Contribution of zooplankton associated with detritus to sediment trap 'swimmer' carbon in Monterey Bay, California, USA

A major food source to the deep sea is thought to be organic detritus derived ultimately from surface production. Much of the detritus is lost as it settles, presumably both to microbial and to metazoan consumers. In the last several decades much research has been devoted to quantifying this vertical particle flux using sediment traps. In processing samples from sediment traps 'swimmers', i.e. zooplankton that are thought to actively enter sediment traps and artificially augment the trap contents, are traditionally removed. However, some zooplankton caught in the traps are likely to be genuine associates and decomposers of detritus and thus similar to microorganisms on detritus which are, mostly for practical reasons, included in the 'particle flux'. We determine what proportion of the swimmer assemblage is detrital associates, and how these detrital associates may affect mesopelagic particle flux measurements, by comparing zooplankton taxa removed from sediment trap samples obtained at 450 m depth in Monterey Bay, California, with taxa observed and collected on large detrital particles (giant larvacean houses) in situ with a submersible ROV (remotely operated vehicle). Trap swimmer composition in Monterey Bay was diverse. Calanoid copepods and adult Hyperia mesudarium amphipods contributed the most to total swimmer carbon; and juvenile hyperiid amphipods, calanoid copepods, and Oncaea sp. copepods were the most: numerous of all groups. Polychaete larvae were also a conspicuous part of the assemblage. The entire swimmer component averaged 24 % of total trap POC (= detrital + swimmer C) in traps, but only 1.5% of total trap POC was due to those we consider detrital associates (e.g, copepods: Oncaea spp., Microsetella spp., and Scopalatum sp.; polychaetes; juvenile hyperiid amphipods). Thus zooplankton on detritus are a relatively small percentage of the POC flux in these traps, and their removal introduces Little ambiguity to trap flux measurements in this area. Important impediments to understanding the contribution of such zooplankton are our limited knowledge of the zooplankton decomposer community at depth, their different contributions in low versus high flux environments, and the behavior of zooplankton on encountering a trap.