Ecology and distribution of living planktonic foraminifera of the south Texas outer continental shelf

Abstract

A seasonal study of the ecology and distribution of planktonic foraminifera was undertaken for the south Texas outer continental shelf. Twelve stations were occupied for winter, spring, and summer seasons. A total of 36 vertical plankton tows were obtained using a 76 micron mesh Nansen net. Fifteen live planktonic foraminiferal species were identified. In addition, seventeen live benthonic foraminiferal species were also encountered in the plankton tows. One benthonic species, Bolivina lowmani. was a major constituent of inner shelf samples. This species is believed to undergo a meroplanktonic stage. Total live planktonic foraminiferal densities averaged 7.2, 3.91, and 8.15 individuals/m for winter, spring, and summer, respectively. The largest standing crop occurred contemporaneously with the smallest densitiy of empty planktonic foraminiferal tests. Both densities and diversities were found to increase seaward across the shelf. The approximate ecologie niche of planktonic foraminifera was ascertained from their relationship to large, solitary, centric diatoms. Multivariate numerical techniques of R and Q mode cluster analysis were utilized to discern subtle seasonal assemblages and distributional patterns. Assemblages of planktonic foraminifera and their spatial distributions across the shelf were found to be correlabe to physical oceanography and, thus, to exhibit seasonal variations. Twelve sediment samples from the study area were also examined to compare populations of planktonic foraminifera in the water column with those preserved in the fossil record. A striking discrepancy was noted between tests of planktonic foraminifera in the plankton and those in the sen diments. Sediment forams were at least two to three times larger than individuals of the same species in the plankton. Absence of these small individuals is believed caused by rapid dissolution of the fragile tests from acidic conditions initiated by decomposition of the organism's protoplasm shortly after death (McMillen, 1976). Chi-square test and cluster analysis were also utilized in plankton to sediment comparisons. Results from these statistical methods indicated that the two populations were not the same. Thus, it can be inferred that the bottom sediment samples represent relict shelf sediments.