Resource partitioning among fish mesoconsumers along a marsh-mangrove ecotone: A response to a pulsed seasonal resource subsidy

Background/Question/Methods

Consumers throughout natural ecosystems are often subsidized by pulsed resources. At the onset of the pulse, high numbers of consumers are able to simultaneously exploit additional food sources. However, as resources become depleted, competition increases such that the extent to which organisms are supported by these pulsed resource subsidies will likely depend on the competitive abilities of individuals and species.

In the Greater Everglades Ecosystem, distinct hydrologic seasonality influences ecosystem structure and function, including prey availability along the marsh-mangrove ecotone. In the dry season, decreasing water levels in upstream marshes force fishes into tidal mangrove creeks. At the same time, ecotonal abundances of estuarine transient consumers, dominated by snook (Centropomus undecimalis) triple, likely in response to increased freshwater prey abundance. In this study, we examine how snook seasonally partition resources with spatially co-occurring freshwater mesoconsumers (e.g, largemouth bass (Micropterus salmoides), Florida gar (Lepisosteus platyrhincus), and bowfin (Amia calva) in the upper Shark River estuary, Everglades National Park. We examined percentage of empty stomachs, gut fullness, diet breadth and percent diet overlap from stomach contents obtained from the 4  species via pulsed gastric lavage. We hypothesize that 1) when resources are highly available at the onset of the pulse, feeding intensity (reflected through stomach fullness and percentage of stomachs with contents), diet breadth and percent dietary overlap will be high and will not differ across species, however 2) in the late dry season when resources diminish, consumers will begin to differentially select resources based on their competitive abilities.

 Results/Conclusions

Results indicate that in the early dry season, consumer diets overlap, as all species heavily exploit marsh-based prey. But, as freshwater prey become less abundant later in the dry season, diets segregate strongly. Freshwater consumer feeding intensity significantly decreases and their diets appear to shift to invertebrate prey, while snook continue to show a high degree of feeding intensity and still consume marsh-based prey subsidies, perhaps indicating a superior competitive ability. These data suggest that marsh hydrology can dramatically impact trophic dynamics and energy flow in the upper estuary.