Eelgrass meadow response to heat stress. I. Temperature threshold for ecosystem production derived from in situ aquatic eddy covariance measurements

Abstract

As seagrass meadows are increasingly threatened by warming oceans and extreme heating events, it is critical that we enhance our understanding of their ecosystem response to heat stress. This study relied on our extensive database of hourly eelgrass Zostera marina ecosystem metabolism to determine, for the first time, the temperature stress threshold (Tth) of Z. marina meadows under naturally varying in situ conditions. Eelgrass ecosystem metabolism was measured using the aquatic eddy covariance technique in a 20 km2 meadow at the Virginia Coast Reserve (USA). We constructed and fitted a non-linear multivariate model to identify 28.6°C as the threshold above which substantial negative effects on net photosynthesis occur. On average, daytime oxygen fluxes decreased by 50% on afternoons when Tth was exceeded, which shifted daily net ecosystem metabolism from metabolic balance to net heterotrophy and therefore a loss in carbon. This study highlights the vulnerability of eelgrass meadows to future warming projections.