Supplementary Table S1.Nutrient data analysed for the GEOVIDE transect relevant to this study.All nutrient data are concentrations (M) and pressure was measured as decibars (equivalent to meters below surface).Note: concentration of DSi recorded in this table was collected from a different cast than the samples collected for analysis of 30 SiDSi.The 30 SiDSi samples were analysed independently for DSi (see table S2).
The fraction of net primary production that is exported from the euphotic zone as sinking particulate organic carbon (POC) varies notably through time and from region to region. Phytoplankton containing biominerals, such as silicified diatoms have long been associated with high export fluxes. However, recent reviews point out that the magnitude of export is not controlled by diatoms alone, but determined by the whole plankton community structure. The combined effect of phytoplankton community composition and zooplankton abundance on export flux dynamics, were explored using a set of 12 large outdoor mesocosms. All mesocosms received a daily addition of minor amounts of nitrate and phosphate, while only 6 mesocosms received silicic acid (dSi). This resulted in a dominance of diatoms and dinoflagellate in the +Si mesocosms and a dominance of dinoflagellate in the –Si mesocosms. Simultaneously, half of the mesocosms had decreased mesozooplankton populations whereas the other half were supplemented with additional zooplankton. In all mesocosms, POC fluxes were positively correlated to Si/C ratios measured in the surface community and additions of dSi globally increased the export fluxes in all treatments highlighting the role of diatoms in C export. The presence of additional copepods resulted in higher standing stocks of POC, most probably through trophic cascades. However it only resulted in higher export fluxes for the –Si mesocosms. In the +Si with copepod addition (+Si +Cops) export was dominated by large diatoms with higher Si/C ratios in sinking material than in standing stocks. During non-bloom situations, the grazing activity of copepods decrease the export efficiency in diatom dominated systems by changing the structure of the phytoplankton community and/or preventing their aggregation. However, in flagellate-dominated system, the copepods increased phytoplankton growth, aggregation and fecal pellet production, with overall higher net export not always visible in term of export efficiency.
Significant variations in the isotopic composition of marine calcium have occurred over the last 80 million years. These variations reflect deviations in the balance between inputs of calcium to the ocean from weathering and outputs due to carbonate sedimentation, processes that are important in controlling the concentration of carbon dioxide in the atmosphere and, hence, global climate. The calcium isotopic ratio of paleo-seawater is an indicator of past changes in atmospheric carbon dioxide when coupled with determinations of paleo-pH.
The δ 30 Si and δ 15 N diatom of diatom opal provide a view of nutrient utilization in past oceans and are used to formulate and test hypotheses concerning Southern Ocean productivity and fluctuations in atmospheric CO 2 over glacial cycles. Water column profiles of the Si and N isotopic composition of nutrients and the δ 15 N diatom of sediment core tops support the use of δ 30 Si and δ 15 N diatom as tracers of silicic acid and nitrate utilization, but some issues remain concerning the use of these proxies for paleoceanographic reconstructions. If average marine δ 30 Si changes over time, it could contribute to the observed down core variations in δ 30 Si. Reconstruction of deepwater δ 30 Si using opal from sponges or deep‐dwelling radiolarians would address this concern. Cleaning and measurement methods for δ 15 N diatom need to be standardized between laboratories and in general suffer from our lack of knowledge of how much organic matter a clean diatom frustule should contain and what its C/N ratio should be. Corresponding shifts in the δ 15 N and C/N of diatom opal with species composition suggests that changes in species composition contributes to the measured down core variations in N and possibly Si as well. This could be due to changes in the ecological niche represented in the sediments or, in the case of N, to species specific fractionation factors. Separation of opal sediments into something more closely resembling monospecific samples is a key development that needs to be made and may be possible using laminar flow systems like “split‐flow lateral‐transport thin fractionation” (SPLITT). In the meantime, information on the species composition of each sieved and cleaned sample analyzed needs to be collected alongside the isotopic data.
Abstract. To examine the potentially competing influences of microzooplankton and calcite mineral ballast on organic matter remineralization, we incubated diatoms in darkness in rolling tanks with and without added calcite minerals (coccoliths) and microzooplankton (rotifers). Concentrations of particulate organic matter (POM in suspension or in aggregates), of dissolved organic matter (DOM), and of dissolved inorganic nutrients were monitored over 8 days. The presence of rotifers enhanced the remineralization of ammonium and phosphate, but not dissolved silicon, from the biogenic particulate matter, up to 40% of which became incorporated into aggregates early in the experiment. Added calcite resulted in rates of excretion of ammonium and phosphate by rotifers that were depressed by 67% and 36%, respectively, demonstrating the potential for minerals to inhibit the destruction of POM by zooplankton in the water column. Lastly, the presence of the rotifers and added calcite minerals resulted in a more rapid initial rate of aggregation, although not a greater overall amount of aggregation during the experiment.
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