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1. Seagrass Recovery Following Marine Heat Wave Influences Sediment Carbon Stocks

   https://doi.org/10.3389/fmars.2020.576784  

   Aoki, Lillian R.; McGlathery, Karen J.; Wiberg, Patricia L.; Oreska, Matthew P.; Berger, Amelie C.; Berg, Peter; Orth, Robert J. (January 2021, Frontiers in Marine Science)

   null (Ed.)

   Worldwide, seagrass meadows accumulate significant stocks of organic carbon (C), known as “blue” carbon, which can remain buried for decades to centuries. However, when seagrass meadows are disturbed, these C stocks may be remineralized, leading to significant CO 2 emissions. Increasing ocean temperatures, and increasing frequency and severity of heat waves, threaten seagrass meadows and their sediment blue C. To date, no study has directly measured the impact of seagrass declines from high temperatures on sediment C stocks. Here, we use a long-term record of sediment C stocks from a 7-km 2 , restored eelgrass ( Zostera marina ) meadow to show that seagrass dieback following a single marine heat wave (MHW) led to significant losses of sediment C. Patterns of sediment C loss and re-accumulation lagged patterns of seagrass recovery. Sediment C losses were concentrated within the central area of the meadow, where sites experienced extreme shoot density declines of 90% during the MHW and net losses of 20% of sediment C over the following 3 years. However, this effect was not uniform; outer meadow sites showed little evidence of shoot declines during the MHW and had net increases of 60% of sediment C over the following 3 years. Overall, sites with higher seagrass recovery maintained 1.7x as much C compared to sites with lower recovery. Our study demonstrates that while seagrass blue C is vulnerable to MHWs, localization of seagrass loss can prevent meadow-wide C losses. Long-term (decadal and beyond) stability of seagrass blue C depends on seagrass resilience to short-term disturbance events. 

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

   https://doi.org/10.3354/meps14588  

   Berger, AC; Berg, P; McGlathery, KJ; Aoki, LR; Kerns, K (May 2024, Marine Ecology Progress Series)

   In June 2015, a marine heatwave triggered a severe eelgrassZostera marinadie-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. 

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

   https://doi.org/10.3354/meps14587  

   Berger, AC; Berg, P (May 2024, Marine Ecology Progress Series)

   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 eelgrassZostera marinaecosystem metabolism to determine, for the first time, the temperature stress threshold (T_th) ofZ.marinameadows under naturally varyingin situconditions. Eelgrass ecosystem metabolism was measured using the aquatic eddy covariance technique in a 20 km²meadow 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 T_thwas 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. 

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4. Seasonal growth and senescence of seagrass alters sediment accumulation rates and carbon burial in a coastal lagoon

   https://doi.org/10.1002/lno.12178  

   Zhu, Qingguang; Wiberg, Patricia L.; McGlathery, Karen J. (September 2022, Limnology and Oceanography)

   Abstract Seagrass meadows are important carbon sinks in the global coastal carbon cycle yet are also among the most rapidly declining marine habitats. Their ability to sequester carbon depends on flow–sediment–vegetation interactions that facilitate net deposition, as well as high rates of primary production. However, the effects of seasonal and episodic variations in seagrass density on net sediment and carbon accumulation have not been well quantified. Understanding these dynamics provides insight into how carbon accumulation in seagrass meadows responds to disturbance events and climate change. Here, we apply a spatially resolved sediment transport model that includes coupling of seagrass effects on flow, waves, and sediment resuspension in a seagrass meadow to quantify seasonal rates of sediment and carbon accumulation in the meadow. Our results show that organic carbon accumulation rates were largely determined by sediment accumulation and that they both changed non‐linearly as a function of seagrass shoot density. While seagrass meadows effectively trapped sediment at meadow edges during spring–summer growth seasons, during winter senescence low‐density meadows (< 160 shoots m⁻²) were erosional with rates sensitive to density. Small variations in winter densities resulted in large changes in annual sediment and carbon accumulation in the meadow; meadow‐scale (hundreds of square meters) summer seagrass dieback due to marine heatwaves can result in annual erosion and carbon loss. Our findings highlight the strong temporal and spatial variability in sediment accumulation within seagrass meadows and the implications for annual sediment carbon burial rates and the resilience of seagrass carbon stocks under future climate change. 

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5. Edge effects of a fragmented seagrass habitat on flow, bivalve recruitment, and sediment dynamics

   https://doi.org/10.3354/meps14545  

   Turrietta, E; Reidenbach, MA (March 2024, Marine Ecology Progress Series)

   In both continuous and fragmented seagrass ecosystems, the vegetation edge can be a location of abrupt hydrodynamic change, with impacts to both ecological and physical processes. We address how flow and wave activity change across seagrass meadow edges and the effects of vegetation on sediment dynamics and bivalve recruitment. TwoZostera marinaseagrass meadow sites were monitored: a high-density site with >500 shoots m^-2and a low-density site with <250 shoots m^-2. Mean flow velocities were significantly reduced in seagrass vegetation adjacent to edges, with reductions compared to unvegetated areas ranging from 30-75%. Recruitment of juvenile bivalves was significantly elevated within vegetation. No significant differences in wave activity or sediment suspension and/or deposition were found spatially across a 10 m distance from a seagrass edge, but significant temporal variability was observed, caused by periodic storms. Wave height was a major predictor for sediment movement along seagrass edges, with an observed 10-fold increase in sediment collection within benthic traps following severe storms. These results were found across various heterogeneous edge configurations and reveal abrupt hydrodynamic responses of both mean flow and turbulence to occur at short spatial scales (1-10 m), with changes to wave and sediment deposition and/or suspension conditions only occurring over larger spatial distances (~100 m). Changes to the hydrodynamic regime were therefore found to be driven by meteorological conditions (e.g. winds, storms) on daily timescales and by changes in seagrass shoot density, altering both bivalve recruitment and sediment dynamics on longer temporal and/or spatial timescales. 

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