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Resolving community metabolism of eelgrass Zostera marina meadows by benthic flume-chambers and eddy covariance in dynamic coastal environments

Nicola Camillini*, Karl M. Attard, Bradley D. Eyre, Ronnie N. Glud

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ABSTRACT: Sediment resuspension is a common process in dynamic coastal settings, but its implications for remineralization and carbon turnover in seagrass meadows are poorly understood. Here, we assessed eelgrass Zostera marina metabolism in the Baltic Sea (SW Finland) using benthic flume-chambers and aquatic eddy covariance to critically evaluate the drivers of benthic O2 exchange during dynamic flow conditions. During quiescent weather conditions, the 2 methods resolved similar metabolic rates and net ecosystem autotrophy (±11% of each other). However, the occurrence of elevated flow speeds and sediment resuspension halfway through the study induced a 5-fold increase in the O2 uptake rates measured by eddy covariance, whereas chamber fluxes remained relatively unchanged. Following particle resettlement, instruments were redeployed and the resolved benthic O2 uptake was just ⁓30% of that measured before resuspension. Laboratory investigations revealed sediment resuspension increased benthic O2 uptake up to 6-fold due to the reoxidation of reduced compounds during resuspension (e.g. FeSx). This process was fully captured by the eddy O2 fluxes, but not by the chamber incubation approach. As a result, the monthly carbon productivity rates eastimated for the chamber and eddy differred by 75-fold. The rapid reoxidation and long-term effects of resuspension on benthic O2 dynamics highlights the importance of fully capturing dynamic conditions when assessing the overall carbon turnover in seagrass communities using O2 flux measurements. Future studies on the biogeochemical functioning of coastal environments, and in particular seagrass habitats, should aim to capture the natural frequency and duration of resuspension events.