High turnover rates of copepod fecal pellets due to Noctiluca scintillans grazing

Copepod fecal pellet production and vertical flux, as well as vertical distributions of copepods, fecal pellets and the heterotrophic dinoflagellate Noctiluca scintillans were monitored in an upwelling plume off the coast of Brazil during 5 d in austral spring. Less than half (20 to 45%) of the pellets produced in the overlying water column reached sediment traps positioned at 30 to 60 m depth, and specific sinking losses computed from steady-state considerations varied inversely with trap depth between 0.3 and 2.9 d–1. Total specific losses varied between 0.6 and 16 d–1, and the major part of these losses were thus unaccounted for by sinking and must have been due to remineralization in the water column; estimated specific remineralization rates increased with the ageing of the plume and varied between 0.3 and 13 d–1. N. scintillans occurred in increasing concentrations in the upwelling plume as the latter aged, up to 5 × 105 cells m–2, and fecal pellets occurred commonly in the food vacuoles of N. scintillans. Specific fecal pellet remineralization rates were linearly related to the abundance of N. scintillans. This relation can be quantitatively accounted for if N. scintillans clears the water for fecal pellets at about 0.6 l cell–1 d–1. A simple encounter model suggests that such high clearance rates are feasible. Since N. scintillans occurs at typical abundances of about 106 cells m–2 in temperate seas during spring, summer and autumn, it may contribute significantly to the recycling of rapidly sinking fecal pellets in the water column.