Pheophorbide a in Hydrobia ulvae faecal pellets as a measure of microphytobenthos ingestion: variation over season and period of day

The microphytobenthos (MPB) - Hydrobia ulvae trophic interaction is one of the main channels of material transfer to higher trophic levels in intertidal mudflats. A new non-invasive approach to evaluate the grazing activity of H. ulvae on microphytobenthos is proposed. The effects of season and period (combination of tide and day/night) on ingestion rates of H. ulvae (using (14)C-labeled MPB) and egested pheopigments a (using HPLC pigment analysis) were also investigated. H. ulvae ingestion rate was found to vary significantly over season and period, being higher in summer and during diurnal low tide periods. This is possibly related to higher growth rates of H. ulvae in summer, as well as to an increase in surface MPB biomass during diurnal low tides. A highly significant relationship was found between ingested chl a and egested pheophorbide a, allowing the estimation of ingestion rate from the amount of egested pheophorbide a on H. ulvae faecal pellets. This new non-invasive methodology may allow the improvement of long-term studies of consumption rates and the evaluation of grazing of H. ulvae on MPB.