatures. Cassin's auklets, off the coast of British Columbia in Canada, normally time their breeding cycle so their chicks hatch right after the zooplankton bloom but now the chicks hatch a month before the peak of zooplankton production to avoid the warmer temperatures. The survival rate of both adults and chicks is affected by these changes.

The predicted increase in storm frequency and severity with climate change may directly affect some seabird populations. Studies on common guillemots in the north Atlantic show that storms appear to impede fishing activities, reducing the size and number of fish that adults can feed to their chicks.

The mobile nature of seabirds means that they will be more able to shift their distribution away from the warmer areas towards the poles. Adelie penguins, however, are resident in the Antarctic and have nowhere cooler to migrate to. Indeed a decrease in the availability of sea ice, where this species overwinters, has led to its decline in numbers. Conversely, South Polar skuas and blue-eyed shags prefer open water and have been increasing in numbers and extending their geographic ranges further south.


Marine mammals

The predicted reduction in primary productivity that could result from increased sea temperatures would inevitably reach the top of the food web with supplies for marine mammals being affected. Natural climate variability during an El Niño Southern Oscillation (ENSO) event has reduced the abundance of important prey species in certain regions of the world and the impacts on marine mammals have been studied.


Young female seals and sea lions and the young themselves were most severely affected in the eastern Pacific area during the 1982-83 ENSO. These females were required to travel away from their young in search of food and with reductions in prey they had to travel further and expend more energy to achieve this. This situation has resulted in a decline in the physical condition of females as well as reduced milk production and pregnancy rates. Young seals and sea lions suffered with higher than usual death rates and reduced growth rates as their mothers were less able to produce milk and left them alone for longer periods. In addition it took several years for the fish stocks and hence the pinniped populations to recover. Other species, such as the Antarctic crabeater seal, are declining in numbers due to their dependence on the decreasing areas of sea ice at high latitudes.

Animals that prefer a more open water situation, like the Southern elephant and Southern fur seals, appear to be increasing in numbers. Shifts in species composition such as these could become more common place as environmental conditions change to favour one lifestyle above another.

Sea ice supports other marine mammals like walruses and ringed seals by providing critical breeding and resting-places. Polar bears spend considerable periods on the ice while catching their preferred prey, seals. Temperature increases in the Arctic have caused sea ice to break up earlier in the summer allowing polar bears less time to build up vital fat reserves to last them through the winter months. If this trend continues a decline in the weight of adult polar bears and birthrates, already documented in Hudson Bay populations, will become a serious threat to their survival.

The impacts of climate change on whales and dolphins could be considerable but difficult to quantify at this trophic level and currently little evidence exists to show how they have been affected. The availability of food may pose a problem for some species of cetaceans especially those visiting important feeding grounds in polar regions. The increase in temperature already experienced in these areas has led to a reduction in the thickness and extent of sea ice and could decrease the abundance of phytoplankton. In the Southern Ocean krill (Euphausea superba), are an important invertebrate eaten by many species of whales and they rely on sea ice for food and protection. In the Arctic key prey species of cetaceans, such as copepods and plankton-feeding fish, could also be reduced. Some cetacean species may be able to adapt to another food source but others, such as belugas and narwhals, may be more dependent on the plankton-feeding prey.


An increase in temperature could have some secondary effects on cetaceans. Melting sea ice would open up the North West Passage into the Arctic and expose cetaceans to increased ship traffic and mineral exploitation. Collisions with ships, disruption of migration patterns, increased mortality through stress and interference with communications would be just a few of the inevitable results.

Changing Oceans introduction
Physical and chemical effects of climate change
Physical effects of ozone depletion and enhanced ultraviolet radiation
Effects on important ecosystems



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