What drives variation in benthic abundance along an environmental gradient—physical, recruitment, top-down, or bottom-up factors?

What drives variation in benthic abundance along an environmental gradient—physical, recruitment, top-down, or bottom-up factors?

Ecologists seek to elucidate patterns in the distribution of species and determine causal mechanisms that underlie them. Important factors structuring shallow-water communities include physical stress, recruitment, predation, competition, and nutrient availability. We tested various hypotheses concerning the effects of these forces upon benthic abundance in a series of field experiments with the Baltic clam (Macoma balthica). We examined densities in both mud and sand habitats spanning a broad spatial scale (50 km in the York River, Chesapeake Bay) along an environmental gradient. Clam density was moderate in mud at all locations, suggesting that mud was beneficial for clams regardless of salinity. Clam density in sand was variable but highest in the low salinity zone. Regression analyses confirmed that neither salinity nor sediment type explained substantial variation in clam density. A predator-exclusion experiment confirmed that predation did not explain variation in clam density. Hydrodynamic models suggested that recruitment did not explain variation in clam density. Measurements of food availability indicated, however, that clam distribution is controlled predominantly by bottom-up factors.

Andrew O. Shelton,¹* Jon Witman,¹ Kyle Hebert,² and Douglas A. Woodby.^{2 1}Department of Ecology and Evolutionary Biology, Brown University, Providence, RI, USA; ²Alaska Department of Fish and Game, Commercial Fisheries Division, Petersburg and Juneau, AK, USA.

The red sea urchin (Strongylocentrotus franciscanus) is a dominant grazer in Pacific nearshore subtidal communities and is the object of an active fishery. Accurate estimates of variation in growth and recruitment within fisheries management areas are vital for establishing sustainable harvest rates and patterns. The goals of this study were to 1) characterize urchin growth in SE Alaska 2) test the validity of an aging technique and 3) relate growth to spatial differences in food, conspecific density and size. As a means of aging we counted growth rings on Aristotle's Lantern at three sites. The Tanaka growth curve (Tanaka 1982, Ebert 1999) was fit for all sites using non-linear regression. Urchins with multiple years of Passively Integrated Transponder(PIT) recapture data and a ring count were overlaid on growth curves. Residual analysis indicated that growth of PIT individuals approximate ring counts well for all but the smallest urchins, where ring-predicted growth slightly overestimated observed(PIT) growth. Rings counts approximated yearly growth well. The Tanaka curve was fit for five additional sites along a fjord (scale: 10s of kilometers). Highest and lowest growth corresponded to sites with extremes of food availability, indicating that the ring technique is sensitive to environmental changes. Overall urchin density declined with distance from the fjord mouth while growth did not vary significantly with distance. The proportion of urchins represented by recruits (defined as urchins < 30 mm in diameter, 2-3 years old) varied significantly over time but differences were especially striking between areas. Proportion of recruits declined from outer to inner fjord sites. Recruitment appears to be a large scale regulator of urchin density while growth varies locally as a result of food availability.


Differential dissolution and sorption of ²³⁴Th, ²¹⁰Pb, and ⁷Be in deposit-feeder digestive fluids

David H. Shull¹* and Lawrence M. Mayer.^{2 1}Department of Biology, Gordon College, Wenham, MA 01984, USA; ²Darling Marine Center, University of Maine, Walpole, ME 04573, USA.