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  1. Although seagrass ecosystems are valued for the services they provide, anthropogenic impacts have led to global declines in seagrass area. South Florida harbors one of the most extensive and iconic seagrass landscapes in the world, but historic seagrass losses appeared to threaten their integrity. The establishment of the Florida Keys National Marine Sanctuary (FKNMS) in 1995 created a benthic community and water quality monitoring network to aid management efforts. With this study, we report on the status and trajectories of benthic communities in South Florida using 25 years of monitoring data. Overall, most of our permanent monitoring sites maintained stable benthic communities over the period of observation. However, for areas that did experience decline, we identified mechanisms for loss of the climax seagrass Thalassia testudinum in the FKNMS with no or only partial recovery over decadal timescales. We observed a shift towards fast-growing Halodule wrightii meadows at anthropogenically nutrient-enriched nearshore sites along the Florida Keys. In addition, we describe almost complete loss of seagrass meadows at some exposed, back-reef sites offshore from the Florida Keys resulting from physical disturbance by major hurricanes. This study demonstrates the utility of long-term monitoring programs for the identification of benthic community trajectories and their putative drivers on the seascape scale, offering valuable lessons for the design of future seagrass monitoring programs. 
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  2. The organic carbon (Corg) stored in seagrass meadows is globally significant and could be relevant in strategies to mitigate increasing CO2 concentration in the atmosphere. Most of that stored Corg is in the soils that underlie the seagrasses. We explored how seagrass and soil characteristics vary among seagrass meadows across the geographic range of turtlegrass (Thalassia testudinum) with a goal of illuminating the processes controlling soil organic carbon (Corg) storage spanning 23° of latitude. Seagrass abundance (percent cover, biomass, and canopy height) varied by over an order of magnitude across sites, and we found high variability in soil characteristics, with Corg ranging from 0.08 to 12.59% dry weight. Seagrass abundance was a good predictor of the Corg stocks in surficial soils, and the relative importance of seagrass-derived soil Corg increased as abundance increased. These relationships suggest that first-order estimates of surficial soil Corg stocks can be made by measuring seagrass abundance and applying a linear transfer function. The relative availability of the nutrients N and P to support plant growth was also correlated with soil Corg stocks. Stocks were lower at N-limited sites than at P-limited ones, but the importance of seagrass-derived organic matter to soil Corg stocks was not a function of nutrient limitation status. This finding seemed at odds with our observation that labile standard substrates decomposed more slowly at N-limited than at P-limited sites, since even though decomposition rates were 55% lower at N-limited sites, less Corg was accumulating in the soils. The dependence of Corg stocks and decomposition rates on nutrient availability suggests that eutrophication is likely to exert a strong influence on carbon storage in seagrass meadows. 
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  3. Abstract. The net ecosystem productivity (NEP) of two seagrassmeadows within one of the largest seagrass ecosystems in the world, FloridaBay, was assessed using direct measurements over consecutive diel cyclesduring a short study in the fall of 2018. We report significant differencesbetween NEP determined by dissolved inorganic carbon (NEPDIC) and bydissolved oxygen (NEPDO), likely driven by differences in air–water gasexchange and contrasting responses to variations in light intensity. We alsoacknowledge the impact of advective exchange on metabolic calculations ofNEP and net ecosystem calcification (NEC) using the “open-water” approachand attempt to quantify this effect. In this first direct determination ofNEPDIC in seagrass, we found that both seagrass ecosystems were netheterotrophic, on average, despite large differences in seagrass netabove-ground primary productivity. NEC was also negative, indicating thatboth sites were net dissolving carbonate minerals. We suggest that acombination of carbonate dissolution and respiration in sediments exceededseagrass primary production and calcification, supporting our negative NEPand NEC measurements. However, given the limited spatial (two sites) andtemporal (8 d) extent of this study, our results may not berepresentative of Florida Bay as a whole and may be season-specific. Theresults of this study highlight the need for better temporal resolution,accurate carbonate chemistry accounting, and an improved understanding ofphysical mixing processes in future seagrass metabolism studies. 
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  4. Hurricanes are some of the largest environmental drivers of change in coastal systems. We investigated the impacts of Hurricane Irma on benthic macrophyte communities in Florida Bay (FB) and the Florida Keys National Marine Sanctuary (FKNMS), USA. Spatiotemporal analyses were performed at multiple hierarchical levels (site, zone, region) to identify potential changes in the Braun-Blanquet (BB) densities of total seagrass (TSG) and total calcareous green macroalgae (TCAL) post-disturbance and to determine whether changes were attributable to hurricane impacts or normal seasonal and inter-annual variability. There were significant decreases in TSG in one of five zones in FKNMS and in one of six zones in FB, but no change in TCAL was recorded in either system. TSG in the Lower Keys Bayside declined from a mean BB score of 2.6 to 1.2, resulting from storm-induced erosion, whereas TSG in coastal FB declined from 1.05–2.4 to 0.36–2.0, likely due to prolonged hyposalinity and low dissolved oxygen following stormwater drainage. Overall, impacts to South Florida benthic macrophyte communities from Hurricane Irma were not catastrophic and were limited in spatial extent. Our results suggest that coastal areas hit by a storm with heavy winds are more likely to sustain direct physical impacts to the benthos, whereas estuarine areas with longer residence times are more at risk of the indirect effects of stormwater runoff and retention. Our analyses placed putative hurricane impacts within the context of recent variability and historical system baselines through the use of long-term monitoring data coordinated by multiple governmental and academic entities. 
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  5. null (Ed.)
    Abstract Susan Lynn Williams (1951–2018) was an exceptional marine ecologist whose research focused broadly on the ecology of benthic nearshore environments dominated by seagrasses, seaweeds, and coral reefs. She took an empirical approach founded in techniques of physiological ecology. Susan was committed to applying her research results to ocean management through outreach to decision-makers and resource managers. Susan’s career included research throughout the USA in tropical, temperate, and polar regions, but she specialized in tropical marine ecology. Susan’s scholarship, leadership, and friendship touched many people, leading to this multi-authored paper. Susan’s scholarship was multi-faceted, and she excelled in scientific discovery, integration of scientific results, application of science for conservation, and teaching, especially as a mentor to undergraduate and graduate students and postdoctoral scholars. Susan served in a variety of leadership positions throughout her career. She embodied all facets of leadership; leading by example, listening to others, committing to the “long haul,” maintaining trust, and creating a platform for all to shine. Susan was an important role model for women in science. Susan was also a loyal friend, maintaining friendships for many decades. Susan loved cooking and entertaining with friends. This paper provides an overview of the accomplishments of Susan in the broad categories of scholarship, leadership, and friendship. 
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  6. Abstract

    Extreme climatic events (ECEs) and predator removal represent some of the most widespread stressors to ecosystems. Though species interactions can alter ecological effects of climate change (and vice versa), it is less understood whether, when and how predator removal can interact with ECEs to exacerbate their effects. Understanding the circumstances under which such interactions might occur is critical because predator loss is widespread and ECEs can generate rapid phase shifts in ecosystems which can ultimately lead to tropicalization.

    Our goal was to determine whether loss of predation risk may be an important mechanism governing ecosystem responses to extreme events, and whether the effects of such events, such as tropicalization, can occur even when species range shifts do not. Specifically, our goal was to experimentally simulate the loss of an apex predator, the tiger sharkGaleocerdo cuviereffects on a recently damaged seagrass ecosystem of Shark Bay, Australia by applying documented changes to risk‐sensitive grazing of dugongDugong dugonherbivores.

    Using a 16‐month‐field experiment established in recently disturbed seagrass meadows, we used previous estimates of risk‐sensitive dugong foraging behaviour to simulate altered risk‐sensitive foraging densities and strategies of dugongs consistent with apex predator loss, and tracked seagrass responses to the simulated grazing.

    Grazing treatments targeted and removed tropical seagrasses, which declined. However, like in other mixed‐bed habitats where dugongs forage, treatments also incidentally accelerated temperate seagrass losses, revealing that herbivore behavioural changes in response to predator loss can exacerbate ECE and promote tropicalization, even without range expansions or introductions of novel species.

    Our results suggest that changes to herbivore behaviours triggered by loss of predation risk can undermine ecological resilience to ECEs, particularly where long‐lived herbivores are abundant. By implication, ongoing losses of apex predators may combine with increasingly frequent ECEs to amplify climate change impacts across diverse ecosystems and large spatial scales.

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  7. Abstract

    Subtropical seagrass meadows play a major role in the coastal carbon cycle, but the nature of air–water CO2exchanges over these ecosystems is still poorly understood. The complex physical forcing of air–water exchange in coastal waters challenges our ability to quantify bulk exchanges of CO2and water (evaporation), emphasizing the need for direct measurements. We describe the first direct measurements of evaporation and CO2flux over a calcifying seagrass meadow near Bob Allen Keys, Florida. Over the 78‐d study, CO2emissions were 36% greater during the day than at night, and the site was a net CO2source to the atmosphere of 0.27 ± 0.17 μmol m−2s−1(x̅ ± standard deviation). A quarter (23%) of the diurnal variability in CO2flux was caused by the effect of changing water temperature on gas solubility. Furthermore, evaporation rates were ~ 10 times greater than precipitation, causing a 14% increase in salinity, a potential precursor of seagrass die‐offs. Evaporation rates were not correlated with solar radiation, but instead with air–water temperature gradient and wind shear. We also confirm the role of convective forcing on night‐time enhancement and day‐time suppression of gas transfer. At this site, temperature trends are regulated by solar heating, combined with shallow water depth and relatively consistent air temperature. Our findings indicate that evaporation and air–water CO2exchange over shallow, tropical, and subtropical seagrass ecosystems may be fundamentally different than in submerged vegetated environments elsewhere, in part due to the complex physical forcing of coastal air–sea gas transfer.

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