Local-scale nutrient regeneration facilitates seaweed growth on wave-exposed rocky shores in an upwelling system

This study shows that, even on exposed, wave-swept, rocky shores in a nutrient-replete upwelling ecosystem, mussels (Mytilus californianus) facilitate the growth of the seaweed Porphyra perforata by enhancing nutrient concentrations in the nearby water column. In field surveys on emergent substrate in the mid-intertidal zone, we found ten times greater abundance of P. perforata on mussels than on adjacent rock. In field experiments, P. perforata accumulated and grew more quickly on mussels than on bare rock or on mussel mimics, suggesting that nutrients excreted by mussels might be responsible for greater P. perforata cover. At high tide, water column ammonium concentrations over mussel beds were nearly double those found over bare rock. Correspondingly, tissue nitrogen concentrations were higher, and carbon-to-nitrogen ratios were lower in P. perforata growing on mussels compared to bare rock. Given the dominance of mussels in mid-intertidal regions of temperate coasts worldwide, ammonium regeneration could be a general contributor to local-scale nutrient availability, even in high-flow systems characterized by high nutrient concentrations. Variation in nutrient availability can affect the growth, composition, and biomass of primary producer communities (Tilman and Wedin 1991; Leibold 1999; Worm et al. 2002). However, the extent to which biologically important variation in nutrient availability emerges from inputs of new vs. regenerated nutrients is often uncertain. Regenerated nitrogen can compose a large proportion of nutrients in systems with low external inputs of nitrogen, but is generally thought to be less important in systems dominated by allochthonous nitrogen inputs (Miller 2004). It is possible that nutrient regeneration may be important in relatively high-nutrient systems open to flow; however, the prevalence of ecologically important autochthonous nitrogen inputs, especially in very high-flow marine systems exposed to strong waves or currents, remains unclear. In intertidal and shallow, subtidal marine systems, nutrient variation was historically ignored in favor of species interactions as explanations for patterns of species distribution and abundance (Menge 2000). However, recent studies have demonstrated that large-scale variation in allochthonous nutrients associated with coastal upwelling can alter species interactions and community structure in nearshore environments (Dayton et al. 1999; Blanchette et al. 2006). For example, Nielsen and Navarrete (2004) described how mesoscale (10 s of km) variation in nitrogen concentration associated with variability in the strength of upwelling along the Chilean coast affects the interaction between molluscan herbivores and macroalgae, altering algal community structure. Similarly, increases in anthropogenic nutrient input can favor fast-growing macroalgae in estuaries (Hauxwell 2001) and on rocky shores (Worm and Lotze 2006), altering the structure and composition of algal assemblages, which can have indirect effects on the structure of the entire community. Whereas upwelling or anthropogenic inputs are important sources of nutrients over relatively large scales, smallscale nutrient inputs, such as nitrogen regeneration by invertebrates, also influence community structure and Acknowledgments We thank S. Williams for lab space and advice; we thank A. Carranza for analytical expertise; we thank K. Edwards, K. Sellheim, and C. Searcy for assistance with data collection. We thank A. Chaudoin and C. Sorte for help preparing the field experiments, and we thank J. D. Ackerman and three anonymous reviewers for helpful comments on the manuscript. The National Science Foundation provided funding for this work (a graduate research fellowship to K.M.A., grant OCE-0351778 to J.J.S., and grant OCE-0549944 to S. Williams and M.E.S.B.). This is contribution 2425 from the Bodega Marine Laboratory and contribution 262 from the Marine Science Center. Limnol. Oceanogr., 54(1), 2009, 309–317 E 2009, by the American Society of Limnology and Oceanography, Inc.