Role of benthic filter feeders in pelagic-benthic coupling: Assimilation, biodeposition and particle flux

Abstract

Potter Cove is characterized by two clearly distinct areas, the outer cove with high benthic primary production and the inner cove with low primary but high secondary benthic production. The substrate of the outer cove is comprised of solid rocks and big boulders which support a dense canopy of phaeophyte macroalgae down to 25 m depth (Klöser et al., 1994). The inner cove is dominated by sessile macrofaunal assemblages on a muddy substrate (Sahade et al., 1998). A rich filter-feeding community occupies this area, in spite of what appears to be unfavorable conditions for their their feeding activities. These conditions include high sedimentation and low pelagic primary production. In Phlebobranchiate ascidians, high suspended inorganic particles reduce the amount of food assimilated by limiting the intake of utilizable food by a process of dilution and/or reducing the efficiency of assimilation of such food (Robbins, 1985). In summer, creeks formed by melting ice transport large amounts of inorganic material from the surrounding land into the cove, resulting in high sedimentation rates of about 18-30 g m-2 day-1 in the inner cove, (Ferreyra et al., 2003). In Potter Cove, the sediment load in the water column is frequently enhanced in the summer by resuspension of the fine bottom material, when the sea surface is ice-free and winds result in turbulent mixing that affects the bottom to 30 m depth (Schloss et al., 1999). The suspended sediment then affects the penetration of light into the water column which together with the deep vertical mixing explains the low phytoplankton biomass within inner Potter Cove (Schloss and Ferreyra, 2002). Water-column analyses from Potter Cove suggested that some organic material in the deposited sediments is released from the water-bottom interface even during winter (Kowalke, 1999; Tatián et al., 2002). Although scarce, this material seems to be the food source for benthic organisms, especially long-lived species that undergo growth and reproduction during winter (Sahade et al., 2004). The origin and composition of this material is, nevertheless, not completely understood. One of the most reasonable hypotheses is that part of this material may be macroalgal debris that originated in the outer cove, which is then continuously carried into the inner cove by clockwise water currents (Ferreyra et al., 2003). This material is sometimes 118easy to observe as little brown-greenish particles floating on the sea surface and a thick bed of brown material that is patchily present on the bottom. Filter feeding animals can play a significant control on phytoplankton biomass, coupling the pelagic and benthic systems. The uptake of phytoplankton and particulate organic matter from the water column, followed by biodeposition by filter feeding animals is known to be one of the major trophic pathways in marine ecosystems (Kowalke, 1999; Ahn, 1993). In phytoplankton-impoverished coastal waters, as Potter Cove, the role of benthic suspension feeders is important, since their fecal depositions can nourish the benthic fauna. The focus of this study was to measure assimilation efficiencies of natural and provided seston (powdered macroalgae) by a series of experiments performed with one bivalve and two ascidian species. The aim was to reveal insights into the possible response of filter feeders to a food source other than phytoplankton. The mass-specific deposition rate was measured, as well as the C:N composition of deposits (feces) should make it possible to determine the contribution of suspension feeders to the particle flux near muddy bottoms in Potter Cove.