Populations of the ecologically important sea urchin Diadema antillarum suffered severe mass mortalities throughout the Caribbean. This mortality was first observed at Panama in January 1983; by January 1984 it had spread to the rest of the Caribbean and to Bermuda. The sequence of mortality events in most areas is consistent with the hypothesis that the causative agent was dispersed by major surface currents over large distances. However, some of the late die-offs in the southeastern Caribbean do not fit this pattern. Several lines of indirect evidence suggest that the phenomenon is due to a water-borne pathogen. If so, this is the most extensive epidemic documented for a marine invertebrate.

Many of the annual or ephemeral algae of the mid to high intertidal zones have heteromorphic life histories, existing as upright morphs during seasonal algal blooms and as crustose or boring morphs during other portions of the year. Experimental removal of herbivores on the coasts of New England and Oregon resulted in the occurrence of the upright morphs in the times of year when they were normally absent (summer in our areas), demonstrating that such uprights can survive the summertime physical regime (contrary to earlier speculation). We suggest that the upright and crustose or boring stages of these algae represent mutually exclusive adaptations to fluctuations in grazing pressure: the upright stages are adapted for high rates of growth and reproduction when grazing pressure is low, and the crustose and boring stages are adapted for surviving through times of high grazing pressure. We predict isomorphic species of algae would predominate in these sorts of habitats if grazing pressure were more constant.

In 1986 more than 8 million liters of crude oil spilled into a complex region of mangroves, seagrasses, and coral reefs just east of the Caribbean entrance to the Panama Canal. This was the largest recorded spill into coastal habitats in the tropical Americas. Many population of plants and animals in both oiled and unoiled sites had been studied previously, thereby providing an unprecedented measure of ecological variation before the spill. Documenation of the spread of oil and its biological begun immediately. Intertidal mangroves, algae, and associated invertebrates were covered by oil and died soon after. More surprisingly, there was also extensive mortality of shallow subtidal reef corals and infauna of seagrass beds. After 1.5 years only some organisms in areas exposed to the open sea have recovered.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org. This content downloaded from 138.Abstract. This paper examines the contribution of herbivory to seasonal variations in algal coverage at the uppermost intertidal levels of a rocky shore on the outer coast of Oregon, USA. In this habitat, simple, fast-growing algae are abundant during the wetter, less sun-exposed, and cooler weather of the winter months, but disappear in the drier, sunnier, and warmer conditions of the summer. Acmaeid limpets, the predominant herbivores in the habitat, are present throughout the year. Previous studies of the high intertidal zone have suggested that physical stresses alone prevent the survivial of algae through the summer months, and that grazing affects algal abundance when physical conditions are more benign.To test these hypotheses, cageless methods were used to exclude the limpets from plots on the uppermost intertidal rocks. The experiments were repeated at 3-4 mo intervals over a period of several years. This partial reduction in herbivory resulted in the development and persistence of dense covers of algae through the summer months, as well as increasing algal abundance during the other times of year. Apparently the primary cause of the seasonal fluctuations in algal abundance is variation in rates of algal production rather than change in absolute rates of algal loss to herbivory or physical stresses. Evidence from this study and the literature indicates that the limpets in this habitat are probably consuming more, not less, algae in the winter months. However, higher rates of algal production in winter more than compensate for the increased herbivory, and the algae become more abundant. During the summer, drier conditions cause rates of production by the algae to fall below rates of consumption by limpets, and the limpets remove most of the standing crop of algae. Thus, increased grazing pressure coincides with increased physical stress. To be a perennial or summer annual in this habitat, an alga not only must be resistant to desiccation, insolation, and high temperatures, but also must be inaccessible to herbivores or be resistant to herbivory. In the uppermost intertidal zone, the abundance of algae in winter evidently depends on the pattern of changes in primary production over the entire year. Scarcity of forage in the summer months reduces populations of the resident limpets, and the favorable conditions for primary production in winter months allow the transient species of algae to establish rapidly, grow, and reproduce before the populations of limpets recover. If even the most favorable physical conditions for algal growth remained constant through the year, the transient algae would probably disappear.It is speculated that under pr...

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