Natural Reserve System University of California 2008 MATHIAS SYMPOSIUM

17 Effects of fine sediment and grazers on periphyton and nutrient cycling in a coastal river Michael Limm, Department of Integrative Biology University of California, Berkeley Excessive loading of fine sediment is one of the most serious impacts of land-use on stream ecosystems in arid and semi-arid environments. Detrimental direct effects of excessive fine sediment are well documented for fish, invertebrates, and buried periphyton. Periphyton growing on clear surfaces may also be indirectly affected by deposited fine sediment, but these indirect effects remain poorly known. Two hypothesized mechanisms for indirect effects of fine deposited sediment on periphyton include hydraulic changes resulting from a smoothed riverbed, and changes in ambient grazer biota resulting from embedded substrates. We manipulated fine sediment in experimental channels and quantified periphyton accrual, metabolism, and nutrient uptake under both natural and reduced grazer conditions. Grazers were reduced by electric fence chargers surrounding experimental tiles in each sediment treatment, allowing us to contrast the indirect effect of altered food web dynamics from the direct effect of altered hydraulic conditions. Results after thirty and sixty days suggest periphyton was not affected by hydraulic contrasts imposed by our sediment manipulation. While periphyton accrual, metabolism, and nutrient uptake did not differ among sediment manipulations, ambient grazing stimulated greater nitrogen uptake and greater metabolism per unit mass. Our results suggest under low flow summer conditions grazer effects outweigh any indirect hydraulic effects of a smoother bed due to deposited fine sediment, and food web composition and/or dynamics can alter nutrient cycling in a coastal river. SUNDAY MORNING SESSION Protection from grazers: an associational defense between kelp and an epiphytic bryozoan? Sarah Bryson, Department of Ecology and Evolutionary Biology University of California, Los Angeles Due to close proximity, hosts and epiphytes potentially exert strong ecological influences on each other. In the case of kelp, associated epiphytes may negatively impact the host due to shading and blocking nutrient uptake. However, an epiphyte can reduce the susceptibility of its host to herbivory. This research examines whether an epiphytic bryozoan, Membranipora membranacea, provides an associational refuge from herbivory for its host giant kelp. I hypothesized that the presence of Membranipora reduces the grazing damage on kelp inflicted by small grazers. Individual invertebrate grazer species were tested in the laboratory to determine their feeding preferences for kelp with or without Membranipora. Almost all grazers (Norris’ topsnail, top snails, Lacuna snails, amphipods, and kelp isopods) were found to have a strong preference for kelp without the bryozoan. However, kelp crabs showed a preference for kelp encrusted with Membranipora. Field surveys indicate a slight reduction of grazing on kelp blades heavily encrusted with Membranipora in areas dominated by small grazers in the canopy. These results suggest that while some protection from herbivores may be gained when Membranipora is present, it is unlikely to outweigh the overall costs of hosting the epiphyte.