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Marine Ecology Progress Series

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MEPS 736:47-62 (2024)  -  DOI:

Eelgrass meadow response to heat stress. II. Impacts of ocean warming and marine heatwaves measured by novel metrics

Amelie C. Berger1,*, Peter Berg1, Karen J. McGlathery1, Lillian R. Aoki2, Kylor Kerns1

1Department of Environmental Sciences, University of Virginia, Charlottesville, VA 22903, USA
2Department of Environmental Studies, University of Oregon, Eugene, OR 97403, USA
*Corresponding author:

ABSTRACT: In June 2015, a marine heatwave triggered a severe eelgrass Zostera marina die-off event at the Virginia Coast Reserve (USA), followed by a slow and spatially heterogeneous recovery. We investigated the effects of heat stress on seagrass loss and recovery. Using hourly summer water temperature measurements from 2016-2020, we developed a novel approach to quantifying the stress of ocean warming on seagrass meadows. We defined 2 metrics: cumulative heat stress (as heating degree-hours, HDHs) and heat stress relief (as cooling degree-hours, CDHs), relative to a 28.6°C eelgrass ecosystem thermal tolerance threshold previously determined at this site from aquatic eddy covariance measurements. These metrics were compared to spatiotemporal patterns of summertime eelgrass shoot density and length. We found that the healthiest parts of the meadow benefited from greater heat stress relief (2-3×) due to tidal cooling (inputs of cooler ocean water through ocean inlets) during warm periods, resulting in ~65% higher shoot densities compared to the center of the meadow, which experienced higher heat stress (2×) and less relief. We also calculated the amount of heat stress preceding the eelgrass die-off in summer 2015, and found that this event was triggered by a cumulative heat stress of ~100-200°C-hours during the peak growing season. Sulfur isotope analyses of eelgrass leaves and sediment also suggested that sulfide toxicity likely contributed to eelgrass decline. Overall, our metrics incorporate physiological heat tolerances with the duration and intensity of heat stress and relief, and thus lay the groundwork for forecasting seagrass meadow vulnerability and resilience to future warming oceans.

KEY WORDS: Heat stress · Resilience · Sulfide toxicity · Eelgrass · Zostera marina · Marine heatwaves · Ocean warming · Metrics · Seagrass recovery

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Cite this article as: Berger AC, Berg P, McGlathery KJ, Aoki LR, Kerns K (2024) Eelgrass meadow response to heat stress. II. Impacts of ocean warming and marine heatwaves measured by novel metrics. Mar Ecol Prog Ser 736:47-62.

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