Volker Mohrholz

Detoxification of sulphidic African shelf waters by blooming chemolithotrophs

Coastal waters support ∼90 per cent of global fisheries and are therefore an important food reserve for our planet. Eutrophication of these waters, due to human activity, leads to severe oxygen depletion and the episodic occurrence of hydrogen sulphide—toxic to multi-cellular life—with disastrous consequences for coastal ecosytems. Here we show that an area of ∼7,000 km2 of African shelf, covered by sulphidic water, was detoxified by blooming bacteria that oxidized the biologically harmful sulphide to environmentally harmless colloidal sulphur and sulphate. Combined chemical analyses, stoichiometric modelling, isotopic incubations, comparative 16S ribosomal RNA, functional gene sequence analyses and fluorescence in situ hybridization indicate that the detoxification proceeded by chemolithotrophic oxidation of sulphide with nitrate and was mainly catalysed by two discrete populations of γ- and ε-proteobacteria. Chemolithotrophic bacteria, accounting for ∼20 per cent of the bacterioplankton in sulphidic waters, created a buffer zone between the toxic sulphidic subsurface waters and the oxic surface waters, where fish and other nekton live. This is the first time that large-scale detoxification of sulphidic waters by chemolithotrophs has been observed in an open-ocean system. The data suggest that sulphide can be completely consumed by bacteria in the subsurface waters and, thus, can be overlooked by remote sensing or monitoring of shallow coastal waters. Consequently, sulphidic bottom waters on continental shelves may be more common than previously believed, and could therefore have an important but as yet neglected effect on benthic communities.

Transverse structure of turbulence in a rotating gravity current

[1] Synoptic, high-resolution, measurements of turbulent kinetic energy dissipation, current velocity and water column stratification across a fast (up to 0.7 m s -1 ) oceanic saline gravity current are presented. Our data provide, for the first time, a detailed two-dimensional picture of the turbulence structure inside a gravity current. Strong boundary-layer and interfacial turbulence can be distinguished from a quiet core, and a strong asymmetry of mixing near the outer edges of the gravity current is apparent. This asymmetry is mirrored by the computed entrainment velocities, varying approximately by a factor of 5 across the gravity current. It is argued that the asymmetry is due to rotational effects that can be clearly identified also in the velocity and density fields.

On the dynamics of the Pomeranian Bight

Abstract CTD and ADCP measurements were obtained from the Pomeranian Bight, on cruises of 2–3 weeks duration together with time-series measurements of currents and salinity, from 1993 to 1996, in different seasons of the year. The observations were supported by high-resolution circulation model runs, representing the whole of the Baltic Sea. The model results and current/salinity observations in the Pomeranian Bight reveal the remote impact of both barotropic and baroclinic processes. The remote barotropic impact is driven by the exchange processes between the North Sea and the Baltic Sea. The process is controlled by a coherent band of intensive barotropic jets, which follow the north coast of Rugen Island, the northern edge of the Pomeranian Bight, and the central and easterly parts of the Pomeranian coast. This barotropic circulation pattern shelters the southern part of the Pomeranian Bight, from intensive currents. The remote baroclinic impact is due to the density difference between the surface waters of the Arkona Basin and the Pomeranian Bight. It drives a southward-directed current along the east coast of Rugen Island, as far south as Greifswalder Oie Island. This baroclinic current is essential for maintaining the salt balance of the Pomeranian Bight, but it hampers the wind-driven flow of water from moving along the northwestern rim of the Pomeranian Bight, along the eastern coast of Rugen Island in the direction of the Arkona Basin. The dynamical regime of the southern part of the Pomeranian Bight is governed by a locally wind-driven Ekman current and a compensating bottom current, as well as by coastal jets; these are weakest near the Swina river mouth and increase eastwards. Basic differences were observed in the development of the river plumes in response to westerly and easterly wind forcing. The lagoon waters were transported in a narrow band along the Pomeranian coast, during westerly winds. Easterly winds of more than a few days duration cause the band of lagoon waters to be dispersed, by upwelling filaments in the offshore direction into the Bornholm Basin. Simultaneously, outflowing lagoon waters form stable plumes off the Usedom coast, which are restricted by a topographical ridge to the west of the Greifswalder Oie Island and the southward-directed salt flow along the east coast of Rugen Island, from flowing directly northwards into the Arkona Basin. However, Ekman transport associated with long-lasting southeasterly wind transports the plume towards the western edge of the Oderbank, where a northward-directed edge current carries the lagoon water plume into the Arkona Basin.

The exceptional oder flood in summer 1997 — riverine mass and nutrient transport into the Pomeranian Bight

The discharge of river water and the amount of nutrients released into the Pomeranian Bight by the Oder river were investigated during the flood event in summer 1997. A barotropic box model of the lower Oder estuary was used to calculate the transport of riverine water through the channels connecting the Szczecin Lagoon and the Pomeranian Bight. The nutrient transport into the Baltic Sea was estimated, based on daily measurements of nutrient concentrations (NO3, NO2, NH4, Ntotal, PO4, Ptotal and SiO4) in the Swina and the Szczecin Lagoon and on the modelled water transport.

Anatomizing one of the largest saltwater inflows into the Baltic Sea in December 2014

In December 2014, an exceptional inflow event into the Baltic Sea was observed, a so-called Major Baltic Inflow (MBI). Such inflow events are important for the deep water ventilation in the Baltic Sea and typically occur every 3–10 years. Based on first observational data sets, this inflow had been ranked as the third largest since 100 years. With the help of a multinested modeling system, reaching from the North Atlantic (8 km resolution) to the Western Baltic Sea (600 m resolution, which is baroclinic eddy resolving), this event is reproduced in detail. The model gave a slightly lower salt transport of 3.8 Gt, compared to the observational estimate of four Gt. Moreover, by using passive tracers to mark the different inflowing water masses, including an age tracer, the inflowing water masses could be tracked and their paths and timing through the different basins could be reproduced and investigated. The analysis is supported by the recently developed Total Exchange Flow (TEF) to quantify the volume transport in different salinity classes. To account for uncertainties in the modeled velocity and tracer fields, a Monte Carlo Analysis (MCA) is applied to correct possible biases and errors. With the help of the MCA, 95% confidence intervals are computed for the transport estimates. Based on the MCA, the “best guess” of the volume transport is 291.0 ± 13.65 km3 and 3.89 ± 0.18 Gt for the total salt transport.

Numerical analysis of stratification and destratification processes in a tidally energetic inlet with an ebb tidal delta

Stratification and destratification processes in a tidally energetic, weakly stratified inlet in the Wadden Sea (south eastern North Sea) are investigated in this modeling study. Observations of current velocity and vertical density structure show strain-induced periodic stratification for the southern shoal of the tidal channel. In contrast to this, in the nearby central region of the channel, increased stratification is already observed directly after full flood. To investigate the processes leading to this different behavior, a nested model system using GETM is set up and successfully validated against field data. The simulated density development along a cross section that includes both stations shows that cross-channel stratification is strongly increasing during flood, such that available potential energy is released in the deeper part of the channel during flood. An analysis of the potential energy anomaly budget confirms that the early onset of vertical stratification during flood at the deeper station is mainly controlled by the stratifying cross-channel straining of the density field. In contrast to this, in the shallow part of the channel, the relatively weak cross-channel straining is balanced by along-channel straining and vertical mixing. An idealized analytical model confirms the following hypothesis: The laterally convergent flood current advecting laterally stratified water masses from the shallow and wide ebb tidal delta to the deep and narrow tidal channel has the tendency to substantially increase cross-channel density gradients in the tidal channel. This process leads to stratification during flood.

Nutrient Dynamics in the Pomeranian Bay (Southern Baltic): Impact of the Oder River Outflow

The Pomeranian Bay is a coastal region fed by the Oder River, one of the seven largest Baltic rivers, whose waters flow through a large and complex estuarine system before entering the bay. Nutrients (NO3−, NO2−, NH4+, Ntot, PO43−, Ptot, DSi), chlorophylla concentrations, oxygen content, salinity, and temperature were measured in the Pomeranian Bay in nine seasonally distributed cruises during 1993–1997. Strong spatial and temporal patterns were observed and they were governed by: the seasonally variable riverine water-nutrient discharges, the seasonally variable uptake of nutrients and their cycling in the river estuary and the Bay, the character of water exchange between the Pomeranian Bay and the Szczecin Lagoon, and the water flow patterns in the Bay that are dominated by wind-driven circulation. Easterly winds resulted in water and nutrient transport along the German coastline, while westerly winds confined the nutrient rich riverine waters to the Polish coast and transported them eastward beyond the study area. Two water masses, coastal and open, characterized by different chemical and physical parameters and chla content were found in the Bay independently of the season. The role of the Oder estuary in nutrient transformation, as well as the role of temperature in transformation processes is stressed in the paper. The DIN:DIP:DSi ratio indicated that phosphorus most probably played a limiting role in phytoplankton production in the Bay in spring, while nitrogen did the same in summer. During the spring bloom, predominated by diatoms, the DSi:DIN ratio dropped to 0.1 in the coastal waters and to 0.6 in the open bay waters, pointing to silicon limitation of diatom growth, similar to what is being observed in other Baltic regions.

Interannual variability of sulphur plumes off the Namibian coast

Coastal sulphur plumes in the upper water layer are frequently observed off the Namibian coast. However, their temporal and spatial development, strength, size and impact on marine life differ at a wide range. This study compares the events of years 2007/2008 to the years 2004/2005 along the Namibian coast, including regional features on the basis of remote sensing satellite data, in situ measurements of a mooring and local observations. The remotely sensed derived intensity of coastal sulphur plumes of year 2008 was very weak compared to 2004 and especially to 2005, the year with the highest activity during the study period. In 2008, the overall maximum of sulphur plumes was observed in February, about 2 months earlier than in 2004 and 2005 when highest intensity was detected in April. Locally, differences in timing and strength of sulphur plumes occurred in the area of the Walvis Bay. There, the maximum intensity was observed in March 2004 and in April 2005, but in 2008 no clear maximum was found. The remotely sensed identified coastal sulphur plumes correlate with periods of low oxygen concentrations in bottom water and high fraction of South Atlantic central water (SACW) at a mooring 20 nautical miles off the central Namibian coast. An exception is the area in the vicinity of the Walvis Bay lagoon that seems to be driven by different local dynamics. The total variability of the sulphur plumes could not be explained by the observed oceanographic conditions alone. Additionally, microbiological and chemical processes in the sediment and the bottom water layer should be included in future investigations related to annual and interannual variability. Disastrous local events were observed in the year 2008 although the averaged intensity was low compared to the years 2004/2005. Therefore, the remotely sensed derived averaged intensity gives probably no real impression of the local impact of sulphur plumes on marine life. A combination of all available methods like the detection by local observations, in situ measurements and remote sensing approaches is required.

Onshore–offshore distribution of Thecosomata (Gastropoda) in the Benguela Current upwelling region off Namibia: species diversity and trophic position

Many Thecosomata (Gastropoda) produce an aragonite shell and are potentially threatened by the increasing ocean acidification. Information about these species is very important for future monitoring of the fate of this group. This paper investigates the distribution, species composition and trophic role of Thecosomata along a transect from the coast into the open ocean off Walvis Bay, Namibia, in September 2010 and January/February 2011. Twenty species were detected, but three taxa ( Limacina bulimoides , Limacina inflata and Desmopterus papilio ) dominated the community with more than 80% of the total standing stock. Diel vertical migration was observed for both Limacina taxa with higher concentrations in surface waters during night. Desmopterus papilio revealed almost no day/night differences. The highest diversities and abundances were detected at the slope and offshore stations, indicating the oceanic preference of this group; some taxa aggregated at the shelf–open ocean interface. δ 15 N measurements confirmed the first trophic level of this group; however, significant differences were detected between seasons with higher values in February 2011. This can be related to differences in seston values as the primary food source. Possible biogeochemical causes for these differences like an exhaustion of the nitrate pool or denitrification processes under suboxic conditions are discussed.

The Impact of the Major Baltic Inflow of December 2014 on the Mercury Species Distribution in the Baltic Sea

The Baltic Sea is a marginal sea characterized by stagnation periods of several years. Oxygen consumption in its deep waters leads to the buildup of sulfide from sulfate reduction. Some of the microorganisms responsible for these processes also transform reactive ionic mercury to neurotoxic methylmercury. Episodic inflows of oxygenated saline water from the North Sea temporally re-establish oxic life in deep waters of the Baltic Sea. Thus, this sea is an especially important region to better understand mercury species distributions in connection with variable redox conditions. Mercury species were measured on three Baltic Sea campaigns, during the preinflow, ongoing inflow, and subsiding inflow of water, respectively, to the central basin. The inflowing water caused the removal of total mercury by 600 nmol m-2 and of methylmercury by 214 nmol m-2 in the Gotland Deep, probably via attachment of the mercury compounds to sinking particles. It appears likely that the consequences of the oxygenation of Baltic Sea deep waters, which are the coprecipitation of mercury species and the resettlement of the oxic deep waters, could lead to the enhanced transfer of accumulated mercury and methylmercury to the planktonic food chain and finally to fish.