ABSTRACT

Radiolarians, planktonic and benthonic foraminiferans, and pteropods collected in plankton tows from waters overlying the outer continental shelf of south Texas have been correlated with biological, physical and chemical data collected during the same period, 1975 through 1977. Planktonic foraminiferans and radiolarians appear to indicate open-marine conditions, and occur in abundance over the shelf during periods of intrusion of shallow or deep (upwelling) water from the open Gulf. Pteropods and benthonic foraminiferans characterize the water column of shelfal waters. Globigerina falconensis Blow, Globigerina pachyderma (Ehrenberg) Globigerina quinqueloba Natland are the dominant planktonic foramiriferans in the well mixed water (to a depth of 70 m) of the winter. In late winter or early spring of 1977, presence of the radiolarian Spongotrochus glacialis Popofsky indicated midshelf upwelling of deep open-Gulf water. Continental runoff dominates the shallow water during the spring and carries Brizalina lowmani (Phleger and Parker) beyond the shelf break in water overlying the pycnocline. This continental runoff seemed to create an "open-ocean estuarine type of upwelling" and brings mesopelagic forms onto the shelf under the pycnocline. Globigerina bulloides d'Orbigny and Globigerinoides ruber (d'Orbigny) dominate shelf waters above the thermocline in summer, and offshore radiolarians and Globigerina falconensis occur beneath the thermocline. During the summer of 1976, an anticyclonic gyre (detached from the Loop Current) grounded itself on the shelf. This gyre or ring carried radiolarians indicative of Subtropical Underwater onto the shelf, and its rotation seems to have been the dominant circulation pattern during that summer. The gyre transported a population of Brizalina lowmani northward. The shelled microzooplankton appear to be excellent indices for current and water-mass movements on the shelf, and for intrusions of open-ocean water masses. These studies, therefore, should aid in deciphering similar parameters on comparable ancient shelves.