Spatial and temporal variation in trophic structure of the Nueces Marsh, TX
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Salt marsh food webs are complex systems, with high levels of in situ primary production supporting a wealth of resident and migratory species. In this study, we use stable isotopes as a tool to trace organic matter utilization within the Nueces Marsh food web. Specifically, we were interested in (1) the use of treated wastewater as a ¹⁵N tracer, and (2) seasonal and interannual variation in food web structure. We hypothesized that treated wastewater would selectively label detrital resourse use within the food web, allowing us to trace grazing vs. detrital pathways within the marsh system. We also hypothesized that marsh consumers would exhibit distinct differences in isotopic composition between summer and winter, and between different years. We found that the Nueces Marsh food web consists of 3.5 consumer trophic levels. The [delta]¹³C values of consumer organisms were similar across the spatial extent of the low marsh, regardless of proximity to wastewater inflow. However, a majority of the organisms collected from the wastewater channel were significantly (p<0.05) enriched in ¹⁵N compared to their reference counterparts. We propose that ¹⁵N-enriched nitrogen is entering the Nueces Marsh food web through detrital rather than grazing-based pathways, making wastewater effluent an effective tracer of detrital integration into a marsh food web system. Hydrologic data indicate that isotopic shifts between seasons and between years reflected larger scale shifts between drought and wet years. During drought years, decreased production by phytoplankton and emergent plants led consumers to rely more heavily on ¹³C-enriched cyanobacterial carbon. In contrast, wet years encourage phytoplankton and emergent plant production, making cyanobacterial carbon relatively less exploited. While the Nueces Marsh food web is supported by a stable detrital carbon pool, it may still be susceptible to larger scale hydrologic events.