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

Abstract

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 pg larva-' d" for P. malmgreniand 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-' d-' for P. m a h g r e n i 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 ?.(l%, 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%, correspond~ng 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.

Trophic Level Stability-Inducing Effects of Predaceous Early Juvenile Fish in an Estuarine Mesocosm Study

“…This grazing dynamic was consistent with predator-prey cascades [25], [54]. Trophic interactions of ‘larger’ plankton (copepods, micro-flagellates, and ciliates) on smaller phytoplankton was less clear however, and confounded in part by early copepod life stages (copepodites and nauplii) and the presence of polychaetes, which share overlapping prey size distributions with copepods [55], [56], [57]. When copepods increased in abundance, ciliates and micro-flagellate numbers decreased, presumably a result of increased copepod grazing.…”

“…In turn, they decreased nano-flagellate and bacterial abundances. When Prionospio polychaetes were included, as in T2, they appeared to dominate predation on micro-flagellates and potentially grazed on phytoplankton [57]. Very little change was seen in ciliate abundances with polychaetes present, suggesting that polychaetes did not feed on ciliates.…”

“…When copepods increased in abundance, ciliates and micro-flagellate numbers decreased, presumably a result of increased copepod grazing. While the micro-flagellates, ciliates, polychaetes and early life-history stage copepods could be capable of consuming the nano-flagellates [20], [56], [57], the latter would be largely unavailable for direct consumption by the adult calanoids as they are likely smaller than the prey size range of these copepods [55].…”

Zooplankton response to storm runoff in a tropical estuary: bottom-up and top-down controls

“…Thus, at the population densities encountered in this study, appendicularians had the potential to clear an average of 83 ± 32% (range 30 to 260%) of the water column daily. Copepods, bivalve larvae, polychaete larvae and Penilia avirostris also are all capable of consuming micro-and nanoplankton prey, albeit at much lower rates (Bayne 1983, Turner et al 1988, Martin et al 1996, Calbet et al 2000. However, the grazing impacts of different zooplankton groups may have been modulated, in part, by changes in phytoplankton community composition.…”

Larval growth in the dominant polychaete Polydora ciliata is food-limited in a eutrophic Danish estuary (Isefjord)

“…The trophic impact of other meroplanktonic larvae has been reported in Isefjord previously by Jørgensen (1981), where, based on ECD, a cohort of bivalve larvae (Mytilus edulis) daily cleared 40 to 50% of the surrounding water mass for small particles (probably flagellates). Although more information about the trophic impact of meroplankton on microbial assemblages is required, meroplanktonic larvae potentially play an important trophic role in terms of both their direct control of the microbial community and as a source of cascading effects in the microbial plankton food web (Martin et al 1996). Hence we propose that meroplanktonic larvae need more attention in terms of future research and as key components structuring the planktonic microbial assemblage in boreal coastal waters.…”