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Seagrasses are recognized as important plant communities in coastal estuaries and lagoons across both tropical and temperate climes; thus, large-scale seagrass die-off events worldwide are of general concern. In Florida Bay, at the southern terminus of the Florida peninsula, seagrass die-off events up to 4000 ha have been reported and smaller scale mortality events are noted annually. In the present study, we examined several hypothesized causative factors (high temperature, hypersalinity, sulfide toxicity) of seagrass ( Thalassia testudinum ) mortality in Florida Bay. To test sulfide effects, in situ sulfide production was stimulated by applying a labile carbon source (glucose) to sulfate reducers in the sediment at five sites across the bay (northeastern, northcentral, and southwestern basins). During the one year study, high temperature (32–36 °C) and salinity (> 50 psu) were recorded in the bay associated with a regional drought. We also experienced major seagrass die-off events at two of our southwestern bay sites. These field conditions provided an excellent opportunity to closely examine cause–effect relationships among stressors and die-off events in the field, and verify results of our previous mesocosm experiments. Even though glucose amendments stimulated porewater sulfides in bay sediments (4–8 mmol L − 1 ), no significant differences in biomass, short shoot density or final growth rates were found between control and glucose plots. In addition, the highest growth rates and shoot densities were concomitant with maximum water column salinity (> 50 psu) and temperature (32–36 °C), when porewater sulfides were also in the millimolar range. Large-scale seagrass mortality events, encompassing ∼ 50% of the entire meadow at one site, occurred at southwestern bay sites when plants were down regulating (slower growth and shoot density), probably in response to shorter day length and lower temperature (30–34 to 23–26 °C) from October, 2004 to January, 2005. Sulfate reduction rates (SRR) were also 2-fold higher in the southwestern (214–488 nmol cm − 3 d − 1 ) versus northcentral and northeastern (97–240 nmol cm − 3 d − 1 ) bay sites, possibly limited by labile carbon, which we found to stimulate SRR 3-fold in northeastern and northcentral bay sites (461–708 nmol cm − 3 d − 1 ) and 4-fold at southwestern bay sites (1211–2036 nmol cm − 3 d − 1 ). Based on a synthesis of the field data reported herein, our mesocosm experiments to date, and contributions by others, we present a conceptual model of seagrass die-off in Florida Bay outlining a cascade of stressors, stimulated by P enrichment, which leads to high O 2 consumption in the system triggering a seagrass die-off event.

To examine the synergism of high temperature and sulfide on two dominant tropical seagrass species, a large-scale mesocosm experiment was conducted in which sulfide accumulation rates (SAR) were increased by adding labile carbon (glucose) to intact seagrass sediment cores across a range of temperatures. During the initial 10 d of the 38 d experiment, porewater SAR in cores increased 2- to 3-fold from 44 and 136 μmol L − 1 d − 1 at 28–29 °C to 80 and 308 μmol L − 1 d − 1 at 34–35 °C in Halodule wrightii and Thalassia testudinum cores, respectively. Labile C additions to the sediment resulted in SAR of 443 and 601 μmol L − 1 d − 1 at 28–29 °C and 758 to 1,557 μmol L − 1 d − 1 at 34–35 °C in H. wrightii and T. testudinum cores, respectively. Both T. testudinum and H. wrightii were highly thermal tolerant, demonstrating their tropical affinities and potential to adapt to high temperatures. While plants survived the 38 d temperature treatments, there was a clear thermal threshold above 33 °C where T. testudinum growth declined and leaf quantum efficiencies (Fv/Fm) fell below 0.7. At this threshold temperature, H. wrightii maintained shoot densities and leaf quantum efficiencies. Although H. wrightii showed a greater tolerance to high temperature, T. testudinum had a greater capacity to sustain biomass and short shoots under thermal stress with labile C enrichment, regardless of the fact that sulfide levels in the T. testudinum cores were 2 times higher than in the H. wrightii cores. Tropical seagrass tolerance to elevated temperatures, predicted in the future with global warming, should be considered in the context of the sediment-plant complex which incorporates the synergism of plant physiological responses and shifts in sulfur biogeochemistry leading to increased plant exposure to sulfides, a known toxin.

The long-term sustainability of seagrasses in the subtropics and tropics depends on their ability to adapt to shifts in salinity regimes, particularlyinlightofpresentincreasesincoastalfreshwaterextractionsandfutureclimatechangescenarios.Althoughtherearemajorconcerns world-wideonincreasedsalinityincoastalestuaries,thereislittlequantitativeinformationonthespecificuppersalinitytoleranceoftropicaland subtropical seagrass species. We examined seagrass hypersalinity tolerance under two scenarios: (1) when salinity is raised rapidly simulating a pulsed event, such as exposure to brine effluent, and (2) when salinity is raised slowly, characteristic of field conditions in shallow evaporative basins; the first in hydroponics (Experiments I and II) and the second in large mesocosms using intactsediment cores from the field (Experiment III). The three tropical seagrass species investigated in this study were highly tolerant of hypersaline conditions with a slow rate of salinity increase (1 psu d 1 ). None of the three species elicited total shoot mortality across the range of salinities examined (35‐70 psu over 30 days exposures);representinginsituexposurerangesinFloridaBay,ashallowsemi-enclosedsubtropicallagoonwithrestrictedcirculation.Basedon stress indicators, shoot decline, growth rates, and PAM florescence, all three species were able to tolerate salinities up to 55 psu, with Thalassia testudinum(60 psu) and Halodule wrightii(65 psu) eliciting a slightly higher salinity threshold than Ruppia maritima(55 psu). However, when salinitywas pulsed,withouta slowosmotic adjustmentperiod,thresholdlevelsdropped20 psutoapproximately45 psufor T. testudinum.While we found these three seagrass species to be highly tolerant of high salinity, and conclude that hypersalinity probably does not solely cause seagrass dieoff events in Florida Bay, high salinity can modify carbon and O2 balance in the plant, potentially affecting the long-term health of the seagrass community. # 2006 Elsevier B.V. All rights reserved.