Trophic Response of Estuarine Fishes to Long-Term Changes of River Runoff
A long-term (13 year) study was carried out to analyze the response of estuarine fishes (numbers, biomass and trophic organization) to seasonal and interannual variations of Apalachicola River flow and associated habitat changes of the receiving estuary (East Bay, Apalachicola Bay system, Florida). Periodic peak floods and prolonged droughts were important events that led to altered patterns of individual fish distribution in terms of numerical abundance and biomass. There was considerable interannual variation in the temporal distribution of the dominant fish species over the study period. Individual estuarine fish species use the estuary as a nursery ground with species-specific ontogenetic feeding patterns that are defined by the complex productivity patterns of the system. In East Bay, there was a dichotomous response of the estuarine trophic organization with herbivores and omnivores (dominated by infaunal and epifaunal macroinvertebrates) directly responsive to river-associated physicochemical factors whereas the carnivores (dominated by the fishes) responded to biological factors such as predation and competition. Estuarine fish organization was indirectly responsive to changes of river flow through prey responses to state habitat and productivity variables associated with river flows. This suggests that the fish associations were strongly dependent on interannual patterns of Apalachicola River flow but that such relationships were primarily caused by biological interactions as defined by specific predator/prey relationships. A prolonged drought led to reduced fish species richness and trophic diversity; such habitat stress was related to enhanced instability of the biological components of the estuary as a function of changes in nutrient cycling. The food web was simplified while overall fish biomass and individual species populations were numerically reduced. The trophic response times of fish assemblages were measured in years from the point of the initiation of the drought. Changes in flow rates that exceeded specific natural levels of variance could be followed by identification of the subtle yet important changes in estuarine productivity and related changes of fish representation within the food web. The use of individual fish species as indicators of such responses is not consistent with the processes that define the long-term behavior of estuarine populations.
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Document Type: Research Article
Publication date: July 1, 1997
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