DAHLHOFF, E.P.; FREETO, J.C.; KEIL, A.P.; MELLO, J.D.; MENGE, B.A.: Biochemical indices of bottom-up effects in Pacific rocky intertidal habitats

Previous studies of nearshore benthic communities have shown that food availability influences the metabolic activity of suspension feeding invertebrates. These studies have shown that RNA:DNA ratios respond rapidly to increases in nearshore concentrations of phytoplankton at some sites. Since RNA:DNA ratios are an indirect measure of growth and dietary status, they may be useful for examining the connection between the intertidal food web and nearshore processes. In this study, we measured RNA:DNA ratios for Mytilus californianus collected monthly at 5-7 sites within three regions defined by the Partnership for Interdisciplinary Study of Coastal Oceans (PISCO): Oregon, Central California, and Southern California. Oregon mussels had higher RNA:DNA ratios than California mussels, especially during the summer (3-way ANOVA: site*time, p<0.001). These results are consistent with other PISCO data, which suggest food availability, recruitment, and growth are higher in Oregon than in California. We also observed large variations in RNA:DNA ratio between sites within each region. For Southern California, RNA:DNA ratios were consistently higher for mussels at sites north and south of Pt. Conception. In Central California, RNA:DNA ratios were highest for mussels at northern, open coast sites. In Oregon, RNA:DNA ratios were highest for mussels at high upwelling sites. The relationship between RNA:DNA ratio, food availability and coastal upwelling was strong in some locales and weak in others. Thus, the impact of "bottom-up" effects on rocky intertidal community structure may depend on the strength of linkages between metabolic activity of key suspension feeding invertebrates and nearshore food availability.