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

N Camillini,Rn Glud,Bd Eyre,Km Attard

doi:10.3354/meps13616

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, 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 benthic O2 uptake resolved by both techniques was just ~30% of the values measured before resuspension. Laboratory investigations revealed sediment resuspension could potentially increase benthic O2 uptake up to 6fold, mainly due to the reoxidation of reduced compounds (e.g. FeSx). This process was fully captured by the eddy O2 fluxes, but not by the chamber incubation. Consequently, the chamber and eddy net ecosystem metabolism amounted to -17 and -824 mmol C m-2, respectively, throughout the study period. The rapid reoxidation and long-term effects of resuspension on benthic O2 dynamics highlight the importance of fully capturing dynamic conditions when assessing the overall carbon turnover in coastal habitats. Future studies on the biogeochemical functioning of coastal environments should aim to capture the natural frequency and duration of resuspension events.

Highlights

Seagrasses are aquatic flowering plants that form extensive and diversity-rich coastal habitats (Duarte et al 2008), wherein dampened hydrodynamics facilitate the entrapment of allochthonous material (Hendriks et al 2008)
The sediment organic matter content rapidly decreased from 1.3 ± 0.4% dry weight (DW) at the surface to 0.8 ± 0.3% DW at 1.5 cm depth; thereafter, it gradually decreased to 0.3 ± 0.1% DW at 9.5 cm depth
Previous studies have demonstrated that deriving accurate benthic O2 fluxes in dynamic settings may require accounting for variations in the mean O2 concentration within the benthic boundary layer by applying a storage correction term (Rheuban et al 2014, Koopmans et al 2020)

Summary

Introduction

Seagrasses are aquatic flowering plants that form extensive and diversity-rich coastal habitats (Duarte et al 2008), wherein dampened hydrodynamics facilitate the entrapment of allochthonous material (Hendriks et al 2008). It is crucial to capture the natural effects of sediment resuspension to assess mineralization and retention of organic material in seagrass habitats. Sediment−water oxygen (O2) exchange rate (or benthic O2 flux) is the most widely used proxy for assessing benthic carbon turnover and metabolic rates (Canfield 1993, Glud 2008), such as respiration (R, negative flux), net daytime production (NDP), and gross primary production (GPP = NDP + |R|). Reduced equivalents transiently accumulate in many coastal sediments (i.e. RQ > 1), as re-oxidation processes are shifted to periods with reduced mineralization intensity and/or enhanced benthic O2 availability (i.e. RQ < 1) (Therkildsen & Lomstein 1993). It is crucial to capture natural variation in benthic O2 fluxes over appropriate timescales to accurately assess benthic carbon turnover of coastal ecosystems. Benthic O2 fluxes tend to exhibit a relatively high day-to-day variability in ecosystems exposed to dynamic flow conditions (Berg et al 2013, Rheuban et al 2014, Attard et al 2015, Gruber et al 2017), while seasonal variability is more common in sheltered settings (Glud et al 2003, Chipman et al 2016)

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