Life history and population dynamics of the opportunistic polychaete <i>Capitella</i> sp. I in relation to sediment organic matter

Capitella sp. I is a deposit-feeding polychaete that occurs in estuarine and marine environments and that shows boom-bust population dynamics in response to organic pollution. A lifecycle experiment was carried out to quantify the relative importance of individual life-history traits for driving the dynamics of Capitella sp. I populations in response to changes in sediment organic matter concentrations. Individual-level effects were estimated as changes in survival, reproductive characteristics, and growth. Effects at the population-level were assessed by integrating the measured life-history traits in a simple 2-stage demographic model that was used to estimate the population growth rate (A-). Low organic content considerably impaired individual life-history traits and, as a consequence, A-. The results of the present study suggest that the minimum organic matter concentration for the maintenance, survival and growth of Capitella sp. I is 0.2 % TOM (total organic matter), and that sediments with < 0.5 % TOM would not support Capitella sp. I reproduction. The change in Ain response to sediment organic concentration was primarily attributable to effects on time to first reproduction, which explained, on average, 47% of the variance in A-. Treatment effects on juvenile survival and fecundity explained, on average, 26 and 24 %, respectively, of the observed treatment effect on A-, whereas adult survival and time between broods together contributed about 3 % to the effect on A-. The threshold between population extinction and population increase was sharp, which suggests that relatively small changes in organic loading can have dramatic consequences for the population dynamics in this species.