Grazing by meroplanktonic polychaete larvae may help to control nanoplankton in the NW Mediterranean littoral: In situ experimental evidence

In this paper, we test the potential meroplankton grazing pressure on the nanoplanktonic community at the Bay of Blanes (NW Mediterranean). We used 2 abundant polychaete larvae (Prionospio malmgreni and Loimia medusa) cultured in situ in dialysis bags from which possible nanoplankton grazers other than larvae had been removed by filtration.Ingestion rates on chlorophyll a ranged from 0,001 to 0,04 and from 0,045 to 0,117 larva-1 d-1 for P. malmgreni and L. medusa, respectively. Ingestion rates on heterotrophic nanoflagellates (HNF) ranged from 2,4 to 32,3 x 103 and from 20,8 to 128,0 x 103 cells larva-1 d-1 for P. malmgreni and L. medusa, respectively. Scaling larval grazing to the natural plankton community results in: (1) a consumption on nanophytoplankton production ranging between 2,5 and 7,0%, corresponding to percentages of nanophytoplankton loss in ambient waters of 2,3 to 6,4%, respectively; (2) a consumption on HNF production ranging between 3,1 and 12,4%, corresponding to percentages of HNF loss in ambient waters of 2,9 to 11,7%, respectively. Although limited in time, the presence of planktotrophic larvae could cause an impact on nanophytoplankton populations which could be of similar relevance to that of holoplanktonic grazers, at least in shallow littoral waters of the Mediterranean Sea. Although potentially high, the estimated ingestion of nanophytoplankton by polychaete larvae was not sufficient to explain the major losses observed in the field, suggesting that these losses must be a result of consumption by other zooplanktonic grazers as well, and/or from processes other than herbivory. Contrary to nanophytoplankton and HNF, bacteria showed enhanced abundance in the presence of both polychaete larvae. Grazing by polychaete larvae caused a disruption of the grazing control exerted by HNF on bacteria. In conclusion, our results strongly indicate an important trophic role of meroplankton both as a direct control of the microbial holoplanktonic populations, and as a source of cascading effects in the microbial planktonic food webs.