EFFECT OF SURFACE DEPOSIT-FEEDER (MACOMA-BALTHICA L) DENSITY ON SEDIMENTARY CHLOROPHYLL-ALPHA CONCENTRATIONS

The effect of surface deposit-feeder (Macoma balthica L.) density on c hlorophyll a concentrations in shallow (2.5-3.5 cm deep) sediment micr ocosms was examined. The experimental design simulated subtidal, unill uminated sediments without benthic algal primary production, where sed imentary chlorophyll a is not necessarily within viable algal cells. C hanges in depth of the visual Redox Potential Discontinuity (RPD), flu orometrically-determined levels of chlorophyll a and phaeopigments and bivalve condition were monitored at weekly intervals for 42 days. Sam pling was conducted by multiple coring in replicate containers at 4 Ma coma densities (0, 370, 741 and 1111 individuals . m-2 ). Cores were s ectioned vertically before analysis. Bivalve density had no effect on the depth of the visual RPD. M. balthica had no persistent effect on s edimentary chlorophyll a levels at natural bivalve field densities. Ev idence that bivalves were either not using chlorophyll a-containing pa rticles as a primary food source, or the absorption efficiency of chlo rophyll a by the bivalves was low includes: (1) chlorophyll a concentr ations did not decline specifically within the surface feeding area, ( 2) phaeopigment concentrations were not elevated, and (3) bivalve cond ition was affected negatively in some treatments without a correspondi ng change in chlorophyll a concentrations. In benthic habitats lacking primary production, where chlorophyll a input depends upon sedimentat ion, abundant, surface deposit-feeding bivalves, such as M. balthica, may have little influence on sedimentary chlorophyll a concentrations.