Increased production of faecal pellets by the benthic harpacticoid Paramphiascella fulvofasciata: importance of the food source

Abstract

The re-use of faecal pellets in the water column before sinking to the seaXoor is known as an important pathway in marine food webs. Especially planktonic cope- pods seems to be actively use their faecal pellets. Since benthic copepods (order Harpacticoida) live in the vicinity of their pellets, it remains unclear how important these pel- lets are for their feeding ecology. In the present study a lab- oratory experiment was conducted to analyse the importance of faecal pellets for the feeding ecology of the harpacticoid Paramphiascella fulvofasciata and its grazing pressure on two diatom species (Seminavis robusta, Navic- ula phyllepta). By quantifying the amount and volume of the produced faecel pellets in diVerent treatments, it was tested to what extent the food source and the lack of faecal pellets would inXuence the production of faecal pellets. We found that the grazing pressure of P. fulvofasciata depended on the diatom density since only a top-down eVect could be found on the smaller Navicula cells during its initial exponential growth phase. The grazer had a nega- tive eVect on the diatom growth and controlled the cell den- sity to about 4,000 cells/cm 2 . In spite of the fact that the addition of faecal pellets did not show a signiWcant positive eVect on the assimilation of diatoms, the removal of faecal pellets strongly promoted the pellet production. Especially when grazing on Navicula the harpacticoid P. fulvofasciata produced signiWcantly more and smaller faecal pellets when the pellets were removed. This outcome illustrates the need for faecal pellets of this harpacticoid copepod when grazing on the diatom Navicula. Apart from its selection for smaller diatom cells, it was suggested that the colonisation of heterotrophic bacteria enriched these pellets. This study is the Wrst to indicate that trophic upgrading occurs on fae- cal pellets and not only on the initial autotrophic food sources per se.