The effects of long-term manipulation of nutrient supply on competition between the seagrasses Thalassia testudinum and Halodule wrightii in Florida Bay.-Oikos 72: 349-358. Long tenn (8 yr) continuous fertilization (via application of bird feces) of established seagrass beds in Florida Bay, FL, USA caused a change in the dominant seagrass species. Before fertilization, the seagrass beds were a Thalassia testudinum monocullure; after 8 yr of fertilization the seagrass Halodule wrightii made up 97% of the aboveground biomass. Fertilization had a positive effect on the standing crop of T. testudinum for the first two years of the experiment. The transition from T. testudinumdominated to H. wrightii-dominated was dependent on the timing of colonization of the sites by H. wrightii; the decrease in T. testudinum standing crop and density at the fertilized sites occurred only after the colonization of the sites by H. wrightii. There were no trends in the standing crop or density of T. testudinum at control sites, and none of the control sites were colonized by H. wrightii. The effects of fertilization on these seagrass beds persisted at least 8 yr after the cessation of nutrient addition, suggesting that these systems retain and recycle acquired nutrients efficiently. Results of these experiments suggest that Halodule wrightii, the nonnal early-successional seagrass during secondary succession in Caribbean seagrass communities, has a higher nutrient demand than Thalassia testudinum, the nonnallate successional species, and that the replacement of H. wrightii by T. testudinum during secondary succession is due to the ability of T. testudinum to draw nutrient availability below the requirements of H. wrightii.

ABSTRACT-The effect of burial due to sudden sediment loading was examined in a mixed Philippine seagrass meadow through the experimental deployment of sediment (0. 2. 4 , 8, and 16 cm deposited over the experimental plots). The responses in shoot density, vertical growth, and branching of the species present were assessed 2. 4, and 10 mo following disturbance. Shoot density responses were strongly species-specific. The large Enhalus acoroides maintained shoot density at all burial treatments, and only showed evidence of decline by the end of the expenment. Thalassia hempnchiiand, to a lesser extent. Cymodocea rotundata showed a sharp decllne In shoot density even at moderate burial treatments, from which they faded to recover. The accompanying species (Halodule uninervis, Syringodiurn isoetifolium, and Cyrnodocea serrulata) showed an initial decline In shoot density followed by recovery. The small Halophila ovalis showed an opportunistic growth in plots receiving intermediate (buried by 4 and 8 cm sediment) disturbance, reaching shoot densities well in excess of those on control plots. The results suggest a pattern of species loss following disturbance by sediment burial corresponding to the sequence, T hemprichii + (C. rotundata. S. isoetifolium, H uninervis) + C. serrulata + E. acoroides. Vertical growth increased significantly for all species with differentiated vertical shoots, except C, serrulata. The examination of the time course of vertical growth imprinted on the shoots of the dominant species, T hemprichii, revealed a rapid response to bunal through increased internodal length, which was maintained over 8 mo following the disturbance. The resulting cumulative vertical growth along the experiment was linearly correlated with the degree of burial in~posed on the plants. Branching of vertical shoots also increased significantly (73 to 96%, depending on the species) with burial. Experimental burial induced changes in shoot age distribution of some of the species, involving rearrangements, through selective mortality or recruitment, of the contribution of young shoots to the populations. The results obtained show major differences in species response to small-scale disturbance, closely linked to predictions derived from consideration of species growth rate and size, and provide evidence of the importance of small-scale disturbance in the maintenance of multispecific seagrass meadows.

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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.

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