No free lunch: Resource partitioning among an estuarine and two freshwater mesoconsumers at a marsh-mangrove ecotone in response to a seasonal marsh subsidy

Background/Question/Methods

Pulse subsidies account for a substantial proportion of resource availability in many systems, having persistent and cascading effects on consumer population dynamics as well as routing of energy within/across ecosystem boundaries. Although the importance of pulsed subsidies is well established, how consumers from donor communities respond and regulate the flow of energy across ecosystem boundaries is not well understood. In the Everglades, seasonal rainfall patterns dictate the movements of fishes. During periods of high rain, vast expanses of marsh become inundated, spawning the growth of small bodied fishes as well as harbor larger bodied piscivorous fishes. However, as rainfall decreases, marshes begin to dry, forcing fishes into the deepest habitats in the landscape including ecotonal estuarine creeks. At the ecotone during marsh drying, the abundance of estuarine consumers triple, presumably in response to increased prey availability. However, the magnitude of the subsidy to estuarine consumers is unknown since they are forced to compete with marsh consumers. The aim of our study was to 1) identify a fish prey subsidy in first and second order ecotonal creeks 2) determine if both estuarine and marsh consumers numerically respond to increased prey availability and 3) examine the extent that marsh consumers reduce subsidies to estuarine consumers.

Results/Conclusions

We identified a pulse of marsh cyprinodontoid, invertebrate, and sunfish prey entering an estuary that was met by an influx of both marsh and estuarine predators.  At the onset of the subsidy, 92% of prey consumed by both marsh and estuarine consumers were of marsh origin. Nearly half of the marsh prey entering the estuary was eaten by marsh consumers. Intense predation by marsh consumers on this prey subsidy indicates that marsh consumers may play a significant role in regulating the flow of energy to estuarine consumers. Consumers also showed marked diet segregation among marsh prey. Bass consumed significantly more cyprinodontoids, bowfin consumed significantly more invertebrates, and snook almost exclusively targeted sunfishes. From this subsidy, both bass and snook increased in physical condition, while bowfin did not. The diversity of the resource pulse may subsidize multiple consumers, routing allochthonous energy through various trophic pathways and across ecosystem boundaries. Preserving complex trophic linkages like those at the Everglades ecotone can be vital to maintaining ecosystem function and the provisioning of services such as recreational fisheries.