In situ differences in nitrogen cycling related to presence of submerged aquatic vegetation in a Gulf of Mexico estuary

TL;DR

This study explores how submerged aquatic vegetation affects nitrogen cycling in a Gulf of Mexico estuary, showing differences in nitrogen processing and microbial communities between vegetated and bare sediment areas.

Contribution

The study provides in situ evidence of how SAV influences nitrogen cycling and microbial community structure in estuaries.

Findings

Sediment–water exchange of dissolved N2 and microbial community structure differ between SAV and bare sediment.

Habitat differences were more consistent in low salinity and high organic content sediments.

Microbial composition, including genes like nifH, varied significantly between SAV and bare sediment habitats.

Abstract

Estuaries provide a suite of ecosystem services to people but are also under heavy stress from human development including excess nutrient loading and alterations in benthic habitat that affect nutrient cycling. Here we examine the interaction of two important and common ecosystem management priorities in estuaries: limiting eutrophication and restoration of submerged aquatic vegetation (SAV). Rates of benthic nitrogen processing can vary by habitat type and there is need for more complete data on the contribution of SAV to overall nitrogen cycling in estuaries, as well as a need to examine nitrogen cycling in situ to better characterize the role of SAV areal coverage in mediating estuarine eutrophication. We compare nitrogen cycling between two common and adjacent habitat types (SAV and adjacent bare sediment [BS]) in an index coastal estuary using an in situ chamber-based approach to better capture realized habitat differences. We also examined genomic community structure of sediment bacteria and archaea to identify biological indicators of nitrogen exchange. Both mean sediment–water exchange of dissolved N2 and microbial functional community structure differed between SAV and BS. Habitat differences were more consistent with lower variability at locations with low salinity and when sediment organic content was highest, which aligns with findings in other studies. Habitat types differed significantly in microbial composition, including functional groups and genes, like nifH, that may contribute to observed differences in nitrogen cycling. Overall, habitat type appeared most important to nitrogen cycling near the river mouth where sediment nitrogen was higher, and this information has implications for integrated management of habitat restoration/conservation and nutrient loading.

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