Living benthonic foraminifera of the south Texas shelf: temporal variations and associated ecologic and paleoecologic implications
Hueni, Camille D.
Casey, Richard E.
Master of Arts
Seasonal and monthly samples for living benthonlc foraminiferan analysis were taken from the south Texas outer continental shelf. The study area was bounded on the north by the entrance to Matagorda Bay, and on the south by the U.S.-Mexico border, and extended seaward from the 1.3 mile Texas state limit to and past the shelf break. One hundred and eleven species were counted and noted to the lowest taxonomic level. Conclusions derived from observations of this data includet Overall density and diversity were higher in the coarser sediments than in the modern homogenous mud sector. Bank stations, located to the north of the sample area, contained high densityhigh diversity populations characterized by shared dominance of all species. Hospital Rock and Southern Bank contained average densities lower than those of the northern banks, but slightly higher than densities of surrounding areas. The south Texas continental shelf can be subdivided into three faunal depth breaks. The inner shelf (to 5 m) is characterized by high density, moderate diversity, species dominance, and "eutrophic” conditions defined by a Brlzallna lowmani-Ammonia beccarli-Bullmlnella elegantisslma dominant assemblage. The mid-shelf (5-8 m) is defined by moderate density, maximum diversity, an area of dominance transition, and "mesotrophic" conditions. Low density, low diversity, and no species dominance is characteristic of the outer shelf (8-18 m). "Oligotrophic” conditions of the outer shelf are associate with Florilus grateloupi, Nonionella.basiloba, Fursenkoina pontoni, Virgullnella pertusa. and Buliminella cf. B. bassendorfensis. Greatest effects of seasonality occur in the inner and midshelf regions. High denstiy of the inner and mid-shelf in the north in Winter, 1976, switched to the south in Spring. In Summer/Fall, this density high migrated back to the north via a mid-shelf route and re-established itself. In the Winter, 1976, inner shelf forms were found out to the shelf edge. Outer shelf forms were brought in to the mid-shelf region in Spring. Distribution of assemblages were related to oceanographic parameters of the bottom and surface movement and related primary productivity in Spring, 1976 by the extent of the mixed seasonal layer in Winter and effect of an anticyclonic gyre the previous Summer/Fall; and by "open estuarine upwelling" in Spring, 1976. Brlzalina lowmanl was used to delineate eutrophic conditions at the sediment-water interface and in the water column. It is defined as an opportunist in the sense that it takes advantage of eutrophic conditions at the substrate interface in the Spring and Winter, 1976 as a primary consumer or detritlvore, but develops in a meroplanktonic form to take advantage of eutrqphism in the water column as a primary producer, or primary consumer, when bottom conditions were generally oligotrphic in the Summer/Fall. Analysis of gravity cores from the south Texas shelf area illustrated paleotemperature fluctuations for Fleistocene-Holocene sediments.