Detrital shadows: Stable isotopes reveal estuarine food web connectivity depends on fluvial influence and consumer feeding mode

Background/Question/Methods

Recent studies in estuarine detritus-based food webs have documented strong gradients in the sources of organic matter assimilated by consumers across diverse landscape scales. These results challenge prior concepts (i.e., estuarine outwelling paradigm) regarding the scale of food web connections across estuarine ecotones, as they infer greater compartmentalization of food webs in relation to landscape setting.  However, these recent studies were conducted in microtidal environments and/or estuaries receiving little riverine input, both of which may minimize detrital transport.  Given how little attention has been paid to distinguishing the scale of food web connections in relation to landscape setting and river flow regimes, the goal of this project is to quantify the strength, temporal, and spatial scales of food web connectivity among adjacent ecosystems and across different gradients of fluvial forcing.

We use multiple stable isotopes in combination with Bayesian multiple source mixing models to trace primary producer contributions to benthic-deposit and filter feeding consumers transplanted to specific locations across estuarine ecotones, including emergent marsh, mudflat, Japanese eelgrass, and native eelgrass. The study was conducted in five Pacific Northwest, USA estuaries representing a gradient of fluvial forcing. By tracing the lengths and strengths of ecosystem connectivity, it is possible to determine the spatial extent of trophic linkages required to adequately capture functional food web processes.

Results/Conclusions

The comparison among estuaries and between consumer indicator species provides insight into the role that river flow plays in connecting the food webs of adjacent ecosystems. Contrary to expectations, emerging results suggest that for benthic-deposit feeders, food webs of more strongly fluvial estuaries are more compartmentalized than those of embayment type estuaries.  Filter feeding consumers, however, reflect broad connectivity as fluvial influence increases.  Thus, while freshwater discharge may act to move and integrate organic matter across space in the water column, other factors drive patterns in organic matter deposition and retention.  As a result, the degree of food web connectivity varies among consumer indicators representative of different feeding modes.