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1. Long‐term trends and resilience of seagrass metabolism: A decadal aquatic eddy covariance study

   https://doi.org/10.1002/lno.11397  

   Berger, Amelie C. ; Berg, Peter ; McGlathery, Karen J. ; Delgard, Marie Lise ( January 2020 , Limnology and Oceanography)

   Seagrass meadows are valued for their ecosystem services, including their role in mitigating anthropogenic CO₂emissions through ‘blue carbon’ sequestration and storage. This study quantifies the dynamics of whole ecosystem metabolism on daily to interannual timescales for an eelgrass (Zostera marina) meadow using in situ benthic O₂flux measurements by aquatic eddy covariance over a period of 11 yr. The measurements were part of the Virginia Coast Reserve Long‐Term Ecological Research study, and covered a relatively stable period of seagrass ecosystem metabolism 6–13 yr after restoration by seeding (2007–2014), a die‐off event likely related to persistently high temperatures during peak growing season in 2015, and a partial recovery from 2016 to 2018. This unique sequence provides an unprecedented opportunity to study seagrass resilience to temperature stress. With this extensive data set covering 115 full diel cycles, we constructed an average annual oxygen budget that indicated the meadow was in metabolic balance when averaged over the entire period, with gross primary production and respiration equal to 95 and −94 mmol O₂m⁻²d⁻¹, respectively. On an interannual scale, there was a shift in trophic status from balanced to net heterotrophy during the die‐off event in 2015, then to net autotrophy as the meadow recovered. The highly dynamic and variable nature of seagrass metabolism captured by our aquatic eddy covariance data emphasizes the importance of using frequent measurements throughout the year to correctly estimate trophic status of seagrass meadows.

    

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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. Seagrass restoration reestablishes the coastal nitrogen filter through enhanced burial

   https://doi.org/10.1002/lno.11241  

   Aoki, Lillian R. ; McGlathery, Karen J. ; Oreska, Matthew P. J. ( July 2019 , Limnology and Oceanography)

   Seagrass meadows perform an important ecological function as filters for incoming nutrients from surrounding watersheds, especially nitrogen (N). By enhancing N removal processes, including N burial in sediments and denitrification, seagrass meadows improve water quality. With accelerating losses of seagrass meadows worldwide, seagrass restoration plays a key role in reestablishing these coastal ecosystem functions. However, few measurements exist of N burial rates in temperate seagrass meadows and none have been published for restored meadows. In this study, we measured N burial rates in a large (6.9 km²) restored eelgrass (Zostera marina) meadow and compared N removal through burial to previous measurements of removal via denitrification. We also compared N removal to inputs from external loading and fixation and to N assimilation in seagrass biomass. We found that, in this meadow, burial was the dominant process of N removal; the burial rate of 3.52 g N m⁻²yr⁻¹was comparable to rates in natural meadows within 10 yr after seeding and was more than 20× the rate in adjacent bare sediments (0.17 g N m⁻²yr⁻¹). We also found that the high rates of N assimilation (2.62 g N m⁻²yr⁻¹) created a substantial though temporary sink for nitrogen during the growing season. Our results highlight how seagrass meadows mediate N cycling through high rates of burial, which to date has been understudied in the literature. The successful return of the N filter function after restoration, shown here for the first time, can motivate continued efforts for seagrass restoration and conservation.

    

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4. Taking the Pulse of Resilience in Conserving Seagrass Meadows

   https://doi.org/10.1093/icb/icae120  

   Graham, Olivia_J ; Harvell, Drew ; Christiaen, Bart ; Gaeckle, Jeff ; Aoki, Lillian_R ; Ratliff, Baylen ; Vinton, Audrey ; Rappazzo, Brendan_H ; Whitman, Tina ( July 2024 , Integrative And Comparative Biology)

   Foundational habitats such as seagrasses and coral reefs are at severe risk globally from climate warming. Infectious disease associated with warming events is both a cause of decline and an indicator of stress in both habitats. Since new approaches are needed to detect refugia and design climate-smart networks of marine protected areas, we test the hypothesis that the health of eelgrass (Zostera marina) in temperate ecosystems can serve as a proxy indicative of higher resilience and help pinpoint refugia. Eelgrass meadows worldwide are at risk from environmental stressors, including climate warming and disease. Disease outbreaks of Labyrinthula zosterae are associated with recent, widespread declines in eelgrass meadows throughout the San Juan Islands, Washington, USA. Machine language learning, drone surveys, and molecular diagnostics reveal climate impacts on seagrass wasting disease prevalence (proportion of infected individuals) and severity (proportion of infected leaf area) from San Diego, California, to Alaska. Given that warmer temperatures favor many pathogens such as L. zosterae, we hypothesize that absent or low disease severity in meadows could indicate eelgrass resilience to climate and pathogenic stressors. Regional surveys showed the San Juan Islands as a hotspot for both high disease prevalence and severity, and surveys throughout the Northeast Pacific indicated higher prevalence and severity in intertidal, rather than subtidal, meadows. Further, among sites with eelgrass declines, losses were more pronounced at sites with shallower eelgrass meadows. We suggest that deeper meadows with the lowest disease severity will be refuges from future warming and pathogenic stressors in the Northeast Pacific. Disease monitoring may be a useful conservation approach for marine foundation species, as low or absent disease severity can pinpoint resilient refugia that should be prioritized for future conservation efforts. Even in declining or at-risk habitats, disease surveys can help identify meadows that may contain especially resilient individuals for future restoration efforts. Our approach of using disease as a pulse point for eelgrass resilience to multiple stressors could be applied to other habitats such as coral reefs to inform conservation and management decisions.

    

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5. Seagrass ecosystem metabolic carbon capture in response to green turtle grazing across Caribbean meadows

   https://doi.org/10.1111/1365-2745.13306  

   Johnson, Robert A. ; Gulick, Alexandra G. ; Constant, Nerine ; Bolten, Alan B. ; Smulders, Fee O. H. ; Christianen, Marjolijn J. A. ; Nava, Mabel I. ; Kolasa, Keith ; Bjorndal, Karen A. ; Randall Hughes, ed., A. ( November 2019 , Journal of Ecology)

   Increasing green turtle abundance will lead to increased grazing within seagrass habitats—ecosystems that are important for carbon sequestration and storage. However, it is not well understood how carbon dynamics in these ecosystems respond to grazing and whether a response differs among meadows or locations.

   We measured seagrass ecosystem metabolism in grazed and ungrazed areas ofThalassia testudinummeadows with established green turtle foraging areas across the Greater Caribbean and Gulf of Mexico. We sampled meadows from five locations that differed in seagrass and environmental characteristics. Established meadows of the invasive seagrassHalophila stipulaceawere also present at two of these locations, and we measured ecosystem metabolism in these meadows for comparison to grazed and ungrazed areas of the nativeT. testudinum.

   Across all individual sites, rates of net ecosystem production (NEP) ranged from 56% to 96% lower in grazed areas than ungrazed areas ofT. testudinummeadows. Rates of NEP were also strongly, positively correlated with above‐ground seagrass biomass across sites. While metabolic carbon capture rates were lower in grazed areas, heterotrophic respiration was not stimulated, and grazing therefore did not result in significant metabolic remineralization of carbon in these meadows. NEP inH. stipulaceameadows was similar to rates inT. testudinummeadows at all three sites, suggesting that metabolic carbon capture may remain similar in Caribbean meadows where this invasive seagrass is replacing native species.

   Synthesis. Our results show that there is a consistent response in metabolic carbon dynamics to green turtle grazing inT. testudinummeadows across the Greater Caribbean region. An increase in grazing will not likely stimulate remineralization of carbon as these important habitats are returned to a natural grazed state.

    

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