The origin of sedimentinrr. detrital matter in a coastal system

Total sedimentation and the fraction due to copepod fecal pellets were measured during the growth season (March-October 1989) in the southern Kattegat, Denmark. In this period the sedimentation of detritus made up 52 g C rn-‘, equal to 82% of the sedimenting matter from the euphotic zone, but fecal pellets (11 g C mpZ) constituted only a minor fraction. The remaining detrital matter was produced by other heterotrophs than copepods. Published data on heterotrophic biomass and grazing obtained during the investigation in the Kattegat are reviewed in order to relate the sedimentation to processes in the pelagic system. Copepod defecation nearly equaled the sedimentation of fecal pellets, indicating that retention of this matter in the pelagic system was insignificant. A considerable fraction (lo-24%) of the carbon flow processed by heterotrophic pica-, nano-, and microplankton was converted to detritus that was lost from the mixed system by sedimentation. The microbial food web is thus not an exclusively regenerating system. Sedimentation of autochthonous matter from the pelagic system is quantitatively related to the input of nutrients that are available for primary production (Eppley and Peterson 1979). The connection is based on a mass balance consideration whereby new production in the euphotic pelagic system is matched by an equivalent output of sedimenting organic matter. Estimates of nutrient fluxes in oceans and in coastal waters accord with this mass balance (Eppley and Peterson 1979; Olesen and Lundsgaard 1995). However, the functional relationship between sedimentation and the pelagic structure and productivity is only partly understood. One major problem is the identification of material collected in sediment traps and the sources of this matter. The identification has been based mainly on microscopy or on analysis for chemical markers of characteristic components. This has provided information on the sedimentation especially of phytoplankton and fecal pellets that can be quantified by pigment analysis and by microscopy. Other recognizable components also contribute to the flux (e.g. molts from zooplankton, eggs, cysts, and dead organisms), but a large detrital fraction usually remains unidentified. The seasonal sedimentation of phytoplankton and total organic matter from the euphotic zone was measured in the southern Kattegat, Denmark (Olesen and Lundsgaard 1995). In the present paper the fecal component of edimentation is estimated from both pigment data and by direct microscopy of material collected during this investigation. The sedimentation of fecal matter and other detritus is related to previously published independent estimates of the activity of the dominant heterotrophic components. The sampling station (56”17’30”N, 1 l”59’54”E) was visited 52 d during 1989. Water was sampled from the mixed zone (extending to a depth of lo--22 m) and sediment traps were deployed in the pycnocline as described in Olesen and Lundsgaard (1995). The fecal pellets in the sediment trap material were quantified by microscopy on samples fixed in 1% glutaraldehyde. The peritrophic membrane was destroyed on most of the pellets and the content was swelling and disintegrating, but both intact and partly disintegrated pellets were counted. The average volumes of intact pellets were estimated using a semiautomatic image analysis system (MOP-Videoplan Kontron Bildanalyse) assuming a cylindrical shape with spherical ends. The number of pellets (intact and partly disintegrated) was multiplied by the average volume of intact