Abstract

The hydrodynamic effects on trapping efficiency of sediment trap cross-frame position, baffles and brine volume were evaluated in three short-term (<1 week) experiments in a temperate shallow marine environment (Evans Bay, Wellington Harbour, New Zealand). The effects of trap position and brine were further investigated during two open ocean, free-floating sediment trap deployments (1-2 days) near the Subtropical Front (STF), east of New Zealand. In the Evans Bay experiments (numbered I-III), cross-frames, each holding 12 cylindrical traps (inside diameter 9 cm, height 95 cm), were moored 3 meters above the seafloor in 15-18 m water depths at three randomly selected inner harbor sites. Triplicate subsamples from each cylinder were analyzed for total dry weight and mass fluxes calculated. The STF deployments utilized JGOFS MULTI-traps (inside diameter 7 cm, height 58 cm) attached to cross-frames moored at three depths (120, 300 and 550 m) on drifting arrays (Experiments IV and V). MULTI-trap samples were analyzed for total particulate mass, carbon and nitrogen. Results from Experiments I and V indicate that a spacing of about 3-trap diameters was sufficient to minimize inter-trap interactions and maintain trapping efficiency among traps suspended on a cross-frame at the same depth. Furthermore, baffles had no effect on trapping efficiency and an undetectable impact on zooplankton "swimmer" populations also collected in traps (Experiment II). In Experiment III, traps that were filled completely with high-density salt brine (50‰ excess NaCl) collected 2-3 times less material than traps with a basal brine height equivalent to 1- and 2.5-trap diameters. In contrast, high levels of inter-site variability confounded the STF MULTI-trap deployments during Experiment IV. However, variability in flux measurements from both Experiments III and IV increased 2 to 3-fold in brine-filled traps. Thus, the potential for brine-filled traps to undercollect material with higher levels of variability could possibly explain previously reported inaccuracies in the sediment trap method.

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