This review provides an assessment of sediment trap accuracy issues by gathering data to address trap hydrodynamics, the problem of zooplankton “swimmers,” and the solubilization of material after collection. For each topic, the problem is identified, its magnitude and causes reviewed using selected examples, and an update on methods to correct for the potential bias or minimize the problem using new technologies is presented. To minimize hydrodynamic biases due to flow over the trap mouth, the use of neutrally buoyant sediment traps is encouraged. The influence of swimmers is best minimized using traps that limit zooplankton access to the sample collection chamber. New data on the impact of different swimmer removal protocols at the US time-series sites HOT and BATS are compared and shown to be important. Recent data on solubilization are compiled and assessed suggesting selective losses from sinking particles to the trap supernatant after collection, which may alter both fluxes and ratios of elements in long term and typically deeper trap deployments. Different methods are needed to assess shallow and short- term trap solubilization effects, but thus far new incubation experiments suggest these impacts to be small for most elements. A discussion of trap calibration methods reviews independent assessments of flux, including elemental budgets, particle abundance and flux modeling, and emphasizes the utility of U-Th radionuclide calibration methods.

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An assessment of the use of sediment traps for estimating upper ocean particle fluxes

In light of evidence suggesting that both phytoplankton and bacteria in the Mediterranean Sea are limited by the availability of phosphorus rather than of nitrogen, and that most of the P in the photic zone during summer stratification exists as dissolved organic compounds (DOP), we address the question of how these observations may interact with the 'biological pump' transporting carbon to deep waters. From theoretical considerations, the C storage via sinking particles should function better in a P- than in an N-limited system. It is argued, however, that the microbial food web during summer stratification has a net accumulation of dissolved organic carbon (DOG) and DOP. The limited data available suggest a high DOC:DOP value which would make downwards transport of DOC with winter deep water formation a potentially effective mechanism in C sequestration from the atmosphere. Part of the DOC accumulating in the photic zone appears to be readily biodegradable. This is in conflict with a simple model of phytoplankton-bacterial competition for phosphate since phytoplankton, as an inferior competitor, would be expected to be reduced in biomass until autochthonous production of organic C falls to a level where bacteria become C-limited. The conflict is resolved by including microzooplankton grazing as a controlling factor of bacterial biomass.

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Nutrient limitations, microbial food webs, and 'biological C-pumps' : suggested interactions in a P-limited Mediterranean

Downward fluxes and the elemental composition of settling particles at 80, 200 and 1000m depth measured during an 18-month sediment trap experiment are presented, together with associated hydrographic and biological data, in order to assess the relative importance of these factors in controlling particle flux. Mass, carbon and nitrogen fluxes were generally higher during the first half of the year. Mass flux maxima occurred at different times of the year depending on trap depth, whereas the peak carbon flux was measured in late June. Most of the total carbon flux was composed of organic carbon. On an annual basis the total particle flux was 32.9, 40.6 and 8.1 g m −2 year −1 at 80, 200 and 1000 m depth, respectively. The organic carbon flux decreased from 4.8 and 5.0 g m −2 year −1 at 80 and 200 m, respectively to 1.2 g m −2 year −1 at 1000 m, and the corresponding nitrogen flux from 0.7 and 0.8 to 0.2 g m −2 year −1 . Organic carbon and nitrogen content of sinking particles decreased with depth, whereas the carbonate carbon content remained constant. The contribution of zooplankton feces to the carbon flux was highly variable throughout the year, ranging from 1.6 to 62%. Swimmers, including fish, that entered the traps in the upper 200 m were abundant and, on average, were twice the weight of the sinking particle flux. Pelagic production was the main source of particles during the late spring and mid-summer sedimentation pulses; however physical forcing, particularly the winter mixing of the water column, was partially responsible for the high sedimentation during the earlier part of the year. Wind gusts and concomitant vertical mixing appeared responsible for the carbon rich particle flux in summer. Separation of data for the mixed and stratified water column periods clearly indicated that most of the downward flux occured in mixed waterswhereas the organic carbon content was higher in particles sedimenting through stratified waters. The export of organic carbon from the euphotic zone (upper 200 m) represented only 5–7% of the estimated phytoplankton production in this region. Furthermore time-series measurements of POC flux averaged on an annual basis were only 14–42% of indirect estimates of carbon export or “new production” for this region. This discrepancy suggests that a good portion of the total downward carbon flux occurs through advection and/or diffusion of DOC.

Dynamics of the downward flux of particles and carbon in the open northwestern Mediterranean Sea

Primary productivity in the oligotrophic Cretan Sea (NE Mediterranean): seasonal and interannual variability

Primary productivity and chlorophyll in the equatorial Pacific are lower than expected based on ambient nutrient concentrations. We tested the hypothesis that these conditions are due to a balance between phytoplankton growth and mesozooplankton grazing. Grazing rates and biomasses of three size classes of zooplankton in the size range of 200-200 μm were measured during March-April and October of 1992 at the equator (140°W). El Nino conditions prevailed in March–April, whereas in October a tropical instability wave (TIW) passed through the study area. Weight-specific pigment ingestion rates of mesozooplankton tended to be higher in March–April than in October while the opposite was the case for biomass. There were no discernible temporal trends in mesozooplankton community grazing rates in March–April, but there were changes in October associated with the passage of the TIW. Mesozooplankton grazing removed 1–9% day−1 of the total chlorophyll and 1–12% of the primary production within the euphotic zone. Therefore, the grazing hypothesis as stated above is rejected, although a simple chlorophyll budget suggests that grazing (mostly by microzooplankton) and phytoplankton growth are roughly in balance. Most of the phytoplankton was 2-μm phytoplankton, we estimate removal rates of the > 2-μm chlorophyll standing stock of up to 27% day−1.

Another question of biogeochemical importance is the rate of the production of large diatoms. Mesozooplankton grazing could have balanced growth of the large (> 20-μm) diatoms in March–April, but not in October. From estimates of respiration and excretion based on water temperature and body size, we infer that: (1) > 80% of the carbon ingested by mesozooplankton is not phytoplankton; (2) mesozooplankton excretion supports < 7% of the nitrogen demands of phytoplankton; (3) the flux of carbon passing through the mesozooplankton would be equivalent to 23% of the primary production; (4) mesozooplankton fecal carbon could account for 100% of the sinking POC flux in this region. Finally, a simple food chain analysis suggests that a significant fraction of the microzooplankton production probably passes through mesozooplankton.

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Papers

Seasonal particulate carbon flux in the coastal northwestern mediterranean-sea, and the role of zooplankton fecal matter