DRILLING FREQUENCY OF BIVALVES INCREASES OVER A GRADIENT OF EUTROPHICATION ON A TEMPERATE CONTINENTAL SHELF

Drilling predation in macrobenthic skeletons, especially mollusks, has been well studied for insights into ancient biotic interactions, including questions of escalation and coevolution. However, few actualistic studies have tested for the effects of specific abiotic factors on drilling frequency (DF) that, if not accounted for, might confound the recognition of important temporal trends. Here, we test for variation in DF as a function of nutrient supply, using an historic gradient of wastewater input to the temperate siliciclastic continental shelf of southern California. The effects of productivity on predation, up to a critically high nutrient level where hypoxia occurs, are unknown, but two alternative hypotheses are possible: (1) no change in DF, because populations of trophic groups increase in proportion to each other, thus maintaining a balanced relationship; or 2) DF increases because higher nutrients support larger prey populations, attracting and supporting more predators. We collected samples at five sites from muddy sands to sandy muds in 50-75 m-water depth: three are within the plume of the Whites Point wastewater outfall (Palos Verdes shelf) and two are at distal sites (Orange County). DF is the percentage of >1mm bivalves sieved from the top ~16 cm of the seabed with naticid gastropod boreholes (corrected for 2 valves/bivalve). DF of bivalve assemblages varies significantly (G test; p<0.01) among sites from 7% to 28%, and increases with nutrient input when grain size is controlled (H2). Among-site variation in DF was also high for trophic subsets (19-60% for chemosymbionts, 8-83% for infaunal suspension feeders, 0-16% for deposit feeders), with each group exhibiting its peak DF at a different site. High DFs were not correlated with sites where the trophic group was most abundant, indicating that high predation is not simply a function of the most common prey. These initial results indicate remarkably large among-site differences in DF within a single, middle-shelf environment as a function of relatively subtle differences in grain size and a comparatively short-lived (~50 year) localized increase in nutrients, given millennial-scale time-averaging. Temporal changes in DF in the fossil record should thus be examined carefully to control for environmental patchiness.