Aquatic ecosystems and the services they generate are threatened by direct and indirect anthropogenic insults. These all-too-familiar threats include pollution, habitat loss and fragmentation, and overuse. The force of humanity over the past century has overwhelmed many aquatic ecosystems, or altered their functioning in ways that compromises delivery of ecosystem services.
Some human alterations of aquatic systems have apparently opposite impacts. Dams and dikes obstruct the connectivity of systems, impairing the movement of organisms. Conversely, canals increase connectivity, and promote the movement of organisms, including exotic or invasive species, often with disastrous consequences. Reservoirs, important for drinking water supplies, hydropower generation, agricultural production, or other uses, sometimes enhance ecosystem services (e.g., wildlife habitat value), but also 'starve'
rivers, estuaries, and coastal zones of sediments and nutrients. Moreover, reservoirs often change in productivity and species richness over time, many becoming warm-water havens undesirable for fish, for example.
Species richness and diversity is threatened in many aquatic ecosystems. The causes include habitat loss or alteration, overharvesting, and pollution. In North America alone, over 350 species or subspecies of fish are considered endangered, threatened, or of special concern. This list includes species that only a few decades ago were so abundant that it was difficult to imagine they would become scarce. We are only beginning to understand the implications of depleted food webs, and their impaired flows of services.
Climate change is a specter that threatens aquatic ecosystems and their services in multiple ways. Calculations have been done to estimate the loss of cold-water habitat under different scenarios of global warming; many temperate and boreal aquatic systems will become unsuitable for coldwater species, such as trout, salmon, and whitefish. Warming will also alter hydrologic cycles, causing more loss of small and ephemeral water bodies and streams. Climate change will also involve more extremes of weather: for example, although the northeastern United States is predicted to become wetter on average, this increase in precipitation will be delivered through more and larger storms. Already, the shallower Laurentian Great Lakes of North America are generating more winter snow storms, as they store more heat and therefore interact more (because of less and shorter duration of ice cover) with passing cold fronts and increased winds. It may be that intact, functioning ecosystems will become increasingly important and valuable as buffers against increasing numbers of catastrophic weather events.
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