The effect of diets of phytoplankton and suspended bottom

Measurements of feeding rate and absorption efficiency were performed to assess the nutritional value of sedimentary organics combined with phytoplankton for raft mussels (Mytilus galloprovincialis Lmk) in Ria de Arosa (Galicia, N.W. Spain). Both clearance rate and absorption efficiency reached a maximum on mixed diets in which the proportions of phytoplankton and sediment particles were similar. Consequently, maximum absorption rates were reached on laboratory suspensions that closely resembled features of natural seston from the Ria. Values of absorption efficiency recorded with suspended sediments (around 13%) do not apply to the detrital component of ingestion in mixed diets, leading to the conclusion that a positive effect of phytoplankton on absorption efficiency of sedimentary organics is taking place. On the other hand, comparison of results for mixed and monoalgal diets also suggests that the occurrence of silt in the ingesta enhances absorption of microalgae, possibly through improving mechanical treatment within the stomach. It was concluded that mixing of bottom material with phytoplankton in the water filtered by raft mussels greatly increases growth potential, provided that no reduction of phytoplankton concentration below 40% of particulate volume takes place. Mussels with different locations in the Ria exhibit variable degrees of infection by the parasitic protozoa Marteilia refringens. Concomitant differences in physiological behaviour could be attributed to digestive impairment caused by parasitization.