The Suite of Stressors Affecting Water Quality, Habitat, and Sustainability of the San Francisco Bay Delta Is Complex and Interactive

Interactions among stressors and between stressors and ecosystem processes are common. Nutrient enrichment, toxic chemicals, and temperature, for example, are affected by hydrology and hydrodynamics, that is, the way tides and freshwater flow interact to determine the temporal and spatial variability of the physical environment of the estuary. This complicates the interpretation and evaluation as to positive, negative, or neutral overall effects of any single stressor on the ecosystem and its attributes. Furthermore, species differ in their individual responses to most types of stress. The result is a complex biological, spatial, and temporal mosaic of impacts from this combination of influences.

To some extent, the evaluation of the impacts of these effects also depends on which ecosystem services and needs are of interest or concern, for example, safe and usable water supplies, recreational and commercial fisheries, habitat condition, or public use of the delta. Thus, while it is politically attractive to attempt to rank stressors so as to prioritize societal investments in their amelioration, that task is much more complex than it might at first seem. To some degree, priorities can be defined if the stress, species, place, and time are first prioritized or defined. The stressors discussed below and shown in Figure 3-1 are highly dynamic; that is, they can quantitatively change in time and space depending on changes in human activities (including future management actions), climate, and combinations thereof.

The Ecosystem and Its Components Do Not Necessarily Respond as a Single Unit to Most Environmental Factors

For example, Chinook salmon (Oncorhynchus tshawytscha) spend several years at sea and then return to pass through the delta as adults to spawn; their eggs and young spend time in delta tributaries before passing through the delta on their way to the ocean to mature. Returning adult Chinook salmon always die after spawning, and so they are not susceptible to chronic environmental factors, because they die before such factors can affect them. They also are strong swimmers and therefore most changes in flow patterns in the delta are reasonably small challenges for them. The eggs and young are susceptible to conditions in the tributaries and are exposed to them for considerable periods, and the outmigrating smolts are not as strong swimmers as are the returning adults, and so probably are more susceptible to changes in flow patterns. By contrast, delta smelt (Hypomesus transpacificus) spend their entire (short) lives in the delta and so they can be chronically exposed to contaminants in the water; being smaller and



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