No abstract

Unusually dense aggregations of the sea urchin Lytechinus variegatus overgrazed at least 0.81 kmz of seagrass habitat in Outer Florida Bay (USA) between August 1997 and I\,Iay 1998. Initially, sea-urchin densities were as high as 364 sea urchins m-', but they steadily declined to within a range of 20 to 50 sea urchins m-2 by December 1998. Prior to this event, sea-urchin densities were <1 sea urchin m-2 in this area of Outer Florida Bay. Seagrasses in Outer Florida Bay consist primarily of manatee grass Syringodium filiforme. of which 82% or 390 g dry weight rn-2 of total seagrass biornass and >95% of the short-shoot apical menstems were removed by sea-urchin grazing in our study area. Such extensive loss may severely limit recovery of this seagrass comrnunity by vegetative reproduction. Effects of the removal of seagrass biomass have already resulted in the depletion of epifaunal-infaunal mollusk assemblages and resuspension of fine-grained (<64 pm) surface sediments-which have caused significant changes in cornrnunity structure and in the physical properties of the Sediments. These changes, coupled with the loss of essential fishery habitat, reductions in primary and secondary production, and degradation of water quality, may lead to additional, longer-term, indirect effects that may extend beyond the boundaries of the grazed areas and into adjacent coastal ecosystems.

No abstract

Widespread mortality of Thalassia testudinum was first documented in Florida Bay, USA, during the summer of 1987. This unprecedented event spanned 3 yr, affected 40 km 2 of seagrass and resulted in more than a decade of ecological disturbances. Initial putative causes for seagrass die-off ranged from climatic anomalies and watershed changes to wasting disease and eutrophication. Subsequent experimental research suggested that hypoxic plant tissue, caused by low water column oxygen content or reduced photosynthesis, allowed intrusion of sulfide leading to plant death. Contributing factors included high temperatures, salinities and T. testudinum biomass, together causing lower oxygen water solubility, higher community respiration rates and elevated nighttime oxygen demand. The Fisheries Habitat Assessment Program (FHAP) has tracked the system's slow recovery since 1995. Recent FHAP data (2012) indicated that T. testudinum had returned to pre-die-off densities in even the most severely affected locations. During the summer of 2015, following several months of drought, National Park Service researchers reported hypersaline conditions and a recurrence of seagrass die-off in north-central Florida Bay. An interagency effort is presently underway to document the duration, extent, impacts and possible factors responsible for the current mortality. Initial field surveys indicate that there is high spatial coincidence between the current and the 1987−1990 events and that hypersalinity, water column stratification and bottom-water anoxia might have once again resulted in mass mortality of T. testudinum in Florida Bay. The goal of this report is to alert the scientific community to the recurrence of this important ecological event.

Seagrass transplanting experiments were conducted in Back Sound, Carteret County, North Carolina, and Tampa Bay, Pinellas County, Florida. In Florida, we compared three planting methods (cores, stapled bare root, and peat‐pot plugs) for shoot addition rate coverage, and labor cost (harvest, fabrication, and deployment) using Halodule wrightii. Only planting methods and development rates were recorded for Syringodium filiforme. Fertilizer additions were made to peat‐pot plantings of H. wrightii and Zostera marina in both North Carolina and Florida. Exclosure cages were tested to attempt to minimize bioturbation of H. wrightii and Z. marina in both North Carolina and Florida. Recovery from harvesting impacts to existing, natural beds of S. filiforme and H. wrightii were assessed in Florida. The peat‐pot method was about 35% and 63% less expensive in work time than staples and core tubes, respectively. Response to fertilizer additions was masked by inconsistent release properties of the fertilizer, although some indication of positive response to phosphorus fertilizer in sediments with low carbonate content, and nitrogen in general, was detected. Complete loss of peat pots, largely ascribed to bioturbation, occurred in a large planting (Tampa Bay) but not in nearby smaller ones where exclosure cages were used. Cages did not affect planting unit survival in North Carolina but did improve number of shoots per planting unit in one of three experiments. No detrimental effects of cages were noted. Existing natura beds used to harvest transplanting stock in Tampa Bay recovered from excavations as large as 0.5 m2 in one year. Significant cost savings were found to be possible through methodological improvement, including planting techniques, bioturbation exclusion, and possibly fertilizer additions.