ABSTRACT

Intellectual Merit: Understanding trophic connections and how resource variability affects consumers is necessary if we are to predict how food webs may shift in the face of environmental change. Macroalgae and phytoplankton support highly productive marine ecosystems. Research based on stable isotope analyses has supported the idea that macroalgal detritus, especially the giant kelp Macrocystis, is a major source of dietary carbon to benthic suspension-feeders. However, recent findings by the investigator's four-year stable isotope study suggest that phytoplankton, not kelp, are the main food resource for benthic suspension-feeders on reefs in the Santa Barbara Channel, and that variation in phytoplankton abundance, combined with feeding selectivity and the scale of consumer tissue turnover times, may drive variability in consumer isotope values. The results suggest that a key assumption made in 'snapshot' isotope studies of coastal ecosystems over the past 20 years, which the isotope signature of coastal phytoplankton can be represented by that of offshore phytoplankton, could be incorrect. This assumption has been made because of the difficulty in separating phytoplankton from detritus to obtain an uncontaminated isotope signature, also a problem in freshwater systems.

The investigator will address two main objectives in this research project: 1) determining the contribution of phytoplankton and giant kelp detritus to the pool of suspended reef POM and whether POM composition varies with distance from kelp forests, and 2) evaluating how different components of the POM are used as food by reef suspension feeders. Two complementary approaches are designed to explore the contribution of phytoplankton and kelp detritus to POM in coastal waters: an advanced flow cytometry and cell-sorting system to separate phytoplankton from bulk POM, and analysis of essential polyunsaturated fatty acids (PUFA) in POM and consumers. The investigators have obtained preliminary data that demonstrate the feasibility of both of these methods. Isotope values of isolated inshore phytoplankton and kelp, and compound-specific PUFA, will be used in mixing models to estimate relative contributions of these two major primary producers to suspension feeder diets. Two hypothesized mechanisms that may influence isotopic composition of consumers will also be tested: selective feeding on particular fractions of the POM, and tissue turnover times.

This project will provide new insights into the trophic support of benthic suspension feeders, an ecologically and economically important guild in coastal ecosystems. The results will test the general hypothesis that giant kelp detritus is an important source of dietary carbon to suspension feeders, a commonly accepted idea that needs re-evaluation in light of key assumptions that have been made in its support. Stable isotope analyses are an ideal tool for testing this hypothesis given the spatial and temporal scales of variability that exist in the abundance of phytoplankton and giant kelp at our study sites. The sampling scheme combined with longer-term data on producer biomass provided by the Santa Barbara Coastal LTER will enable the investigators to capture this variability, which is generally missed by studies based on 'snapshot' stable isotope analyses.

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