Ecology and distribution of microplankton of the south Texas outer continental shelf
Williams, Damon Hughes
Casey, Richard E.
Master of Arts
A seasonal study of the ecology and distribution of microplankton was undertaken for the south Texas outer continental shelf. Twelve stations were occupied for winter, spring, and summer seasons during 1975 and 1976. A total of 199 thirty liter Niskin bottle samples were analyzed; 98 collected during 1975, and 11 collected during 1976. The filtered contents of these samples were examined under a plankton scope to determine the relative abundance of microplanktonic groups, density of shelled microplankton, and. density of general microplanktonic organisms. The average density of microplanktonic organisms in 1/2 photic zone samples for all stations during winter was 2.9 X lQ-5/m3 for 1375 data and 2.2 X 15/m3 for 1976 data; 5.4 X 15/m3 for 1975 data and 4.7 X 1^/m3 for 1976 data during spring; and 4.5 X 13/m3 for 1975 data and 1.5 X l<3?/m3 for 1976 data during summer. Probably a more realistic average for summer 1976 densities is 2.4 X lOfym3 (obtained by excluding one extremely dense sample, 8.7 X 1/m3, from consideration). Microplankton densities exhibited a seaward decrease with the exception of samples taken along one transect. Densities for radiolarian, forarainiferan, ostracod, and pteropod microplankton groups were calculated and were generally similar for the two years with the exception of summer seasonal data during 1976. A sharp increase in nassellarian and spume!larian radiolarians, as well as planktonic foraminiferans, and the presence of species endemic to certain water masses suggested that an anticyclonic ring containing components of Subtropical Underwater was present on the south Texas continental shelf during late spring and summer of 1976. Physical and chemical oceanographic data collected concurrently with the Niskin samples also suggest this possibility. Relative abundances of 35 microplankton groups in 1975 data and 34 microplankton groups in 1976 data were analyzed using multivariate cluster analysis techniques to determine what parameters affected the biological composition of microplankton assemblages. R-mode analyses of the two yearly data sets showed no striking differences in the composition of microplankton assemblages for 1975 and 1976. Q-mode analyses for 1975 and 1976 indicated that location of the shelf and .season of collection were important determinants on' microplankton comnunity structure. Statistical summaries of the clusters defined by these analyses Indicated that nearshore microplankton populations can be characterized by abundant numbers of diatoms and dinoflagellates, shallow offshore waters by abundant planktonic foraminifera, spumellarians, and relatively high numbers of copepods and naupliar larvae, and deeper offshore waters by an abundance of nassellarians, copepods, and naupliar larvae, while spumellarians and planktonic foraminifera are still common. Finally, a combination of Niskin discrete samples and Nansen tow channel samples was found to be very effective in detecting localized physical oceanographic phenomena and resultant changes in microplankton populations. The overall results of this study correlated well with other plankton studies in the area, and they Indicate that trends in microplankton distribution and relative abundance, particularly certain radiolarian and foraminiferan groups, may be very good environmental indicators of major and minor oceanographic phenomena.