Abstract
Field studies in the Baltic Sea indicate that the growth rate of the deposit-feeding ben- thic amphipod Monoporeia affinis (Lindstrom), which is found in densities of up to ≅25 000 ind. m -2 , is negatively density-dependent. We tested the density-dependence of growth in M. affinis under controlled laboratory conditions, using sediment of high organic content (loss on ignition 10.9%) and a wide density range of 2 amphipod year-classes (juveniles, 1150 to 38 000 ind. m -2 , 1 yr old sub- adults, 580 to 18 000 ind. m -2 ). Growth was strongly negatively density-dependent for both juveniles and subadults. We then used sediment of low organic content (loss on ignition 3.7%) to test whether the density-dependence of growth was due to a direct crowding effect at high density, or mainly due to food limitation. Using an intermediate amphipod density (2800 ind. m -2 ) expected to give clear growth depression, we added baker's yeast as supplementary food at 6 levels corresponding to an input of 0 (control) to 147 g C m -2 over an 8 wk period. The 3 highest yeast additions resulted in black, reduced sediment and elevated amphipod mortality in some replicates, but where the sediment did not become reduced, food additions enhanced growth significantly, demonstrating that growth depression at high density was primarily due to food limitation and not to crowding as such.
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