Estuaries exhibit a wide array of human impacts that can compromise their ecological integrity, because of rapid population growth and uncontrolled development in many coastal regions worldwide. Long-term environmental problems plaguing estuaries require remedial actions to improve the viability and health of these valuable coastal systems. Detailed examination of the effects of pollution inputs, the loss and alteration of estuarine habitat, and the role of other anthropogenic stress indicates that water quality in estuaries, particularly urbanized systems, is often compromised by the overloading of nutrients and organic matter, the influx of pathogens, and the accumulation of chemical contaminants. In addition, the destruction of fringing wetlands and the loss and alteration of estuarine habitats usually degrade biotic communities. Estuaries are characterized by high population densities of microbes, plankton, benthic flora and fauna, and nekton; however, these organisms tend to be highly vulnerable to human activities in coastal watersheds and adjoining embayments. Trends suggest that by 2025 estuaries will be most significantly impacted by habitat loss and alteration associated with a burgeoning coastal population, which is expected to approach six billion people. Habitat destruction has far reaching ecological consequences, modifying the structure, function, and controls of estuarine ecosystems and contributing to the decline of biodiversity. Other anticipated high priority problems are excessive nutrient and sewage inputs to estuaries, principally from land-based sources. These inputs will lead to the greater incidence of eutrophication as well as hypoxia and anoxia. During the next 25 years, overfishing is expected to become a more pervasive and significant anthropogenic factor, also capable of mediating global-scale change to estuaries. Chemical contaminants, notably synthetic organic compounds, will remain a serious problem, especially in heavily industrialized areas. Freshwater diversions appear to be an emerging global problem as the expanding coastal population places greater demands on limited freshwater supplies for agricultural, domestic, and industrial needs. Altered freshwater flows could significantly affect nutrient loads, biotic community structure, and the trophodynamics of estuarine systems. Ecological impacts that will be less threatening, but still damaging, are those caused by introduced species, sea level rise, coastal subsidence, and debris/litter. Although all of these disturbances can alter habitats and contribute to shifts in the composition of estuarine biotic communities, the overall effect will be partial changes to these ecosystem components. Several strategies may mitigate future impacts.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0376892902000061

Environmental threats and environmental future of estuaries

We examined the importance of terrestrial detrital inputs to secondary productivity of a headwater stream. Following a year of pretreatment studies on two headwater streams, we excluded terrestrial litter inputs (=treatment) to one stream while using the other as a reference. We excluded litter for 3 yr followed by 1 yr of small woody debris (≤10 cm diameter) removal and litter exclusion. Monthly benthic samples were collected from dominant mixed substrate (cobble, pebble, and sand-silt) as well as from moss-covered bedrock outcrop substrates. We used randomized intervention analysis (RIA) to test the null hypotheses that no change in abundance or biomass of functional feeding groups or specific taxa occurred in the treatment stream relative to the reference stream. Benthic organic matter was significantly lower in mixed substrate habitats of the treatment stream; however, small woody debris did not show a significant reduction prior to manual removal during year 4. At the end of the treatment period, tot...

https://www.jstor.org/stable/pdfplus/2657224.pdf

Effects of resource limitation on a detrital‐based ecosystem

All ecosystems are influenced by physical forcing. Estuarine ecosystems respond most strongly on an interannual timescale to variability in freshwater flow. Several mechanisms for positive or negative flow effects on biological populations in estuaries have been proposed; however, positive effects appear to operate mainly through stimulation of primary production with effects propagating up the food web. In the northern San Francisco Estuary, abundance or survival of several common species of fish and shrimp varied positively with flow-in data through 1992. I re-examined these relationships and those of several additional taxa in an analysis of long-term (20 to 40 yr) mon- itoring data. The spread of the introduced clam Potamocorbula amurensis in 1987 provided an oppor- tunity to examine simultaneously the responses of estuarine species to flow and to changes in the food web. I separated variability into a flow response, a step change after 1987 and other sources of variability. Responses of fish and shrimp contrasted with those of lower trophic levels. All but 1 spe- cies of nekton responded positively to flow, only 2 had clear declines after 1987, and none of the rela- tionships changed in slope after 1987. In contrast with the higher trophic levels, chlorophyll a (chl a) and several species of zooplankton declined markedly after 1987, and had either weak responses to flow or responses that changed after 1987. Thus, the food web appears strongly coupled between benthos and plankton, and weakly coupled between zooplankton and fish, as has been found in other systems. More importantly, the variation with freshwater flow of abundance or survival of organisms in higher trophic levels apparently did not occur through upward trophic transfer, since a similar rela- tionship was lacking in most of the data on lower trophic levels. Rather, this variation may occur through attributes of physical habitat that vary with flow.

/pdf/effects-of-freshwater-flow-on-abundance-of-estuarine-2nqdklattn.pdf

Effects of freshwater flow on abundance of estuarine organisms : physical effects or trophic linkages?

Strong changes in stable isotope tracers commonly occur across estuarine salinity gradients from freshwater to the sea. The tracer gradients reflect the different geochemistries and mixing of freshwater and seawater, and these bottom-up geochemical influences are recorded in estuarine food webs in the isotopic compositions of animals. Conservative mixing calculations suggest that watershed-level inputs of freshwater and nutrients should exert strong influences on isotopic values of estuarine consumers, especially consumers such as bivalves that largely depend on phytoplankton production. Deviations from conservative isotope mixing also occur, and the magnitude of these deviations measures the strength of within-estuary organic matter cycling for estuarine food webs, especially inputs of non-phytoplankton foods such as macrophyte detritus and benthic algae. Measuring consumer isotopes across salinity gradients should be a relatively simple way to monitor effects of watershed nutrient loading and hydrologic flushing in supporting estuarine fisheries production.

https://www.jstor.org/stable/info/1353315

Conservative mixing of stable isotopes across estuarine salinity gradients: A conceptual framework for monitoring watershed influences on downstream fisheries production

River discharge has long been recognized as one of the factors that contributes to the high productivity of estuaries. Although there is little evidence that river inputs of terrestrial carbon make a direct contribution to coastal food webs, such exported nutrients may stimulate in situ production in estuaries and thus enhance the survivorship and growth of fish and crustaceans in these systems. Furthermore, fluctuations in salinity and turbidity may influence the extent of available habitat for fish and crustaceans and therefore their distribution and/or catchability. Despite these potential links between flow and the secondary production of estuaries and coastal waters, there is still a common perception that ‘water going to sea is wasted’ and a continuing trend to regulate the flow of rivers. We review the evidence for links between river flow and the productivity of estuarine/coastal fisheries, drawing on a case study of the Logan River in southeast Queensland, and explore the potential mechanisms for these linkages. Our research, and that of others, confirms that high river discharge can have a strong positive effect on the production of commercial and recreational coastal fisheries. It also shows that the seasonal pattern of flow is equally, if not more important, than the magnitude of flow. River regulation is likely to have a dramatic effect on the production of coastal fisheries and, given the current pressures for water resource development, this is an important avenue for future research and evaluation.

River flows and estuarine ecosystems: Implications for coastal fisheries from a review and a case study of the Logan River, southeast Queensland

A long-term (9.5 yr) study addressed the relationship of the trophic orga- nization of a river-dominated Gulf of Mexico estuary with interannual trends of freshwater input and biological controlling features. Alluvial river flow characteristics were evaluated with respect to seasonal and interannual changes in physical, chemical, and biological trends in the receiving estuary. Infaunal and epifaunal macroinvertebrates and fishes taken over the period of sampling in the Apalachicola Bay system were transformed into their trophic equivalents. The long-term trophic organization of the bay was then related to observed changes in the physical and chemical conditions in the receiving estuary with particular attention to long-term response to a 2-yr drought. Within limited natural bounds of fresh- water flow from the Apalachicola River, there was little change in the trophic organization of the receiving estuary over prolonged periods. The physical instability of the estuary was actually a major component in the continuation of a biologically stable estuarine system. However, when a specific threshold of freshwater reduction was reached during a prolonged natural drought, we suggest that the clarification of the normally turbid and highly colored river-estuarine system led to rapid changes in the pattern of primary production, which, in turn, were associated with major changes in the trophic structure of the system. Increased light penetration due to the cessation of river flow was an important factor in the temporal response of bay productivity and herbivore/omnivore abundance. There was a dichotomous response of the estuarine trophic organization, with herbivores and omnivores responsive to river-dominated physicochemical factors whereas the carnivores responded to biological factors. Trophic response time could be measured in months to years from the point of the initiation of low-flow conditions. The reduction of nutrient loading during the drought period was postulated as a major cause of the loss of productivity of the river-dominated estuary during and after the drought period. Recovery of such productivity with resumption of increased river flows was likewise a long-term event. Based on the observed trends in the bay, postulated permanent reductions of freshwater flows due to anthropogenous ac- tivities could lead to major reductions of biological productivity in the Apalachicola Bay system. The long-term data indicated that, with reduction of freshwater flow below a level specific for the receiving system, the physically controlled, highly productive river-estu- arine system would become a species-rich, biologically controlled bay with substantially reduced productivity.

F. Graham Lewis

Papers

Freshwater input to a gulf estuary: long‐term control of trophic organization