Importance of kelp detritus as a component of suspended particulate organic matter in giant kelp Macrocystis pyrifera forests

To determine the potential of giant kelp Macrocystis pyrifera detritus as a food resource for coastal suspension feeders, we quantified the production and size distribution of small particulate (< 1 cm) kelp detritus in relation to water motion, state of kelp blade erosion, and percent cover of the bryozoan Membranipora serrilamella on blades. We enclosed kelp blades in the laboratory and in the field to collect detritus particles, which we then fractionated by size. Carbon and nitrogen content were quantified for each size fraction. Suspension feeders typically consume particles up to 250 mu m in size, and most kelp detritus particles that we collected were within this size range. Blade degradation state and blade mass were found to be weak but significant predictors of detrital generation. We used the data from our laboratory and field experiments combined with 5 yr of monthly biomass measurements of giant kelp and suspended particulate organic matter (POM) to estimate the contribution of kelp detritus to suspended particulate organic carbon (POC) and nitrogen (PON) in the kelp forest at Mohawk Reef off Santa Barbara. Results showed that detrital kelp carbon contributed < 0.2% of total suspended reef POC and PON. Therefore, despite the high production rate of kelp and the importance of exported kelp detritus to intertidal and deep-water ecosystems, our results suggest that small kelp detrital par ticles do not represent a significant food source to kelp forest suspension feeders.