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The sulphur cycle has a key role on the fate of nutrients through its several interconnected reactions. Although sulphur cycling in aquatic ecosystems has been thoroughly studied since the early 70's, its characterisation in saline endorheic lakes still deserves further exploration. Gallocanta Lake (NE Spain) is an ephemeral saline inland lake whose main sulphate source is found on the lake bed minerals and leads to dissolved sulphate concentrations higher than those of seawater. An integrative study including geochemical and isotopic characterization of surface water, porewater and sediment has been performed to address how sulphur cycling is constrained by the geological background. In freshwater and marine environments, sulphate concentration decreases with depth are commonly associated with bacterial sulphate reduction (BSR). However, in Gallocanta Lake sulphate concentrations in porewater increase from 60 mM at the water–sediment interface to 230 mM at 25 cm depth. This extreme increase could be caused by dissolution of the sulphate rich mineral epsomite (MgSO4·7H2O). Sulphur isotopic data was used to validate this hypothesis and demonstrate the occurrence of BSR near the water–sediment interface. This dynamic prevents methane production and release from the anoxic sediment, which is advantageous in the current context of global warming. These results underline that geological context should be considered in future biogeochemical studies of inland lakes with higher potential availability of electron acceptors in the lake bed compared to the water column. [ABSTRACT FROM AUTHOR]

Isoetid macrophytes such as Lobelia dortmanna and Littorella uniflora are engineering species with an extensive root system and high radial oxygen loss. Despite several studies on these macrophytes, the effect of their oxygenation on methane dynamics has never been investigated. In this study, we hypothesise that isoetids promote dissolved inorganic carbon fixation and methane oxidation in sandy sediments. Our whole-ecosystem approach study lasted 2 years (2013-2014) on two oligo-mesotrophic shallow lakes. Benthic chamber incubations confirmed that, as a result of primary production and methanotrophy, isoetid lawns had consistently lower benthic carbon fluxes than bare sediments. On a daily basis, vegetated areas acted as a carbon sink (−0.7 ± 0.4 g C m days, as DIC + CH), whereas bare sediments acted as a net source (0.6 ± 0.5 g C m days, as DIC + CH). Photosynthetic quotients of <1 indicated that photosynthetically produced oxygen was not released into the water column, but accumulated in leaf lacunae or was transferred to the rhizosphere, that contributing to the alteration of net benthic fluxes at the sediment-water interface. This preliminary study highlights the necessity of further investigating the role that isoetids play in mitigating greenhouse gas emissions from temperate shallow lakes. [ABSTRACT FROM AUTHOR]

From 1989 to 2007, a severe decline in Zostera noltii meadows was reported in the Arcachon Bay, with an accelerated regression after 2005. We investigated the inter-annual variability of the biogeochemistry of the sediment in an area affected by seagrass decline. In late summer and in winter of the years 2006, 2010, and 2011, sediment cores were collected at low tide on vegetated and adjacent non-vegetated sediments located in the eastern part of the Arcachon Bay. The geochemical analyses of sediment solid-phase organic carbon, reactive P and Fe, and the pore water concentrations of Fe, DIP, and NH are presented. The changes in the chemistry of sediment and pore water between 2006 and 2010 are interpreted as a consequence of the decrease in the Z. noltii biomass between 2006 and 2010. The absence of significant seasonal variations in biomass throughout the growth period (March-September) in 2011 is most likely related to the regression of Z. noltii meadow that strongly affects the study area. In contrast to the healthy meadow in 2006, the declining meadow favored the dissolution of sedimentary particulate phosphorus in winter. In late summer, the low biomass of seagrass resulted in a net release of ammonium in the pore water of the upper 20 cm of sediment. This study clearly shows that seagrass decay may enhance nutrient release in sediments, resulting in a significant supply of phosphorus to the water column of a magnitude comparable to annual inputs to the lagoon from the rivers and the tidal pump. [ABSTRACT FROM AUTHOR]

We have studied 15 catchments supplying freshwater to a French Atlantic coastal lagoon, where increase in nitrogen loads due to agriculture is supposed to have destabilized the ecosystem in the last decades. The catchment is a lowland composed of Pleistocene sands with an average slope of 0.25%. To study the nutrient export in relation to land-use surface waters were sampled bi-weekly between October 2006 and January 2009 and land-use was established by plane photographs and Geographic Information System (GIS). Cultivated pine forests represent more than 80% of the total surface and 7% of the catchment area has been deforested recently. Significant areas of some catchments are used for maize crop. Housing is confined to the coastal zone. Maize and forest crop give a robust signature in terms of nitrate export. In view of modeling the nutrient fluxes, we have established the mean export rate for every land-use: forested parcels, deforested parcels, cultivated surfaces, and housing areas export 45, 93, 2850, and 61 kg N-nitrate km year, respectively. Exports of ammonium, dissolved organic N (DON), and dissolved inorganic P (DIP) could not be related to land use. The mean export is 13, 100, and 0.57 kg km year for N-ammonium, DON, and DIP, respectively. The modeling of nitrogen flux is in good agreement with our measures for the largest catchment, which supplies about 90% of the total continental DIN flux. However, small catchments are more dynamic due to hydrological conditions and the model is less accurate. This work has permitted to complete and unify scattered studies about nutrient cycling in this area. Thus we have established and compared the nitrogen budget of cornfields and cultivated pine forest. We have emphasized that (i) fertilizer use should be reduced in cornfields because they stock between 200 and 6400 kg DIN km year, and (ii) the nitrogen budget in pine forest mostly depends on tree harvesting and symbiotic N-fixation, which is poorly constrained. Export of N by rivers represents a small contribution to the N budget of soils. [ABSTRACT FROM AUTHOR]

Manganese is a major redox reactive element of benthic metabolism. We have built a database of existing knowledge on the benthic geochemistry of Mn in the Bay of Biscay, in order to comprehensively assess the behaviour of Mn in a variety of environments during early diagenesis. The database contains vertical profiles of particulate and dissolved Mn species of 59 cores collected during 17 cruises between 1997 and 2006 at nine stations positioned between 140 and 4,800 m water depths. At all studied stations, Mn species follow the conventional distribution, where Mn(III,IV) species are enriched in the oxic layer, and dissolved Mn is present in the anoxic sediments. A minor part of Mn-oxides originates from sedimenting particles. The major part is of diagenetic origin, and derives from the oxidation of upward-diffusing dissolved Mn(II). Mn-oxide inventories are higher at the deeper stations than at the shallower ones. This difference cannot be attributed to different sources of sedimenting particles, but it must depend on sedimentation rate and diagenetic processes. At depth, dissolved Mn(II) concentrations are constant. This probably reflects equilibrium with an authigenic Mn(II) phase, which is the ultimate phase into which Mn is fossilized. The Mn content of deeper anoxic sediments is similarly low in all the cores studied, associated with corresponding trends of Mn content in sedimenting particles of the Bay of Biscay. Bioturbation, rather than redox oscillations, can convey Mn(III,IV) species downwards into the anoxic sediments where they are reduced, associated with a peak of dissolved Mn. Because dissolved Mn(II) is re-oxidized when it diffuses towards the oxic layer, the inventory of the diagenetic Mn(III,IV) phase remains at steady state, especially at stations where the oxic layer is thick. It then becomes possible to calculate the residence time of diagenetic Mn(III,IV) particles within the oxic layer, using the upward-directed flux of pore water Mn(II). By applying this residence time to the accumulation of sediments within the oxic layer, we obtain the sediment mass accumulation rate. The values calculated for the sediments of the Bay of Biscay fit well with accumulation rates obtained from radionuclides or sediment traps. The method has also been validated with data collected in other marine sedimentary environments. [ABSTRACT FROM AUTHOR]