Dissolved carbon dioxide in Dutch coastal waters

The role of shelf seas in global carbon cycling is poorly understood. The dissolved inorganic carbon system and air-sea exchange of carbon dioxide (CO2) are described for the Dutch coastal zone in September 1993. The inorganic carbon chemistry was affected by tidal mixing, wind speed, wind direction, freshwater input, stratification and coastal upwelling. Surface water had a variable fugacity of carbon dioxide (fCO2) between 300 and 800 μatm with short-term changes partly related to the tidal cycle. High contents of dissolved inorganic carbon (DIC) and CO2 in relatively saline water probably originated from mineralisation of accumulated organic matter in water and sediments farther out at sea and transport of water enriched in DIC into the coastal zone by upwelling. Air-sea exchange of CO2 ranged from —20 to 60 mmol m−2 day−1. These fluxes are critically discussed in the light of potential stratification. It is not possible to assess from this study whether the Dutch coastal zone is a net sink or source for atmospheric CO2

The influence of salinity on the solubility of Zn and Cd sulphides in the Scheldt estuary

In the estuary of the river Scheldt, where an oxygen gradient exists in addition to the salinity gradient, redox processes will be of major importance for trace metal mobilisation. In this study, the influence of salinity and pH on the redox processes of dissolved Zn and Cd sulphides is investigated together with the effects on the ratio of the dissolved Zn and Cd concentrations. The speciation of these metals is calculated with the chemical equilibrium programme MINEQL + .Zn sulphides are oxidised at lower oxygen concentrations than Cd sulphides, due to lower stability constants, causing a sudden increase or peak in the dissolved Zn/Cd ratio. The formation of dissolved Cd chloride complexes when oxidation occurs at high salinities (S = 15) increases the mobility of Cd, causing a decrease in the Zn/Cd peak of the total dissolved concentrations. The peak is three to four times smaller at S = 15 than when oxidation occurs at S = 2. The simple model calculations compare very well with field data. The Scheldt estuary is suitable to illustrate these calculations. In the, 1970s, the anoxic part of the estuary reached S = 15-20, but since the early 1980s it has dropped to S = 2-10. Historic data on metals in the estuary from 1978, 1987 and the 1990s were used to compare with the equilibrium calculations. The increase of the dissolved Zn/Cd peak at low salinity as a consequence, of the decreasing anoxic region is confirmed well by the data. The good agreement between model calculations and field data is a proof of the extreme importance of redox processes for the solubility of Zn and Cd sulphides in the estuary

Rapid invasion of anthropogenic CO2 into the deep circulation of the Weddell Gyre

Data are presented for total carbon dioxide (TCO2), oxygen and nutrients from 14 cruises covering two repeat sections across the Weddell Gyre, from 1973 to 2010. Assessments of the rate of increase in anthropogenic CO2 (Cant) are made at three locations. Along the Prime Meridian, TCO2 is observed to steadily increase in the bottom water. Accompanying changes in silicate, nitrate and oxygen confirm the non-steady state of the Weddell circulation. The rate of increase in TCO2 of +0.12±0.05?µmol?kg-1?yr-1 therefore poses an upper limit to the rate of increase in Cant. By contrast, the bottom water located in the central Weddell Sea exhibits no significant increase in TCO2, suggesting that this water is less well ventilated at the southern margins of the Weddell Sea. At the tip of the Antarctic Peninsula (i.e. the formation region of the bottom water found at the Prime Meridian), the high rate of increase in TCO2 over time observed at the lowest temperatures suggests that nearly full equilibration occurs with the anthropogenic CO2 of the atmosphere. This observation constitutes rare evidence for the possibility that ice cover is not a major impediment for uptake of Cant in this prominent deep water formation region

Rapid acidification of mode and intermediate waters in the southwestern Atlantic Ocean

Observations along the southwestern Atlantic WOCE A17 line made during the Dutch GEOTRACES-NL programme (2010–2011) were compared with historical data from 1994 to quantify the changes in the anthropogenic component of the total pool of dissolved inorganic carbon (?Cant). Application of the extended multi-linear regression (eMLR) method shows that the ?Cant from 1994 to 2011 has largely remained confined to the upper 1000 dbar. The greatest changes occur in the upper 200 dbar in the Subantarctic Zone (SAZ), where a maximum increase of 37 µmol kg-1 is found. South Atlantic Central Water (SACW) experienced the highest rate of increase in Cant, at 0.99 ± 0.14 µmol kg-1 yr-1, resulting in a maximum rate of decrease in pH of 0.0016 yr-1. The highest rates of acidification relative to ?Cant, however, were found in Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW). The low buffering capacity of SAMW and AAIW combined with their relatively high rates of Cant, increase of 0.53 ± 0.11 and 0.36 ± 0.06 µmol kg-1 yr-1, respectively, has lead to rapid acidification in the SAZ, and will continue to do so whilst simultaneously reducing the chemical buffering capacity of this significant CO2 sink

Iron-mediated effects on nitrate reductase in marine phytoplankton

The potential activity of nitrate reductase was determined in uni-algal cultures in the laboratory and in natural marine phytoplankton assemblages. In the laboratory bioassays, distinct differences in nitrate reductase activity were observed in iron replete versus depleted cultures for Emiliania huxleyi, Isochrysis galbana and Tetraselmis sp. Cells from iron-depleted cultures had 15 to 50 percent lower enzyme activity than those from iron-replete cultures. Upon addition of iron, nitrate reductase activity was enhanced in depleted cells up to levels comparable to those of the replete cells. Bioassays in the northern North Sea conducted in 1993, under low iron conditions, demonstrated similar results. Upon addition of 2.5 nM iron, a distinct enhancement, to a maximum of three times, of nitrate reductase activity was observed within 32 h after addition. Therefore, iron can stimulate nitrate reductase activity. In spite of the clean techniques used, some nitrate reductase activity was always observed. Iron deficiency was shown to impair nitrate reductase activity, but it is unlikely that nitrate reduction would cease completely

Klimaat onderzoek bij het Nederlands Instituut voor Onderzoek der Zee

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Controls of the surface water partial pressure of CO₂ in the North Sea

The seasonal variability of the partial pressure of CO2 (pCO2) has been investigated in the North Sea, a northwest European shelf sea. Based on a seasonal and high spatial resolution data set the main controlling factors - biological processes and temperature - have been identified and quantified. In the central and northern parts being a CO2- sink all year round, the biological control dominates the temperature control. In the southern part, the temperature control dominates the biological control at an annual scale, since the shallow water column prevents stronger net-CO2 removal from the surface layer due to the absence of seasonal stratification. The consequence is a reversal of the CO2 sea-to- air flux during the spring bloom period, the only time, when CO2 is taken up from the atmosphere in the southern region. Net community production in the mixed layer has been estimated to 4mol Cm−2 yr−1 with higher values (4.3 mol Cm−2 yr−1) in the northern part and lower values in the southern part (2.6 mol Cm−2 yr−1)

Effects of iron stress on chromatic adaptation by natural phytoplankton communities in the Southern Ocean

Effects of iron stress on chromatic adaptation were studied in natural phytoplankton communities collected in the Pacific region of the Southern Ocean. Iron enrichment experiments (48 to 72 h) were performed, incubating plankton communities under white, green and blue light respectively, with and without addition of 2 nM Fe. Pigment ratios were affected by iron addition only to a minor extent. The pigment composition as dictated by the light conditions was similar for both the iron-enriched and the unamended bottles. Upon iron addition, phytoplankton auto-fluorescence, as estimated by flow cytometry, decreased markedly, indicating iron stress of the endemic phytoplankton community. It was concluded that iron did not control chromatic adaptation via the pigment composition, but exerted a clear effect on the efficiency of electron transfer