• European Marine Science
• Other research productsclose
• European Commission close
• EU close
• Biogeosciences (BG) close
• European Marine Science close

Arkona Basin (southwestern Baltic Sea) is a seasonally-hypoxic basin characterized by the presence of free methane gas in its youngest organic-rich muddy stratum. Through the use of reactive transport models, this study tracks the development of the methane geochemistry in Arkona Basin as this muddy sediment became deposited during the last 8 kyr. Four cores are modeled each pertaining to a unique geochemical scenario according to their respective contemporary geochemical profiles. Ultimately the thickness of the muddy sediment and the flux of particulate organic carbon are crucial in determining the advent of both methanogenesis and free methane gas, the timescales over which methanogenesis takes over as a dominant reaction pathway for organic matter degradation, and the timescales required for free methane gas to form.

Macrophytes growing in shallow coastal zones characterised by intense metabolic activity have the capacity to modify pH within their canopy and beyond. We observed diel pH changes in shallow (5–12 m) seagrass (Posidonia oceanica) meadows spanning 0.06 pH units in September to 0.24 units in June. The carbonate system (pH, DIC, and aragonite saturation state (ΩAr)) and O2 within the meadows displayed strong diel variability driven by primary productivity, and changes in chemistry were related to structural parameters of the meadow, in particular, the leaf surface area available for photosynthesis (LAI). LAI was positively correlated to mean, max and range pHNBS and max and range ΩAr. In June, vertical mixing (as Turbulent Kinetic Energy) influenced max and min ΩAr, while in September there was no effect of hydrodynamics on the carbonate system within the canopy. Max and range ΩAr within the meadow showed a positive trend with the calcium carbonate load of the leaves, pointing to a possible link between structural parameters, ΩAr and carbonate deposition. Calcifying organisms, e.g. epiphytes with carbonate skeletons, may benefit from the modification of the carbonate system by the meadow. There is, however, concern for the ability of seagrasses to provide modifications of similar importance in the future. The predicted decline of seagrass meadows may alter the scope for alteration of pH within a seagrass meadow and in the water column above the meadow, particularly if shoot density and biomass decline, on which LAI is based. Organisms associated with seagrass communities may therefore suffer from the loss of pH buffering capacity in degraded meadows.

Ocean acidification from the uptake of anthropogenic carbon is simulated for the industrial period and IPCC SRES emission scenarios A2 and B1 with a global coupled carbon cycle-climate model. Earlier studies identified seawater saturation state with respect to aragonite, a mineral phase of calcium carbonate, as a key variable governing impacts on corals and other shell-forming organisms. Globally in the A2 scenario, water saturated by more than 300%, considered suitable for coral growth, vanishes by 2070 AD (CO2≈630 ppm), and the ocean volume fraction occupied by saturated water decreases from 42% to 25% over this century. The largest simulated pH changes worldwide occur in Arctic surface waters, where hydrogen ion concentration increases by up to 185% (ΔpH=−0.45). Projected climate change amplifies the decrease in Arctic surface mean saturation and pH by more than 20%, mainly due to freshening and increased carbon uptake in response to sea ice retreat. Modeled saturation compares well with observation-based estimates along an Arctic transect and simulated changes have been corrected for remaining model-data differences in this region. Aragonite undersaturation in Arctic surface waters is projected to occur locally within a decade and to become more widespread as atmospheric CO2 continues to grow. The results imply that surface waters in the Arctic Ocean will become corrosive to aragonite, with potentially large implications for the marine ecosystem, if anthropogenic carbon emissions are not reduced and atmospheric CO2 not kept below 450 ppm.

Cable bacteria are multicellular, filamentous microorganisms that are capable of transporting electrons over centimeter-scale distances. Although recently discovered, these bacteria appear to be widely present in the seafloor, and when active they exert a strong imprint on the local geochemistry. In particular, their electrogenic metabolism induces unusually strong pH excursions in aquatic sediments, which induces considerable mineral dissolution, and subsequent mineral reprecipitation. However, at present, it is unknown whether and how cable bacteria play an active or direct role in the mineral reprecipitation process. To this end we present an explorative study of the formation of sedimentary minerals in and near filamentous cable bacteria using a combined approach of electron microscopy and spectroscopic techniques. Our observations reveal the formation of polyphosphate granules within the cells and two different types of biomineral formation directly associated with multicellular filaments of these cable bacteria: (i) the attachment and incorporation of clay particles in a coating surrounding the bacteria and (ii) encrustation of the cell envelope by iron minerals. These findings suggest a complex interaction between cable bacteria and the surrounding sediment matrix, and a substantial imprint of the electrogenic metabolism on mineral diagenesis and sedimentary biogeochemical cycling. In particular, the encrustation process leaves many open questions for further research. For example, we hypothesize that the complete encrustation of filaments might create a diffusion barrier and negatively impact the metabolism of the cable bacteria.

Changes in marine net primary productivity (PP) and export of particulate organic carbon (EP) are projected over the 21st century with four global coupled carbon cycle-climate models. These include representations of marine ecosystems and the carbon cycle of different structure and complexity. All four models show a decrease in global mean PP and EP between 2 and 20% by 2100 relative to preindustrial conditions, for the SRES A2 emission scenario. Two different regimes for productivity changes are consistently identified in all models. The first chain of mechanisms is dominant in the low- and mid-latitude ocean and in the North Atlantic: reduced input of macro-nutrients into the euphotic zone related to enhanced stratification, reduced mixed layer depth, and slowed circulation causes a decrease in macro-nutrient concentrations and in PP and EP. The second regime is projected for parts of the Southern Ocean: an alleviation of light and/or temperature limitation leads to an increase in PP and EP as productivity is fueled by a sustained nutrient input. A region of disagreement among the models is the Arctic, where three models project an increase in PP while one model projects a decrease. Projected changes in seasonal and interannual variability are modest in most regions. Regional model skill metrics are proposed to generate multi-model mean fields that show an improved skill in representing observation-based estimates compared to a simple multi-model average. Model results are compared to recent productivity projections with three different algorithms, usually applied to infer net primary production from satellite observations.

The present study provides new knowledge about the so far largely unexplored Coral Patch seamount which is located in the NE Atlantic Ocean half-way between the Iberian Peninsula and Madeira. For the first time a detailed hydroacoustic mapping (MBES) in conjunction with video surveys (ROV, camera sled) were performed to describe the sedimentological and biological characteristics of this sub-elliptical ENE-WSW elongated seamount. Video observations were restricted to the southwestern summit area of Coral Patch seamount (water depth: 560–760 m) and revealed that this part of the summit is dominated by exposed hard substrate, whereas soft sediment is just a minor substrate component. Although exposed hardgrounds are dominant for this summit area and, thus, offer suitable habitat for settlement by benthic organisms, the benthic megafauna shows rather scarce occurrence. In particular, scleractinian framework-building cold-water corals are apparently rare with very few isolated and small-sized live occurrences of the species Lophelia pertusa and Madrepora oculata. In contrast, dead coral framework and coral rubble are more frequent pointing to a higher abundance of cold-water corals on Coral Patch during the recent past. This is even supported by the observation of fishing lines that got entangled with rather fresh-looking coral frameworks. Overall, long lines and various species of commercially important fish were frequently observed emphasising the potential of Coral Patch as an important target for fisheries that may have impacted the entire benthic community. Hydroacoustic seabed classification covered the entire summit of Coral Patch and its northern and southern flanks (water depth: 560–2660 m) and revealed extended areas dominated by mixed and soft sediments at the northern flank and to a minor degree at its easternmost summit and southern flank. Nevertheless, these data also predict most of the summit area to be dominated by exposed bedrock which would offer suitable habitat for benthic organisms. By comparing the locally restricted video observations and the broad-scale monitoring of a much larger and deeper seafloor area as derived by hydroacoustic seabed classification, it becomes obvious that habitat information obtained by in situ sampling may provide a rather scattered pattern about the entire seamount ecosystem. Solely with a combination of both methods, a satisfactory approach to describe the diverse characteristics of a seamount ecosystem can be derived which is in turn indispensable for future scientific monitoring campaigns as well as management and conservation purposes.

Recent observations and modelling studies suggest that biogeochemical changes can mask atmospheric CO2-induced pH decreases. Data collected by the Dutch monitoring authorities in different coastal systems (North Sea, Wadden Sea, Ems-Dollard, Eastern Scheldt and Scheldt estuary) since 1975 provide an excellent opportunity to test whether this is the case in the Dutch coastal zone. The time-series were analysed using Multi-Resolution Analysis (MRA) which resulted in the identification of system-dependent patterns on both seasonal and intra-annual time scales. The observed rates of pH change greatly exceed those expected from enhanced CO2 uptake, thus suggesting that other biogeochemical processes, possibly related to changes in nutrient loading, can play a dominant role in ocean acidification.

Instrumental records from the Norwegian Sea and the Trondheimsfjord show evidence that changes of bottom water temperature and salinity in the fjord are linked to the salinity and temperature variability of the North Atlantic Current (NAC). Changes in primary productivity and salinity in the surface and intermediate water masses in the Trondheimsfjord as well as the fjord sedimentary budget are mainly driven by changes in riverine input. In this study we use 59 surface sediment samples that are evenly distributed in the fjord to examine whether dinocyst assemblages and stable isotope ratios of benthic foraminifera reflect the present-day hydrology and can be used as palaeoceanographic proxies. In general, modern benthic δ18O and δ13C values decrease from the fjord entrance towards the fjord head with lowest values close to river inlets. This is essentially explained by gradients in the amounts of fresh water and terrigenous organic matter delivered from the hinterland. The distribution of benthic δ13C ratios across the fjord is controlled by the origin (terrigenous vs. marine) of organic matter, local topography-induced variability in organic matter flux at the water–sediment interface, and organic matter degradation. The dinocyst assemblages display the variations in hydrography with respect to the prevailing currents, the topography, and the freshwater and nutrient supply from rivers. The strength and depth of the pycnocline in the fjord strongly vary seasonally and thereby affect water mass characteristics as well as nutrient availability, temporally creating local conditions that explain the observed species distribution. Our results prove that dinocyst assemblages and benthic foraminiferal isotopes reliably mirror the complex fjord hydrology and can be used as proxies of Holocene climatic variability.

About one third of the anthropogenic carbon dioxide (CO2) released into the atmosphere in the past two centuries has been taken up by the ocean. As CO2 invades the surface ocean, carbonate ion concentrations and pH are lowered. Laboratory studies indicate that this reduces the calcification rates of marine calcifying organisms, including planktic foraminifera. Such a reduction in calcification resulting from anthropogenic CO2 emissions has not been observed, or quantified in the field yet. Here we present the findings of a study in the Western Arabian Sea that uses shells of the surface water dwelling planktic foraminifer Globigerinoides ruber in order to test the hypothesis that anthropogenically induced acidification has reduced shell calcification of this species. We found that light, thin-walled shells from the surface sediment are younger (based on 14C and δ13C measurements) than the heavier, thicker-walled shells. Shells in the upper, bioturbated, sediment layer were significantly lighter compared to shells found below this layer. These observations are consistent with a scenario where anthropogenically induced ocean acidification reduced the rate at which foraminifera calcify, resulting in lighter shells. On the other hand, we show that seasonal upwelling in the area also influences their calcification and the stable isotope (δ13C and δ18O) signatures recorded by the foraminifera shells. Plankton tow and sediment trap data show that lighter shells were produced during upwelling and heavier ones during non-upwelling periods. Seasonality alone, however, cannot explain the 14C results, or the increase in shell weight below the bioturbated sediment layer. We therefore must conclude that probably both the processes of acidification and seasonal upwelling are responsible for the presence of light shells in the top of the sediment and the age difference between thick and thin specimens.

Coastal systems can act as filters for anthropogenic nutrient input into marine environments. Here, we assess the processes controlling the removal of phosphorus (P) and nitrogen (N) for four sites in the eutrophic Stockholm Archipelago. Bottom water concentrations of oxygen and P are inversely correlated. This is attributed to the seasonal release of P from iron (Fe)-oxide-bound P in surface sediments and from degrading organic matter. The abundant presence of sulfide in the pore water, linked to prior deposition of organic-rich sediments in a low oxygen setting (legacy of hypoxia), hinders the formation of a larger Fe-oxide-bound P pool in winter. Burial rates of P are high at all sites (0.03–0.3 mol m−2 y−1), a combined result of high sedimentation rates (0.5 to 3.5 cm yr−1) and high sedimentary P at depth (~ 30 to 50 μmol g−1). Organic P accounts for 30–50 % of reactive P burial. Apart from one site in the inner archipelago, where a vivianite-type Fe(II)-P mineral is likely present at depth, there is little evidence for sink-switching of organic or Fe-oxide bound P to authigenic P minerals. Denitrification is the major benthic nitrate-reducing process at all sites (0.09 to 1.7 mmol m−2 d−1), efficiently removing N as N2. Denitrification rates decrease seaward following the decline in bottom water nitrate and sediment organic carbon. Our results explain how sediments in this eutrophic coastal system can efficiently remove land-derived P and N, regardless of whether the bottom waters are oxic or frequently hypoxic. Hence, management strategies involving artificial reoxygenation are not expected to be successful in removing P and N, emphasizing a need for a focus on nutrient load reductions.