Seasonality of living benthic foraminifera from the San Pedro Basin, California borderland

Analysis of Rose Bengal-stained benthic foraminifera from six box cores collected from the San Pedro Basin in April, July, and October, 1988 (similar to 720 m water depth) reveals that the foraminiferal assemblages vary during this six month period. Species diversity is low, with five to six species constituting 90% of the total abundance in the >150 mu m fraction, In the 63-150 mu m fraction, eight species account for 90% of the total abundance in April, decreasing to three species in July, and increasing to seven in October. Species are vertically stratified within the sediments, reflecting microhabitat preferences, and are grouped according to their vertical patterns. In the >150 mu m fraction, three groups occur: 1) taxa having maxima from 0-1 cm, 2) infaunal taxa with maxima from 1.5-3 cm, and 3) species with more complex distributions. In the 63-150 mu m fraction, two groups are identified: 1) infaunal species having maxima between 1-6 cm, and 2) species with variable distributions in April, nearly absent in July, and displaying subsurface maxima in October. No change in species distribution patterns was observed between April and October in either size fraction. A comparison of the two size fractions of Chilostomella ovoidea and Buliminella tenuata shows similar depth profiles, which suggests that there are no ontogenetic changes of microhabitat preferences of these two taxa. Maxima of Nonionella stella and Buliminella tenuata in July and Bolivina spissa, Chilostomella ovoidea, Globobulimina pacifica, Fursenkoina bramlettei, and the agglutinated foraminifera in October suggest that rapid growth occurred over a three month period.