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

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.

Isoprene fluxes from warm temperate and tropical seagrass communities

“…While both species emit isoprene during the day, increased uptake at night resulted in a net uptake over the diel cycle. In Wallis Lake, P. australis was twice as productive as Z. muelleri during the day (mean ± SE 1.5 ± 1.3 nmol m −2 h −1 and 0.9 ± 1.4 nmol m −2 h −1 respectively), which is expected since it is proportionally nearly 3 times the size of Z. muelleri (aboveground biomass: mean ± SE 69.1 ± 40.7 g dry wt m −2 and 25.6 ± 3.8 g dry wt m −2 , respectively) (Camillini 2020). Furthermore, isoprene uptake in Z. muelleri was more than twice the uptake of P. australis at night (mean ± SE −8.6 ± 3.0 nmol m −2 h −1 and −3.2 ± 1.5 nmol m −2 h −1 respectively).…”

“…Benthic isoprene fluxes were measured using 55 l benthic flume-chambers (Eyre et al 2011b, Camillini et al 2021. Briefly, Perspex chambers with rounded ends, 680 × 300 × 300 mm, and sediment surface area of 0.08 m 2 were used in triplicate over seagrass beds or bare sediments (Fig.…”

“…Photosynthetic active radiation (PAR) was recorded every 10 min within all chambers using HOBO loggers (UA-002-64) on Lizard Island (13 h light:11h dark cycle) and Wallis Lake (12 h light:12 h dark cycle). HOBO Lux units were calibrated using a Li-Cor-192 light meter to convert to μmol photons m −2 s −1 (Camillini 2020). Light data for P. australis is not available due to logger being shaded by the leaves within the chamber.…”

Benthic oxygen fluxes in a coastal upwelling system (Ria de Vigo, NW Iberia) measured by aquatic eddy covariance

“…10d) during the October campaign is the key factor enhancing benthic oxygen fluxes compared to the June campaign. Intensification of bottom shear stress produced by strong currents can resuspend surface sediments, mixing the organic matter and reduced compounds into the overlying water and favouring their oxidation (Almroth et al 2009, Niemistö et al 2018, Camillini et al 2021. In fact, intense resuspension events in the Ria de Vigo are more likely to happen in autumn (Villacieros-Robineau et al 2013).…”

In-Situ Estimates of Net Ecosystem Metabolisms in the Rocky Habitats of Dokdo Islets in the East Sea of Korea

“…Aquatic eddy covariance (AEC) has been developed in recent decades [15]. AEC is a noninvasive in-situ measurement technique that has been used to measure O2 flux in various natural water habitats (e.g., coral reefs, seagrass meadows, hard bottom benthic flora beds, oyster reefs) [16][17][18][19][20][21][22][23][24]. The measurement covers a relatively large surface area (10-50 m 2 ) and interferes minimally with the prevailing hydrodynamic conditions [25].…”