Biotic Processes Controlling Vertical Export of Biogenic Matter from Marine Pelagic Systems

Abstract

Pelagic systems are potentially capable of retaining and recycling all biogenic material, although some losses due to sinking particles inevitably occur. However, in coastal areas and on shelfs a major proportion of the phytoplankton production is usually supplied to the sediments (e.g. Smetacek et al., 1984). Relating primary production in the surface layers quantitatively to vertical particle flux is difficult in the open ocean because only a small percentage of the total production (e.g. large phytoplankton cells, various kinds of aggregates and faecal pellets) is lost annually via sinking. A mill, grinding suspended and sinking particles in the mesopelagic zone of the ocean, is usually effective and regulates vertical flux. The physical environment determines nutrient availability (and consequently new production), influences the cell size of phytoplankton, the aggregation of sticky particles and hence the particles potentially available for sedimentation (Alldredge and Jackson, 1995). Thus there is a strong impact of bottom-up regulation of vertical flux, determined by abiotic control. However, there is also an obvious, but in marine ecosystems so far less considered, top-down regulation which is entirely determined by the biota. The strength and variability of top-down control is an important driving force for the fate of organic matter which is reflected in observed geographic and interannual variability. Differences in patterns and magnitude of export production in various pelagic ecosystems may be unexplained when exclusively regarded from an abiotic and bottom-up perspective. The relationship between phytoand zooplankton governs vertical flux seasonality, and zooplankters with different life cycles and feeding strategies further modify the principle patterns of export production (Wassmann et al., 1991). The relationship between various zooplankton communities and life pattems of major forms has not been adequately studied in order to understand entirely the regulation of export of organic matter from the upper layer (Verity and Smetacek, 1996). For example, herbivores with life-cycle strategies which involve overwintering of large biomass and predictable seasonal appearance (copepods, euphausiids) will have a different impact than opportunistic organisms with very low overwintering biomass (e.g. salps, pteropods). In some areas, predictable seasonal appearance by over-wintering zooplankton can be reduced, amplified or be stochastic due to inter-annual variations in advection. The role of omnivory and camivory as well as the coupling between the microbial loop, meso-zooplankton and suspended biomass on vertical flux is poorly known. Also, the role of different zooplankton functional groups to promote (e.g. grazing and production of large faecal pellets) or remove sinking matter (e.g. grazing on aggregates and processes such as coprophagy) influence the export and retention efficiency of various pelagic ecosystems. It is repeatedly assumed that large, ungrazed cells, aggregates and some faecal pellets dominate vertical export in the upper layers, in particular at the end of major phytoplankton blooms. This implies that the physical environment and phytoplankton primarily determine vertical export of biogenic matter. This opinion find support in the notion that so far a majority of vertical flux studies have been carried out in areas where meso-zooplankton is excluded from over-wintering or were other processes result in a mis match between phytoand zooplankton production. The assumption that abiotic and bottom-up regulation of vertical flux predominates gains also support by the fact that many of the biogeochemically oriented vertical flux investigations with long-term deployed multitraps were rarely accompanied by extensive planktonic research. This implies that the vertical export of carbon is estimated, but less is known about the composition of the sedimented matter, let alone the plankton dynamics above the sediment traps which control the vertical flux. To understand the impact of top-down regulation on vertical export of biogenic matter suggests that greater emphasis on research focused on planktonic food webs is needed in basically biogeochemically oriented approaches. The emphasis on biogeochemical focused, long-