Ecosystem, aquatic

This section describes only the most general attributes of aquatic ecosystems. All ecosystems can be functionally characterized in terms of their processing of energy and their cycling of mass (e.g., carbon and nutrients). These functions, as well as the life history aspects of ecosystems, are discussed in the following subsections. [For a more complete discussion of ecosystems, the reader is referred to several texts, such as Odum (1971), Ricklefs (1990), or Curtis (1983)]. 

The reverse of this process is respiration, in which biota release the energy stored in CH20 by reacting it with oxygen or another oxidant. The energy released is then used in activities such as maintenance, growth, mobility, or reproduction of the biota. 

FIGURE 2-18 Pyramids of biomass (g/m2) in three ecosystems. Typically, the biomass of a given trophic level is less than the biomass of lower trophic levels, giving rise to a pyramid when box sizes are drawn proportional to biomass. This is the consequence of decreasing energy availability at successively higher trophic levels [data are from Odum (1971)]. 

FIGURE 2-19 Portion of an aquatic food web in a small stream in South Wales. Note that some organisms (such as the net-spinning caddis shown here) actually feed at more than one trophic level [adapted from Jones (1949). Reproduced with permission of Blackwell Science Ltd.]. 

If ammonium is the predominant nitrogen source, the photosynthesis reaction can be written as 

In this part of Chapter 9 the current situation and progress towards application of biological based methods in ERA is reviewed, including the development of WOE frameworks. For aquatic ecosystems and sediments, bioassay test systems and WOE frameworks already exist and they are currently being used. For the terrestrial environment the situation is still immature. After briefly presenting the aquatic and sediment frameworks, including key references, the developments for the terrestrial environment are discussed. The preliminary terrestrial framework is demonstrated on the basis of the results of investigations at a contaminated site in The Netherlands. 

Application of bioassays for toxicity testing in aquatic ecosystems is well under way. Recently, a risk framework for a preliminary and a refined effect assessment has been suggested (Straetmans et al., 2003). As part of the toxicity measurements, organic contaminants in aquatic samples are lOOx concentrated (using a XAD resin and acetone) and toxicity experiments are performed with dilution series (De Zwart and Sterkenburg, 2002). Major features of this framework are  

Ecological risk assessment approaches currently used in The Netherlands for aquatic sediments have been described recently by Den Besten et al. (2003). Two main goals for sediment quality assessment in Europe are distinguished  

Biological effects-based assessment of in situ risks (in situ BEB A) at sites where sediment quality and potentially sediment management is to be considered. 

Biological effects-based assessment of the ex situ quality of dredged sediments (ex situ BEBA) in order to select sediment management options (e.g. free or confined disposal or treatment options). 

Substances that are soluble in water often are in the form of ionic compounds, consisting of positive and negative ions. Common salts, such as ordinary table salt (sodium 

Salinity is the degree of saltiness of seawater. In the open ocean the concentration of dissolved salts is about 30 g/L of seawater. 

Salt crystals are a familiar sight along the shores of salt lakes such as the Great Salt Lake in Utah, the Salton Sea in southern California, Mono Lake in eastern California, and the Dead Sea in Israel. After swimming in the ocean, you may have hcked your lips and tasted the salt that had precipitated on them. Commercial table salt companies flood shallow basins with salt water, allow the water to evaporate, and dredge the salt that is left behind. 

Carbon dioxide (CO ) is slightly soluble in water (as shown below), giving the familiar carbonic acid (H COj) found in soft drinks. In aquatic systems, H COj dissociates into bicarbonate ions (HCO 3) and carbonate ions (CO 3 ), as shown by the following equations  

These reactions can take place in both directions depending upon temperature, acidity, and the presence of other ions. Notice that when hydrogen ions (H ) are formed, they tend to make natural aquatic ecosystems slightly acidic. 

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Despite the immensity and complexity of known and suspected roles of temperature in aquatic ecosystems, certain thermal criteria have been especially useful in minimizing risks from thermal discharges. More data have been organized at the physiological level than at higher levels of organization. 

Human civilization interferes more and more with the cycles that cormect land, water, and atmosphere, and pollution seriously affects water quahty. In order to assess the stresses caused to aquatic ecosystems by chemical perturbation, the distribution of pollutants and their fate in the environment must be investigated (see Air pollution). 

Pollutant Distribution. Of particular importance for the aquatic ecosystem is the distribution of volatile substances, eg, gases and volatile organic compounds, between the atmosphere and water, and the sorption of compounds at soHd surfaces, eg, settling suspended matter, biological particles, sediments, and soils (41,42). 

METAL SPECIES IN THE AQUATIC ECOSYSTEMS AND THEIR EFFECT ON THE WATER QUALITY... 

Polychlorinated biphenyls (PCBs) and organochlorine pesticides (OCPs) ai e toxic compounds of anthropogenous origin, able to accumulate in tissues of alive organisms and to cause different diseases. These compounds ai e the most dangerous for aquatic ecosystems as they easily adsorb in sludge and ai e included in food chains of biota. Humans consume PCBs and OCPs mostly with fish. 

In addition to reproductive effects, fish exposed to endocrine disrupters may have a decreased response to stress or decreased growth and metabolism which can affect their ability to survive, or to defend themselves against predators. All of these factors can affect the ability of the species to survive and to reproduce itself in sufficient numbers to maintain the stocks on which our commercial and sport fisheries are based. Not all fish species will be equally susceptible to the effects of endocrine disrupters. Selective sensitivity to such effects, especially those affecting reproduction, may well lead to major changes in the flora and fauna of some of our major aquatic ecosystems as the balance between fish, mammals, invertebrates and plants, and between predators and prey, is destabilised... 

One of the major effects of acidic deposition is felt by aquatic ecosystems in mountainous terrain, where considerable precipitation occurs due to orographic lifting. The maximum effect is felt where there is little buffering of the acid by soil or rock structures and where steep lakeshore slopes allow little time for precipitation to remain on the ground surface before entering the lake. Maximum fish kills occur in the early spring due to the "acid shock" of the first meltwater, which releases the pollution accumulated in the winter snowpack. This first melt may be 5-10 times more acidic than rainfall. 

The Reactions and Physical Transport tlie chemical and biological transfornuition, and water movement, that result in different levels of water quality at different locations in time in an aquatic ecosystem. 

Water birds have not been shown to be directly affected by acidification. However, the prey of waterbirds may be of concern as these lower food-chain organisms may have elevated levels of toxic metals related to acidification of their habitat. Moreover, most water birds rely on some component of the aquatic food-chain for their high protein diet. Invertebrates that normally supply caJcium to egg-laying birds or their growing chicks are among the first to disappear as lakes acidify. As these food sources are reduced or eliminated due to acidification, bird habitat is reduced and reproductive rate of the birds is affected. The Common Loon is able to raise fewer chicks, or none at all, on acidic lakes where fish populations are reduced 37 and 5S). However, in some isolated cases, food supplies can be increased when competitive species are eliminated (e.g.. Common Goldeneye ducks can better exploit insects as food when competition from fish is eliminated). The collective influences of acidification are difficult to quantify on a specific area basis but for species that rely on a healthy aquatic ecosystem to breed, acidification remains a continuing threat in thousands of lakes across eastern North America 14). 

On a global scale, the atmosphere serves as the major pathway for the transport and deposition of contaminants from emission sources to terrestrial and aquatic ecosystem receptors (22, 27). Once a contaminant is airborne, the processes of atmospheric di sion, transport, transformation, and deposition act to determine its fate. These processes are complex and the degree to which they influence the fate of a particular contaminant is dependent on its physico-chemical characteristics, the properties and concentrations of coexisting substances, and the prevailing meteorological conditions, including wind, precipitation, humidity, temperature, clouds, fog, and solar irradiation. 

In addition to poisoning humans, pesticides affect agricultural ecosystems and spread beyond into surrounding terrestrial and aquatic ecosystems. As a result, detrimental environmental effects follow pesticide use in the United States. Some of these are delineated below ... 

Nevertheless, the response of water resources will be more complex, as human activities will also change in response to altered climates. The intensity of the pressure put on water resources and aquatic ecosystems by external drivers is related to higher economic income (e.g. expressed by electricity production and consumption) of human societies [5]. The limitation of resources can be qualified by a diversity of terms, varying somewhat in intensity drought, temporality, and... 

Alterations of water flow, independent of the cause, impact the stmcture and function of aquatic ecosystems. Extended drought produces the loss of hydrologic connectivity between stream compartments, and affects the biota. Therefore, flow cessation triggers a chain of cascading effects, eventually affecting community structure and ecosystem functioning. 

McCclain ME, Richey JE, Pimentel TP (2003) Biogeochemical hot spots and hot moments at the interface of terrestrial and aquatic ecosystems. Ecosystems 6 301-312... 

Overall the results reported in this review indicate that water scarcity might increase metal exposure (due to low dilution), metal uptake (due to higher retention under low flow), and metal toxicity and/or accumulation (depending on the dose and time of exposure), but also might cause opposite effects depending on the source of pollution. In addition, water scarcity will influence nutrient loads and will also modulate the fate and effects of metals. Thus, future studies addressing the role of environmental stress on the effects of toxicants at community scale are key to predict the impact of toxicants in the aquatic ecosystems. 

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