| Abstract: | Three studies were made on aspects of physical and biological interactions which affect the structure of a subtropical rocky intertidal community in central Texas. A new video recording and analysis system developed for one of these studies is also presented. In the first study, the upper zonal limits of both Chthamalus fragilis and colonial algal turf species showed a significant linear relationship with wave height. C. fragilis showed more than twice the elevation response than did the algal turf species. The variability of C. fragilis elevations increased with increasing wave height. The second study explored the effects of substrate angle and breaking waves on patterns of distribution and abundance of dominant organisms. Sediment deposition increased on sheltered horizontal slopes. Algal turf mass and infaunal invertebrate abundance was greater on horizontal slopes. The abundance of some pericarideans and the tree oyster Isognom on bicolor was greatest on sheltered horizontal slopes. Xanthid crabs and other bivalves were more abundant at sheltered stations. Results support the hypothesis that changes in wave exposure and substrate angle modify hydrodynamic conditions which are directly and/or indirectly important in structuring intertidal communities. In the third study, the effects of mobile predators on community structure were examined. Results from fall and early winter indicate that neogastropods were responsible for approximately 87% of the observed mortality due to large predators. However, results from spring and early summer show that neogastropod predation was less important and only accounted for approximately 27.5 percent of the mortality observed due to large predators. Additional prey mortality was primarily due to predation by the stone crab Menippe ad in a. Other cage experiments indicate that small xanthid crabs are major predators on small prey. The midlittoral zone trophic structure was similar to tropical communities in that there was an abundance of highly mobile consumers, primarily crabs and fish. However, like temperate communities, prey escaped predators during colder months. Also like temperate communities, a high abundance of algae and sessile invertebrates, and distinct zonation patterns were present throughout much of the year. |
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