Because deposit feeders dominate most sediments and their activities have profound effects on benthic communities, identifying the food resources that they assimilate and predicting their population dynamics are key issues in aquatic ecology. Though the diet of deposit feeders remains largely unknown, recent evidence suggests that juveniles of deposit-feeding species require more labile foods than those utilized by adults. My studies of ontogenetic niche shifts and their implications for population dynamics focused on tentaculate, deposit-feeding polychaetes and had four components.

First, a model of particle contact predicted that thinner feeding palps are biased toward contacting larger particles than thicker palps. Because palp and body size covary, the model predicts that juveniles will contact larger particles than adults. An experiment with glass beads and palp mimics of varying diameters (monofilament line) concurred. A feeding experiment with PSEUDOPOLYDORA KEMPI (Spionidae) also showed the trend. Assuming that particle size and food value correlate negatively, smaller juveniles will be biased toward relatively less-nutritious particles. This foraging constraint associated with palp size and a digestive constraint associated with gut size suggest that juveniles must forage differently than conspecific adults.

Delta (del) C-13 was used to trace ontogenetic changes in diet as juveniles grow. P. KEMPI collected from Skagit Bay and False Bay, Washington showed significant variation in del C-13 with body size, while individuals fed a known diet did not. HOBSONIA FLORIDA (Ampharetidae) from Skagit Bay and POLYDORA LIGNI (Spionidae) from False Bay showed similar trends. Comparisons of the wormsą del C-13 to that of potential foods suggested that detritus from macroalgae dominates adult diets, while benthic diatoms dominate juvenile diets.

The importance of benthic diatoms was tested by reducing their growth in situ. A method to perfuse porewaters with the herbicide DCMU was developed. Achieved DCMU concentrations were below levels known to be toxic to animals and significantly reduced the abundances of diatoms, juvenile H. FLORIDA, and a meiofaunal oligochaete that competes with juvenile H. FLORIDA.

To explore the recruitment implications of the ontogenetic changes in diet, I quantified the energy reserves of individuals of varying sizes using a fluorometric analysis of neutral and polar lipids. Size-specific neutral lipid concentrations increased between body lengths of 2.0 - 6.6 millimeters (mm), but did not vary among worms > 6.6 mm. Because size-dependent changes in particle selection and del C-13 also diminish at ~6 mm, the combined data imply that a food-related bottleneck exists for juveniles < 6 mm. Once juveniles attain this size, they are able to forage as adults and are no longer subjected to the food limitation that existed earlier in development.

The documentation of ontogenetic diet changes and evidence of stage-specific food limitation suggest that variations in the supply of key juvenile foods (e.g. benthic diatoms or fresh phytodetritus) can impose recruitment bottlenecks that might be central to the dynamics of populations that deposit feed as adults and communities that deposit feeders dominate.