Primary, new and export production in the NW Pacific subarctic gyre during the vertigo K2 experiments

Author Posting. © Elsevier B.V., 2008. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Deep Sea Research Part II: Topical Studies in Oceanography 55 (2008): 1594-1604, doi:10.1016/j.dsr2.2008.04.013.This paper presents results on tracer experiments using 13C and 15N to estimate uptake rates of dissolved inorganic carbon (DIC) and nitrogen (DIN). Experiments were carried out at station K2 (47°N, 161°E) in the NW Pacific subarctic gyre during July-August 2005. Our goal was to investigate relationships between new and export production. New production was inferred from the tracer experiments using the f ratio concept (0-50m); while export production was assessed with neutrally buoyant sediment traps (NBSTs) and the e ratio concept (at 150m). During trap deployments, K2 was characterized both by changes in primary production (523 to 404 mg C m-2 d-1), new production (119 to 67 mg C m-2 d-1), export production (68 to 24 mg C m-2 d-1) and phytoplankton composition (high to low proportion of diatoms). The data indicate that 17 to 23% of primary production is exportable to deeper layers (f ratio) but only 6 to 13% collected as a sinking particle flux at 150m (e ratio). Accordingly, > 80% of the carbon fixed by phytoplankton would be mineralized in the upper 50m (1 – f), while < 11% would be within 50-150m (f – e). DIN uptake flux amounted to 0.5 mM m-2 h-1, which was equivalent to about 95% particulate nitrogen (PN) remineralized and/or grazed within the upper 150 m. Most of the shallow PN remineralization occurred just above the depth of the deep chlorophyll maximum (DCM), where a net ammonium production was measured. Below the DCM, while nitrate uptake rates became negligible because of light limitation, ammonium uptake did continue to be significant. The uptake of ammonium by heterotrophic bacteria was estimated to be 14-17% of the DIN assimilation. Less clear are the consequences of this uptake on the phytoplankton community and biogeochemical processes, e.g. new production. It was suggested that competition for ammonium could select for small cells and may force large diatoms to use nitrate. This implies that under Fe stress as observed here, ammonium uptake is preferred and new production progressively suppressed despite the surplus of nitrate.This research was supported by the Research Foundation Flanders through grant G.0021.04 and Vrije Universiteit Brussel via grant GOA 22, as well as the US National Science Foundation programs in Chemical and Biological Oceanography

Arsenic speciation in the River Zenne, Belgium

Arsenic species have been assessed in the Zenne River, a sewage contaminated tributary of the Scheldt estuary, in winter 2003. The highest total dissolved As concentrations were found in the middle part of the river with values up to 3.6 mu g L(-1). Particulate As concentrations increase towards the mouth of the River with highest levels of 2 mu g L(-1). A very good correlation between the % of dissolved As and % of dissolved Fe was observed. They both linearly decrease with the amount of dissolved oxygen.In the middle part of the Zenne River where the oxygen levels were lowest, even below 1 mg L(-1), As(III) was the dominant species. In the other parts (upstream and downstream) of the river, As(V) was dominant. A linear relation between the measured redox values and those calculated via the As(III)/As(V) couple exists, but the range of measured Eh values is much larger than the calculated ones. No methylated dissolved As species were found during our survey

Reliability of N flux rates estimated from ¹⁵N enrichment and dilution experiments in aquatic systems

This paper investigates the estimation behavior of six increasingly complex 15N models, for estimating flux rates between phytoplankton and dissolved N pools in aquatic ecosystems. The development of these models over the last 40 years reflects increasing realism in the pools and fluxes that constitute the N cycle. The purpose of this paper is to assess how the model results are influenced by the underlying assumptions. For example, with respect to uptake of 15N by phytoplankton, is anything gained by assuming that regenerated N become isotopically enriched after the introduction of the 15N label, or is it just as accurate to assume that no source other than the initial 15N label contributes to the enrichment signal in phytoplankton? To conduct an objective assessment of the models, we compared them to (1) a set of reference values generated numerically by a process oriented model, and (2) real experimental data. The results show that for a number of 15N models, properties such as accuracy and precision cannot both be optimized under the same conditions, and a compromise must be struck. Oversimplified models risk bias when their underlying assumptions are violated, but overly complex models can misinterpret part of the random noise as relevant processes. Therefore none of the 15N model solutions can a priori be rejected, but each should carefully be assessed with hypothesis testing. A backward regression strategy based on a statistical interpretation of the cost function (sum of the weighted least squares residuals) was used to select optimal solution subsets corresponding to a given data set