The structure of microplankton communities of the South Texas Outer Continental Shelf
Gust, Linda Wolfgram
Casey, Richard E.
Master of Arts
Microplankton from 12 stations in the waters overlying the south Texas outer continental shelf (STOCS) had previously teen collected in 3 liter Niskin bottles for approximately winter, spring, and summer seasons from December 1974 through the spring of 1977- In this study, microplankton from these samples (especially from samples from half-photic zone) are identified and counted, and the data are analysed to assess variation in community structure (density, diversity and the number of trophic levels). Microplankton density, diversity, and trophic level composition indicate that two extremes in types of microplankton communities exist in the STOCS research area with intermediate "types" between. The first type is a diatom dominated, dense (1 organisms/nP) community having very low diversity and very few trophic levels. This community exists in the winter, 1977, nearshore samples and corresponds with relatively low concentrations of dissolved silicate, nitrate, and phosphate. A less dense (1 organisms/iP), though otherwise similar community exists offshore during spring, 1977- Two examples of a second type of community are found in the summers of 1975 and 1976. Both have low diatom abundance, low general microplankton density (1^ organisms/nr in 1975; 1 organisms/rP in 1976), and very high diversity. Many samples have four trophic levels. This community type occurs in waters having higher concentration of dissoved silicate and phosphate, and in waters that are warmer and have higher salinity than the winter-early spring waters. The diatom dominated, dense community having very low diversity and very few trophic levels appears to he related to the only dramatic upwelling event on the STOCS recorded during this study and is considered to he a true local (STOCS) eutrophism. The low diatom abundanc low general microplankton density, and very high diversity community appears to be related to the incursion of open Gulf waters carrying into the STOCS a community of this structure with it.