Fine-scale vertical profiles of meiofauna in muddy subtidal sediments

The fine-scale vertical profile (2-mm intervals to 2 cm) of meiofauna was surveyed at three water depths (20, 50, and 80 m) on the Louisiana continental shelf and at two depths (25 and 55 m) in a subarctic Alaskan bay. Meiofaunal densities at these sites measured through 4 cm were similar to those of other subtidal muddy-sediment sites, nematodes averaging about 1000 and 1500 and harpacticoid copepods averaging 125 and 30 per 10 cm2 in Louisiana and Alaska, respectively. Overall, harpacticoids were numerous at the sediment–water interface and exceptionally shallow in depth profile; densities decreased with increasing depth, with few individuals below 6 mm. Across all sites and water depths, weighted mean depths of harpacticoids averaged 5.4 mm. Nematodes were distributed to much greater depths, and generally increased in density with depth, with highest densities below 6 mm. In the Gulf of Mexico, nematode density peaked in the second centimetre. Across all sites and depths, nematode weighted mean depth averaged 10.5 mm within the upper 2 cm. Harpacticoids were the most abundant meiofaunal taxon in the upper 4 mm in the Gulf of Mexico at the 50- and 80-m sites. Overall, harpacticoids were disproportionately abundant at the sediment–water interface to a depth of 6 mm, and play a more significant role in events that take place at the surface than their down-core abundance would indicate. For example, harpacticoids are potentially more influenced by bottom-feeding fish and erosional events than are nematodes, and may exert an influence over the initial burial of sedimented phytodetritus.