Meiobenthic communities of the Santa Maria Basin on the California continental shelf

The Santa Maria Basin encompasses the bulk of the continental shelf off the central coast of California. Meiofauna communities on this shelf are very abundant. The average density of metazoa plus protozoans to a sediment depth of 10 cm was 4040 per 10 cm2. Metazoan meiofauna were restricted to the surface sediments, where 84% were found in the top 4 cm. Over a 3-year period the average density within the top 4 cm of sediment was 1910 per 10 cm2 in water depths of 90–410 m. At a depth of 565 m the density was much lower, 218 per 10 cm2. The decrease in density correlated with a decrease in dissolved oxygen with depth. The percent of fine sediments and total organic carbon (TOC) content increased with water depth, therefore clay and carbon content were inversely correlated with density. There appears to be a seasonal cycle of increasing densities in the autumn and winter and decreasing densities in the summer. This cycle could be related to the seasonal cycle of storms and upwelling, which contribute energy to the benthos. The community structure of harpacticoid copepods was very complex. There were 115 species among 371 samples, and about 100 were undescribed species. Community structure was homogeneous between 90 and 410 m, but very different at 565 m. These changes correlated with changes in depth and sediment texture. Harpacticoid diversity also decreased with water depth. A suite of three factors; decreasing oxygen concentration and sediment grain size, and increasing TOC content; correlated with decreasing population density with water depth and with decreasing latitude across the basin.