Kennedy W. Nyongesa and Shem O. Wandiga
Institute of Climate Change and Adaptation, University of Nairobi
Aquatic ecosystems are critical components of the environment; they contribute to biodiversity and ecological productivity, they also provide a variety of ecosystem services for human populations, including water for drinking and irrigation, recreational opportunities and habitat for fish species that are economically important. However, aquatic systems have been threatened increasingly, directly and indirectly by human activities. In addition to the challenges posed by land-use change, environmental pollution and watercourse diversions, aquatic systems are experiencing the added stress of global climate change. Increase in water temperatures because of climate change will alter fundamental ecological processes and the geographic distribution of aquatic species. Such impacts may be ameliorated if species attempt to adapt by migrating to suitable habitats. However, human alteration of potential migratory corridors may limit the ability of species to relocate, increasing the likelihood of species extinction and hence loss of biodiversity. Changes in seasonal patterns of precipitation and runoff will alter hydrologic characteristics of aquatic systems and hydrological regimes, affecting species composition and ecosystem productivity. Communities of aquatic organisms are sensitive to changes in the frequency, duration, and timing of extreme precipitation events such as floods or droughts. Changes in the seasonal timing of snowmelt will alter stream flows, potentially interfering with the reproduction of many aquatic species. Climate change is likely to further stress sensitive freshwater and coastal wetlands, which are already adversely affected by a variety of other human impacts, such as altered flow regimes and deterioration of water quality. This essay reviews Global Climate Change impacts on aquatic ecosystems and biodiversity; a conceptual framework is presented and a conclusive working model is illustrated.
Key Words: Aquatic Ecosystems, Climate Change, Impacts Biodiversity
This paper was presented at the 11th Egerton University International Conference and Peer- reviewed, corrected and recommended for publication : Dates: 29 – 31 March, 2017 “Knowledge and Innovation for Social and Economic Development. “
The diversity of life on earth is dramatically affected by human alterations of ecosystems (IPCC, 2001), biodiversity is continually transformed by a changing climate. Approximately 70% of the earth’s surface is covered by water. Climate change is already changing the distribution and abundance of biota in aquatic ecosystems. Even minor changes to water temperature will result in changes to the current that flow across the earth’s surface. Aquatic ecosystems broadly fall in to two categories:
(a) Marine Ecosystem (b) Fresh water Ecosystem.
a) 2.0 Marine ecosystem
Climate change within the ocean
The increase in greenhouse gases within the earth’s atmosphere is set to change three fundamental variables:
(i). Reduced Total Carbonate alkalinity
Total carbonate alkalinity of seawater will decrease as CO2 increases within the earth’s atmosphere (Gattuso et al., 1998; Kleypas et al., 1999). This particular variable is expected to substantially change the acidity and carbonate ion pool of the global ocean. Doubling carbon dioxide concentrations in the atmosphere will decrease the aragonite saturation state in the tropics by 30% by 2050.
ii). Increased Sea level
Changes in sea level have had major impacts on the abundance and particularly the distribution of both marine and terrestrial biodiversity. Sea level will rise as climate changes pushes planetary temperature higher. This occurs due to the thermal expansion of ocean water, the melting of glaciers, and changes to the distribution of ice sheets. The expected increasing in sea level is approximately 9 – 29 cm over the next 40years or 28 – 29 cm by 2090 (Church et al., 2001; IPPC2001). According to Nichols et al. (1999), sea level rise could cause the loss of up to 22% of the world’s coastal wetlands by 2080. Combined with other human impacts, this number is likely to climb to a loss of 70% of the world’s coastal wetlands by the end of the 21st century.
iii) Sea temperature increase
Significant increase in heat content has not been distributed evenly (IPCC, 2001); sea temperature increase in turn influences the marine environment. Due to its direct effects on the density of seawater, changes in global temperatures can play directly upon the rates and directions of ocean water movement.
b) Freshwater Ecosystems
Gattuso et al. (1998) observe that threats to freshwater fauna fall into several broad categories: – nutrient enrichment, hydrological modifications, habitat loss and degradation, pollution, and the spread of invasive species. They further argue that a changing climate and increasing levels of UV light pose additional risks that superimpose upon existing threats. The combination of rapid land use change, habitat alteration and a changing climate are viewed as serious challenges to aquatic ecosystems. Surface freshwater is a small fraction of global water; healthy freshwater ecosystems provide vital ecosystem services to human societies including the provision of clean water for drinking for agriculture, for fisheries, and for recreation (IPCC, 2001). Many regions, in the world have insufficient clean water to meet even the minimal demands for human survival.
Climate change and the hydrological cycle
Freshwater ecosystems will naturally be sensitive to change in the hydrologic cycle and these are difficult to predict (Gattuso et al., 1998); a warmer climate will result in greater evaporation from water surfaces and greater transpiration by plants that will result in a more vigorous water cycle (IPCC, 2001). Future climate change will directly affect lake ecosystems through warmer temperature and changes to the hydrologic cycle. Biological impacts and rapid climate change has many negative implications for the biodiversity of rivers and streams (Kleypas et al., 1999). Climate change may cause extinction at several taxonomic levels. At the species level, those species that are highly restricted geographically and ecologically are vulnerable to global extinction (Jackson et al., 2001); this is true for all fish species.
The deep sea is recognized as a major reservoir of biodiversity. It is believed that the deep seabed support more species than all other marine environment. Marine biodiversity and ecosystem are threatened by pollution, shipping, military activities and climate change, but today fishing presents the greatest threat. The greatest threat to biodiversity in the deep sea is bottom trawling. This type of high seas fishing is more damaging to seamounts and the cold-water corals they sustain. These habitats are home for several commercial bottoms- dwelling fish species.
Coastal fisheries are critical resources for hundreds of millions of people. Many scientists now point to the dramatic over exploitation of fisheries and the subsequent decline in fish stocks as the major factor in ecosystem change over the past two centuries (Jackson et al., 2001). Recent evidence has revealed that oceanographic and climatic variability may play a dominant role in fish stocks (Klyashtorein, 1998; Hollowed et al., 2001; Attrill and Power, 2002).The relationship between climate variability and fish stocks is probably complex. In some cases, subtle changes may affect conditions and crucial changes in the life history of the fish species. The most widespread effects of climate occur on the primary and secondary production in marine ecosystems.
Klyashtorein (1998) opines that, tropical intertidal and sub tidal regions are dominated by ecosystems that are characterized by a framework of different types of corals. They have undergone major changes over the past 20 years, much of which has been associated with climate change and other stresses (Bryant et al., 1998); despite the lack of external nutrients, these ecosystems form rich and complex food chains that support large populations of fish, birds, turtles and marine mammals. Light, temperature and the carbonate alkalinity of seawater decrease in a pole ward direction, making the formation of carbonate reefs more difficult at higher latitudes (Hollowed et al., 2001). Coral reefs have already experienced major impacts from climate change. Major disturbances to coral reefs have increased dramatically over the past 30 years and have been linked irrefutably to periods of warmer than normal sea temperatures. Coral bleaching occurs when corals rapidly lose the cells (Bryant et al., 1998); reef-building corals experience mortality rates that may exceed 90% changes, which are likely to have huge impacts on marine biodiversity. Corals form the essential framework within which a multitude of other species makes their homes (Hollowed et al., 2001); fish that depend on corals for food, shelter or settlement may experience dramatic changes or go extinct which are likely to have huge impacts on marine biodiversity. Corals form the essential framework within which multitudes of other species make their habitat (Church et al., 2001). Thousands of other organisms that make the bulk of marine biodiversity are also vulnerable to climate change impacts (Attrill and Power 2002). Figure 1 shows the conceptual framework as envisaged in this review paper.
Climate, Environmental Drivers, and Aquatic Ecosystems
Postel (2000) asserts that the ecological consequences of climate change will largely depend on the rate and magnitude of change in two critical environmental drivers: (1) temperature and, (2) water availability from precipitation and runoff. These factors regulate many ecological processes in aquatic ecosystems, both directly and indirectly. Average temperatures are predicted to increase across North America, but more markedly at higher latitudes (Wigley, 1999); some areas of the continent will become wetter and some drier and the variability in the timing and quantity of precipitation will change, altering patterns of runoff to aquatic ecosystems. Although the precise geography of these regional shifts is uncertain at present because of some limitations in climate forecasting, changes in the fundamental character of many aquatic ecosystems in response to a changing climate are highly probable (Attrill and Power 2002).
Aquatic ecosystems occupy low points on the landscape and because water flows downhill, these ecosystems filter materials that move via gravity from the terrestrial environment (Postel, 2000). Accordingly, pollutants and fertilizers flowing off the landscape accumulate in these systems, impairing their health. Humans often modify the physical structure of these systems for commerce, agriculture, and recreation, resulting in habitat destruction, one of the major stresses on the integrity of contemporary freshwater ecosystems (Vörösmarty et al., 2000). Further, these ecosystems are harmed by direct appropriation of freshwater for human consumption, a problem that is likely to grow in the future. Coastal wetlands also receive the pollutants and eroded silts from human activities on land via freshwater inputs (Houghton et al., 2001); likewise, they experience direct habitat loss from coastal development and erosion exacerbated by reduced river inflows.
Effects of Climate Change on Marine Animals and Ocean change effects
Coral reefs are created in shallow tropical waters by millions of tiny animals called corals. Each coral makes a skeleton for itself, and over time, these skeletons build up to create coral reefs, which provide habitat for lots of fish and other ocean creatures (Spalding, 2001). Warmer water has already caused coral bleaching (a type of damage to corals) in many parts of the world. By 2050, live corals could become rare in tropical and sub-tropical reefs due to the combined effects of warmer water and increased ocean acidity caused by more carbon dioxide in the atmosphere (IPCC, 2007). The loss of coral reefs will reduce habitats for many other sea creatures, and it will disrupt the food web that connects all the living things in the ocean.
Rising temperatures, rising sea levels and other trends are having an effect on the world’s sea turtles. A sea level rise of only 50cm could cause sea turtles to lose their nesting beaches (IPCC, 1990). Six species of sea turtles are already are endangered All female turtles come ashore at nesting beaches, dig nests in the sand, lay their eggs and then return to the sea. Erosion of nesting beaches caused by rising sea level and more intense storms adds the potential for further dangers to nesting beaches. Climate change directly affects the reproduction of sea turtles in three ways (WRI, 1996). First, sea level rise will affect significant nesting beach areas on low-level sand beaches. Second, rising temperatures increase the chance that sand temperature will exceed the upper limit for egg incubation, which is 34 degrees C. Third, rising temperatures bias the sex ratio toward females because temperature during incubation determines the sex of the egg. For example Loggerhead turtle nests in Florida are already producing 90 percent females owing to high temperatures, and if warming raises temperatures by an additional 1 degree C or more, no males will be produced there. Adult feeding patterns are also affected by climate change. Sea grass beds are in decline, water temperature is higher on intertidal sea grass flats, and coral reefs, typically feeding grounds for green turtles, are affected by bleaching.
Sea turtles have existed for more than 100 million years and have survived ice ages, sea level fluctuations of more than 100 meters and major changes to the continents and the seas. As a result, they may be able to respond to unfavorable nesting temperatures or inundation of beaches as they have in the past, by seeking out new nesting sites or modifying the seasonality of nesting (WRI, 1996). The report notes that it may however take decades or centuries for sea turtles to re-establish and stabilize their habitats, and steadily encroaching human development of coastal areas makes the availability of new habitat for them very limited.
Whales and Dolphins
A rise in sea level could there could damage shallow coastal areas used annually by whales and dolphins which need shallow, gentle waters in order to rear there small calves (IPCC,2001). For example The North Atlantic right whales, which can grow to more than 55 feet long and weigh 70 tons, range from Nova Scotia to the southeastern United States and migrate the length of the East Coast. Today there are fewer than 500 right whales left in the world. These animals have been protected since 1935 and listed as endangered since the early 1970s.
Apart from whaling and commercial fishing activities, a tiny crustacean, (Calanus finmarchicus), which is a key food source for right whales, as well as for cod, haddock, herring and mackerel has been affected negatively by global warming (WRI,1996). Without dense patches of this zooplankton, female whales cannot bulk up to prepare for calving, carry a pregnancy to term or produce enough milk. When the concentration of this zooplankton is too low, right whales do not feed; highly concentrated patches often occur where currents converge or at the boundary of water of different densities. Changes of seawater temperature, winds and water currents can affect patch formation of the zooplankton. Shifts in zooplankton populations could affect North Atlantic Right Whale ranges.
Between 1997 and 1999, the numbers of this zooplankton plummeted (WRI, 1996). Over 50 years of observation, scientists have learned that the zooplankton is abundant when the North Atlantic Oscillation (NAO) Index, which charts variations in atmospheric pressure centers over the North Atlantic, is predominantly positive. When it becomes negative, the numbers decline. Right whales illustrate the dramatic downstream significance of NAO conditions. In the winter of 1996, the NAO index exhibited its largest drop of the century. This atmospheric phenomenon has a dramatic effect on the great ocean conveyor belt, the mixing of the warm salty waters that move north with the Gulf Stream and the cold less-salty water moving south from the Labrador Sea. Resulting changes in the water in the Northwest Atlantic determine zooplankton ecology. However, because of the right whales’ long reproductive cycle, the consequences of this climatic event were not over yet for the whales. In 1999, only one right whale calf was born, the lowest on record (there were 21 born in 1996). Nevertheless, in 2001, two years after the abundance of the zooplankton increased, 30 right whale calves were born, the most recorded since 1982.
Some scientists suggest that increased climate variability or a prolonged period of negative NAO index, which are both expected under a global warming scenario, would undermine the already tenuous recovery of the North Atlantic right whale (IPCC, 2014).
The Arctic’s top predator, the polar bear, is affected both by the reduction in sea ice and by reduced stocks of its primary food, the ringed seal (WRI, 1996). Polar bears use sea ice as a platform for hunting their prey and for resting. Polar bears are often described as completely dependent on ice for their survival. Polar bears often have to swim long distances between areas of stable ice. As sea ice becomes thinner and multi-year ice disappears, a greater proportion of females make their dens on land, expending more energy to get there. Decreases in the physical condition of females and in reproduction have already been documented. The Arctic Climate Impact Assessment Report in 2005 stated that polar bears are unlikely to survive the complete loss of summer sea-ice cover. As a result of sea-ice decline, the population of about 22,000 polar bears would decrease by two-thirds by the year 2050 (IPCC, 2014). In 2008, the U.S. Fish and Wildlife Service listed the polar bear as a threatened species under the Endangered Species Act, the nation’s primary tool for conserving imperiled plants and animals. A threatened species is defined as likely to become endangered in the foreseeable future. (Parmesan and Yohe, 2003).
Sea ice is decreasing throughout the Arctic range due to climate change (IPCC, 2007; 2014). Ice reduction decreases the abundance of seals, and increases the amount of energy and time needed for hunting, leaving less energy for reproduction. The already endangered Mediterranean Monk Seals need beaches upon which to raise their pups. Rising temperatures mean that large areas of the ocean that were once frozen throughout the year now become open water. Various Arctic wildlife populations have been forced to adapt to changes in their habitats. According to scientists, the retreat of sea ice has reduced the platform that seals traditionally use to rest between searches for fish and mussels. The warming climate is changing the ocean’s ecology to such a degree that the survival of seals and their young has increasingly become a concern for marine biologists. The loss of sea ice in Antarctica has caused a decrease in the amount of algae, plankton and krill, the foundation of the ocean’s food chain. In recent years, scientists have directed their attention to the impacts of climate change in the Bering Sea’s ecosystem, which appears to be showing early climate change effects including the reduction of food for bottom-dwelling creatures. Global warming will greatly affect the Bering Sea’s phytoplankton (IPCC, 2014). Any changes affecting this ecosystem are of crucial importance, as the Bering Sea produces half of the fish caught in the United States, and almost a third worldwide, every year. The Bering Sea has typically had a large presence of phytoplankton. Phytoplankton are eaten by zooplankton, which are in turn eaten by large fishes. The changes observed in the Bering Sea’s ecosystem affect the marine mammals, including seals, which are part of its food chain.
Fish and Lobster
Commercial fish, shellfish, including cod and lobster, have thresholds of water temperature that limit the conditions under which they can reproduce and grow (WRI, 1996). Temperature influences the location and timing of spawning, which affects the growth and survival of young cod. It is believed that a temperature of 54 degrees F is the maximum for cod survival and those temperatures above 47 degrees lead to a decline of growth and survival of cod. Traditionally, the conditions on Georges Bank, an enormous shoal off the Massachusetts coast, have been ideally suited to the growth and reproduction of cod. There, the intersection of the warm Gulf Stream with the cold, nutrient-rich Labrador current provides optimum conditions for phytoplankton, the zooplankton that eat them and the cod larvae that eat them. The Atlantic cod has been a staple of the New England fishing industry. Generally, phases of low production have corresponded with periods of high water temperatures.
The effects of rising temperatures on cod, for example, may be significant. Currently, temperatures in the waters are at the top of cod’s favored range (IPCC, 2007). If, under the IPCC high emissions scenario, the maximum temperature for cod is reached, this species could disappear from the waters south of Cape Cod. Off Georges Bank, cod could be vulnerable to loss of habitat. And the Gulf of Maine, north from Georges Bank, is likely to continue to support adult cod throughout the century, but not juvenile cod (WRI, 1996).
Lobsters are ectoderms, their body temperatures are determined by the water in which they live. Higher temperatures cause cold-blooded animals to use more energy for respiration, leaving less energy for feeding, growth, energy storage, immune response and reproduction (Parmesan and Yohe, 2003). Much of a lobster’s life is related to the temperature on the ocean floor, what it eats, how successfully it breeds and where it migrates. As ocean temperatures fluctuate, so do the lobsters’ behavioral habits. Lobsters can respond to temperature changes by changing their habitat. For example, lobsters are likely to move toward higher latitudes or to areas cooled by tidal mixing. In New England, for example, such populations will move north toward the Bay of Fundy. Warming temperatures increase the lobster’s respiration rate and oxygen needs while reducing the amount of dissolved oxygen available. Research has found that as water temperatures rise above 69 degrees F, lobsters’ respiration rate increases to a point where their demand for oxygen exceeds the supply, causing physiological stress (WRI, 1996).
Scientists have also seen evidence linking rising seawater temperatures to the spread of lobster shell disease in Massachusetts’ waters (WRI, 1996). Caused by a bacterial infection in the carapace, lobster shell disease has become dramatically more common in recent years. A new type of shell disease was observed in New England in the late 1990s, when temperatures were higher than in previous years, and has been steadily moving northward toward Maine and Canada. The incidence of shell disease is strongly related to the number of days during which water temperatures exceed 20 degrees C (68 degrees F). The mechanism by which higher temperatures increase shell disease is not yet known, although it may increase the rate of growth of a bacteria suspected to cause the disease. It also affects the lobsters’ rate of growth so that they attain sexual maturity more quickly.
In contrast to males, which molt every year, mature females do not molt during the year they carry eggs, and only molt every two years (WRI, 1996). This long period provides more time for bacteria to infect females’ shells, and higher sea temperature accelerates the rate at which they attain this vulnerable state. One surprising recent development has been an increase in numbers of lobsters in the Gulf of Maine (IPCC, 2007). This may also be due to ocean warming, as it may be that the newly warmer waters spur a longer growing season, encourage more rapid growth, cause lobsters to hatch earlier and provide better conditions for larval lobsters. Another reason for the larger numbers may be that warmer temperatures and over-fishing have depleted the stocks of cod, a species that preys on lobsters.
Beginning in the mid-1960s and stabilizing in the mid-1980s, there were small temperature increases, ranging from .97 to .3 degrees C per decade (WRI, 1996). These temperature increases correlate, with a two- to nine-year lag, with a severe decline in rock hopper penguins, emperor penguins and Adelie penguins, as well as albatrosses and seals. There was also a dramatic decline in penguins in Antarctica, probably caused by disappearing sea ice over the past century. Adelie and emperor penguins, which are dependent on sea ice, have declined from 300 breeding pairs to 9 in the Western Antarctic peninsula, with a decline of 50 percent in Terre Adelie. Adelie penguins have declined by 70 percent on Anvers Island along the Antarctic Peninsula but are thriving at more southerly Ross Island. Conversely, open-ocean feeding penguins, the chinstrap and the gentoo, have taken advantage of the ice’s retreat, invading southward along the Antarctic Peninsula. Many penguins have survived severe climate change during the last 3 million years, showing the resilience of the species (IPCC, 2014).
This knowledge is important and applicable in designing Adaptive Resource Management in the wake of Impacts of climate change on both terrestrial and aquatic Ecosystems and Biodiversity. For instance, three aspects of the water cycle are of fundamental importance in understanding the structure and function of aquatic ecosystems. First, the amount of water determines livable habitat for aquatic organisms. In addition, more water often translates into more contact with the terrestrial environment (shoreline, floodplain), which increases nutrient supply and influences the types of food available to aquatic organisms. Second, the rate at which water moves through a system has a tremendous influence on the rates at which ecosystem processes occur and on the kinds of organisms that can live in them.
Human society depends on inland freshwater and coastal wetland ecosystems to provide ecological goods and services. Human demands for aquatic ecosystem quantity and quality now pose severe threats sustainable development. The multiple human stressors on aquatic ecosystems will interact with future climate change. Current biodiversity changes are still largely driven by anthropogenic alteration of habitats. Every human being is responsible in developing a sustainable biodiversity. In general, there is strong scientific consensus that aquatic ecosystems are vulnerable to projected climate change. Human activities have severely modified many aquatic ecosystems with actions such as diversion, groundwater pumping, and the building of dikes, levees, and reservoirs, all of which have modified natural processes and increased the vulnerability of aquatic ecosystems to additional stress associated with climate change. Cumulatively, these alterations fragment the aquatic landscape and make dispersal between ecosystems more difficult. A rapidly changing climate introduces uncertainty in water resource management and therefore threatens aquatic ecosystems if they are not adequately considered in human adaptation to climate change.
Aquatic ecosystems have not generally been adequately represented in the formulation of environmental policies, especially the need for water quantity that can sustain aquatic biodiversity and productivity. Human activities should maximize the potential for adaptation of these ecosystems by minimizing environmental stresses as pollution, habitat destruction, fragmentation and introduction of invasive species. These and other actions could enhance resilience of aquatic and wetland ecosystems, regardless of whether climate changes occur with the projected magnitude or not. Climate change offers additional opportunities to implement more science-based management of aquatic resources; this could be considered as Adaptive Resource Management. According to IPCC (2014), examples of such policies would include the following: (1) Maintain riparian forests that shade streams and rivers to control increased temperature and contribute to the maintenance of existing habitat quality. (2) Reduce nutrient loading to rivers, lakes, and estuaries. (3) Locate any new reservoirs only off-channel so as not to disrupt the natural downstream flow of water and sediments critical to riverine ecosystems. (4) Restore aquatic and wetland ecosystems to the maximum extent possible. On regulated rivers and wetlands, natural flow regimes or hydro-periods should be restored, to promote ecosystem resilience to climate change and other stressors. (5) Minimize groundwater pumping for irrigation, human consumption and other activities that removes water from aquatic and wetland ecosystems. Figure 2 shows the Conclusive Working Model as envisaged in this review paper. Nyongesa, Omuya and Sitati (2016) advanced the model that illustrates the relationship between climate change, ecosystems and biodiversity, adaptive resource management and sustainable development in its three dimensions.
Cited as Nyongesa K.W and Wandiga S.O (2019) Climate Change Impacts on Aquatic Ecosystems and Biodiversity. Bureau for Climate Change Resilience and Adaptation; Nairobi