| Global Biogeochem Cycles
This large-scale quasi-synoptic study gives a comprehensive picture of sea-air CO2 fluxes during the melt season in the central and outer Laptev Sea (LS) and East Siberian Sea (ESS). During a 7 week cruise we compiled a continuous record of both surface water and air CO2 concentrations, in total 76,892 measurements. Overall, the central and outer parts of the ESAS constituted a sink for CO2, and we estimate a median uptake of 9.4 g C m−2 yr−1 or 6.6 Tg C yr−1. Our results suggest that while the ESS and shelf break waters adjacent to the LS and ESS are net autotrophic systems, the LS is a net heterotrophic system. CO2 sea-air fluxes for the LS were 4.7 g C m−2 yr−1, and for the ESS we estimate an uptake of 7.2 g C m−2 yr−1. Isotopic composition of dissolved inorganic carbon (δ13CDIC and δ13CCO2) in the water column indicates that the LS is depleted in δ13CDIC compared to the Arctic Ocean (ArcO) and ESS with an offset of 0.5‰ which can be explained by mixing of δ13CDIC-depleted riverine waters and 4.0 Tg yr−1 respiration of OCter; only a minor part (0.72 Tg yr−1) of this respired OCter is exchanged with the atmosphere. Property-mixing diagrams of total organic carbon and isotope ratio (δ13CSPE-DOC) versus dissolved organic carbon (DOC) concentration diagram indicate conservative and nonconservative mixing in the LS and ESS, respectively. We suggest land-derived particulate organic carbon from coastal erosion as an additional significant source for the depleted δ13CDIC.
Export of calcium carbonate corrosive waters from the East Siberian Sea
The Siberian shelf seas are areas of extensive biogeochemical transformation of organic matter, both of marine and terrestrial origin. This in combination with brine production from sea ice formation results in a cold bottom water of relative high salinity and partial pressure of carbon dioxide (pCO2). Data from the SWERUS-C3 expedition compiled on the icebreaker Oden in July to September 2014 show the distribution of such waters at the outer shelf, as well as their export into the deep central Arctic basins. Very high pCO2 water, up to ∼ 1000 µatm, was observed associated with high nutrients and low oxygen concentrations. Consequently, this water had low saturation state with respect to calcium carbonate down to less than 0.8 for calcite and 0.5 for aragonite. Waters undersaturated in aragonite were also observed in the surface in waters at equilibrium with atmospheric CO2; however, at these conditions the cause of under-saturation was low salinity from river runoff and/or sea ice melt. The calcium carbonate corrosive water was observed all along the continental margin and well out into the deep Makarov and Canada basins at a depth from about 50 m depth in the west to about 150 m in the east. These waters of low aragonite saturation state are traced in historic data to the Canada Basin and in the waters flowing out of the Arctic Ocean north of Greenland and in the western Fram Strait, thus potentially impacting the marine life in the North Atlantic Ocean.
Carbon geochemistry of plankton-dominated samples in the Laptev and East Siberian shelves: contrasts in suspended particle composition
Recent Arctic studies suggest that sea ice decline and permafrost thawing will affect phytoplankton dynamics and stimulate heterotrophic communities. However, in what way the plankton composition will change as the warming proceeds remains elusive. Here we investigate the chemical signature of the plankton-dominated fraction of particulate organic matter (POM) collected along the Siberian Shelf. POM (>10 mu m) samples were analysed using molecular biomarkers (CuO oxidation and IP25 ) and dual-carbon isotopes (delta C-13 and Delta C-14). In addition, surface water chemical properties were integrated with the POM (>10 mu m) dataset to understand the link between plankton composition and environmental conditions. delta C-13 and Delta C-14 exhibited a large variability in the POM (> 10 mu m) distribution while the content of terrestrial biomarkers in the POM was negligible. In the Laptev Sea (LS), delta C-13 and Delta C-14 of POM (> 10 mu m) suggested a heterotrophic environment in which dissolved organic carbon (DOC) from the Lena River was the primary source of metabolisable carbon. Within the Lena plume, terrestrial DOC probably became part of the food web via bacteria uptake and subsequently transferred to relatively other heterotrophic communities (e.g. dinoflagellates). Moving eastwards toward the sea-ice-dominated East Siberian Sea (ESS), the system became progressively more autotrophic. Comparison between delta C-13 of POM (> 10 mu m) samples and CO(2)aq concentrations revealed that the carbon isotope fractionation increased moving towards the easternmost and most productive stations. In a warming scenario characterised by enhanced terrestrial DOC release (thawing permafrost) and progressive sea ice decline, heterotrophic conditions might persist in the LS while the nutrient-rich Pacific inflow will likely stimulate greater primary productivity in the ESS. The contrasting trophic conditions will result in a sharp gradient in delta C-13 between the LS and ESS, similar to what is documented in our semi-synoptic study.
Reduction of Baltic Sea Nutrient Inputs and Allocation of Abatement Costs Within the Baltic Sea Catchment
Silicon isotope enrichment in diatoms during nutrient-limited blooms in a eutrophied river system
Sun, XL; Andersson, PS; Humborg, C; Pastuszak, M; Morth, CM
| J. Geochem. Explor.
a eutrophied river system
We examined the Si isotope fractionation by following a massive nutrient limited diatom bloom in a eutrophied natural system. The Oder River, which is a eutrophied river draining the western half of Poland and entering the southern Baltic Sea, exhibits diatom blooms that cause extreme Si isotope fractionation. The rapid nutrient depletion and fast BSi increase observed during the spring bloom suggest a closed system Rayleigh behavior for DSi and BSi in the river at certain time scales. A Si isotope fractionation factor ((30)epsilon(Dsi-Bsi)) of -1.6 +/- 0.31%. (2 sigma) is found based on observations between April and June, 2004. A very high delta Si-30 value of up to +3.05 parts per thousand. is measured in BSi derived from diatoms. This is about 2 times higher than previously recorded delta Si-30 in freshwater diatoms. The Rayleigh model used to predict the delta Si-30 values of DSi suggests that the initial value before the start of the diatom bloom is close to +2 parts per thousand, which is relatively higher than the previously reported values in other river water. This indicates that there is a biological control of the Si isotope compositions entering the river, probably caused by Si isotope fractionation during uptake of Si in phytoliths. Clearly, eutrophied rivers with enhanced diatom blooms deliver Si-30-enriched DSi and BSi to the coastal ocean, which can be used to trace the biogeochemistry of DSi/BSi in estuaries. Crown Copyright (C) 2013 Published by Elsevier B.V. All rights reserved.
Modeling Social–Ecological Scenarios in Marine Systems
Österblom, H.; Merrie, A.; Metian, M.; Boonstra, W.J.; Blenckner, T.; Watson, J.R.; Rykaczewski, R.R.; Ota, Y.; Sarmiento, J.L.; Christensen, V.; Schlüter, M.; Birnbaum, S.; Gustafsson, B.G.; Humborg, C.; Magnus Mörth, C.-M.; Müller-Karulis, B.; Tomczak, M.T.; Troell, M.; Folke, C.
Global carbon dioxide emissions from inland waters
Raymond, P.A.; Hartmann, J.; Lauerwald, R.; Sobek, S.; McDonald, C.; Hoover, M.; Butman, D.; Striegl, R.; Mayorga, E.; Humborg, C.; Kortelainen, P.; Dürr, H.; Meybeck, M.; Ciais, P.; Guth, P.
Nitrogen Fluxes from large watersheds to coastal ecosystems controlled by Net Anthropogenic Nitrogen Inputs and Climate