Capitella sp. I and Streblospio benedicti are infaunal, deposit-feeding polychaetes that occur in estuaries and littoral wetlands throughout much of the United States. Life table response experiments (sensu Caswell 1989a) were carried out in the laboratory to compare the demographic responses of these species to three common sources of estuarine contamination or enrichment: sewage (Milorganite), blue-green algae (Spirulina sp,), and hydrocarbons (No. 2 fuel oil). Life table data were used to generate two population projection models (a fully age-classified model and a simple two-stage model) for each species in each treatment and in a salt marsh sediment control. These models were used to quantify the effects of treatments on survival, reproduction, and age at maturity, and hence on population growth rate. For both species, survival was high in all treatments except the blue-green algae treatment, where oxygen depletion (to <1 mL/L) occurred. Treatments had dramatic effects on age at maturity, fertility, and generation time, which differed between species and among contaminants. Population growth rates (lambda) were higher in Capitella sp. I than in S. benedicti for all treatments, primarily due to earlier maturation and a fertility advantage exhibited by Capitella during the first few weeks of reproduction, In Capitella sp. I, explosive increases in lambda were seen in the sewage (lambda = 5.31) and algae (lambda = 2.81) enrichments relative to the control (lambda = 1.86) and the hydrocarbon treatments (lambda = 1.67), Reduced maturation time and increases in age-specific fertility associated with rapid growth and large body size were responsible, Hydrocarbons reduced lambda primarily through delayed maturation and reduced age-specific fertility. population growth rates of S. benedicti in the hydrocarbon treatment (lambda = 1.11) and algae treatment (lambda = 1.09) were reduced relative to the control (lambda = 1.46) and sewage treatments (lambda = 1.41), The hydrocarbon reduction resulted from delayed maturity and reduced fertility, whereas the algal effects were caused by reductions in both juvenile survival and fertility. Our analyses revealed that Capitella sp. I's population growth rate was less sensitive than that of S. benedicti to these three common forms of estuarine contamination, that different sources of organic enrichment (sewage and blue-green algae) introduced at the same C and N levels could have varying demographic effects, and that when two contaminants (hydrocarbons and blue-green algae) caused similar reductions in population growth rate in a species (Streblospio), the underlying mechanisms may have differed. For both species all demographically important effects of contaminants occurred early in life, suggesting a need to focus on juveniles and young adults in held and laboratory testing. The experiments performed here demonstrated the sensitivity of polychaete demographic properties to the condition of estuarine sediments, This sensitivity may be exploited to evaluate organic enrichment and hydrocarbon contamination in field settings.
