Christa Pohl

Manganese cycling in the Gotland Deep, Baltic Sea

Abstract Manganese plays an important role as both an electron donor and acceptor in redox processes of stratified marine environments. Here we present results on Mn cycling in the water column of the Gotland Deep, Baltic Sea—a basin with periodically observed anoxic conditions in bottom waters. In the period 1999–2001, the deeper part of the Gotland Basin was permanently anoxic, and no inflows of significant amounts of oxygenated North Sea water were recorded below the halocline. The upward Mn(II) flux at the chemocline varied between 28 and 77 μmol m−2 day−1 (avg. 53 μmol m−2 day−1) and it was balanced by the downward flux of oxidized Mn over the entire year. The vertical flux of Mn(II) in the water column was mostly regulated by the flux of settling Mn oxides with a minor contribution of Mn(II) diffusing from bottom sediments (7.1–8.2 μmol m−2 day−1). The potential Mn(II) oxidation contributed no more than 2% to the total transferable electron flux (potential chemosynthesis) from the anoxic into the oxic zone, whereas the flux of settling Mn oxides presumably accounted for 3–30% of sulfide oxidation. Two different morphotypes of Mn-rich particles, formed during Mn(II) oxidation in the Gotland Basin chemocline, were identified using SEM-EDX: amorphous particles (92%) and Metallogenium-like particles. About 68% of all Mn-rich particles were associated with large aggregates containing an organic matrix. Manganese represented more than 60% of the total elemental composition of the particles; calcium, iron and silica were also detected.

The effect of redox processes on the partitioning of Cd, Pb, Cu, and Mn between dissolved and particulate phases in the Baltic Sea

Abstract The concentrations of Cd, Pb, Cu, Mn in the dissolved and particulate phases were measured during monitoring cruises in Gotland Basin between 1992 and 1996. The situation during the investigation period was influenced by a major inflow of dense and oxygen-rich water into the Baltic Sea at the beginning of 1993. This terminated a stagnation period of nearly seventeen years and caused oxygen concentrations to increase, mainly in the deep waters of the Bornholm- and Gotland basins. Beside the change of the redox conditions as a direct effect on the geochemical behaviour of trace metals in the water column the scavenging of Cd and Pb by Mn-precipitates is discussed under oxic conditions. In 1992 dissolved metal concentrations decreased by factors of 3 (Cd), 2.5 (Cu), 2 (Pb) in the anoxic deep waters, compared with surface concentrations of 0.16 nM/kg (Cd), 0.29 nM/kg (Pb) and 10.1 nM/kg (Cu), due to the formation of sulfidic metal species scavenged by particles. After the saltwater inflow had brought oxic conditions, the elimination of dissolved Cd and Pb was influenced by high contents of Mn (IV) precipitates, while for Cu the dissolved species (hydroxo-, chloro-, and organic complexes) predominated.

Luminescent whole-cell cyanobacterial bioreporter for measuring Fe availability in diverse marine environments.

ABSTRACT A Synechococcus sp. strain PCC 7002 Fe bioreporter was constructed containing the isiAB promoter fused to the Vibrio harveyi luxAB genes. Bioreporter luminescence was characterized with respect to the free ferric ion concentration in trace metal-buffered synthetic medium. The applicability of the Fe bioreporter to assess Fe availability in the natural environment was tested by using samples collected from the Baltic Sea and from the high-nutrient, low-chlorophyll subarctic Pacific Ocean. Parallel assessment of dissolved Fe and bioreporter response confirmed that direct chemical measurements of dissolved Fe should not be considered alone when assessing Fe availability to phytoplankton.

The exceptional Oder flood in summer 1997 — distribution patterns of the oder discharge in the Pomeranian Bight

The exceptional Oder flood in summer 1997 was a unique event in order to investigate the impacts on and the consequences for the ecosystem of the Baltic Sea of about 6.5 km3 additional water loaded with nutrients and contaminants and discharged within only 5 weeks. About 15 institutions participated in this investigation in both the Szczecin Lagoon and the Pomeranian Bight. The Baltic Sea Research Institute Warnemunde studied the water and nutrient inflow, the spreading of the Oder discharge, and the impact of the discharge on the ecosystem. The main topic of the presented investigations is a detailed study of the spatial and temporal spreading of the extreme river discharge in the Pomeranian Bight and the southern Baltic Sea by satellite data, ship observations and continuous buoy measurements as well as numerical modelling.

Trace metal impact into the Baltic Sea during the exceptional Oder flood in summer 1997

In July 1997, strong rainfall events in the upper Oder catchment area caused a flood catastrophe. This provided a unique opportunity to investigate the impacts of flood events on the Baltic Sea ecosystem. About 9 km 3 of water discharged into the Pomeranian Bight within only 5 weeks, while the normal mean water discharge of the Oder River is 18 km 3 /year. The floodwaters reached the inner coastal Oder Lagoon on July 20. On July 28, low salinity ( < 3) and a high suspended particulate material (SPM) content (11 mg/dm 3 ) measured at a Baltic Sea coastal station close to the mouth of the Oder Lagoon indicated the presence of lagoon water that had been displaced by the floodwaters. In this water, about 88.9% of Hg and Cd, 94.3% of Pb, 55.5% of Co and 22.5% of Cu were associated with particles, while at the reference station 152 (Arkona Basin), this proportion was much lower (due to 10 times lower particle concentrations). On July 31, lower salinity and SPM concentrations at the coastal station indicated that the floodwaters had reached the Baltic Sea. The percentage of metals bounded to particles had decreased to 66% for Hg, 61% for Cd, 58% for Pb, 37% for Co and 11% for Cu. Compared to the summer of 1995, the mean Hg and Cd discharge during the flood increased by a factor of 4.5-5 due to the 4.5 times higher water discharge. The discharge of Pb increased by a factor of 5.5 and that of Cu and Co by a factor of 4. The inflowing water was rapidly diluted in the Pomeranian Bight, with stations more distance from the mouth of the lagoon showing much lower concentrations. A significant reverse relationship between salinity and the trace metal contents of Pb, Cu, Co and Mn demonstrated conservative mixing during the flood event. Only for dissolved cadmium (Cddiss) was a positive linear relationship with the salinity observed, which is attributed to the Cddiss uptake by the particle-rich flood waters. No elevated concentrations were measured. In conclusion, the trace metals in the floodwaters are not likely to have caused additional adverse effects in the Baltic Sea ecosystem.

Revisiting the biogeochemistry of arsenic in the Baltic Sea: Impact of anthropogenic activity

With the increase in anthropogenic environmental disruption, the behavior of arsenic in the Baltic Sea has received more scientific attention because of its complex forms and toxicity, and was re-visited to determine if there have been measurable changes recently. A cruise was conducted in 10-19 May 2011 to investigate the species and distribution of total dissolved inorganic arsenic (TDIAs: [TDIAs]=[As(V)]+[As(III)]) revealing links between the hydrographic dynamics and biological/chemical reactions in the Baltic Sea. In addition, long-term (2002-2010) time-series investigations of particulate arsenic in the Gotland Basin were also conducted in February every year for monitoring purposes. The behavior of TDIAs was non-conservative due to the removal and regeneration processes occurring in the Baltic Sea. Biological scavenging plays a dominant role as sink for TDIAs, with removal amount of 3.1±1.6nmol/L above the pycnocline of the Baltic Sea. Significant regeneration of TDIAs was observed below the pycnocline of the Baltic Sea, which was closely related to hypoxia. The decomposition of organic arsenic and release from the sediment by desorption of As-bearing Fe and Mn oxides were thought to be two major sources for TDIAs regeneration. The median concentration of TDIAs (8.4nmol/L) was much lower than in most marginal seas and oceans, including the near-bottom water around a chemical weapon dumpsite (13.9nmol/L). The hypoxia in the deep water contributed to the increase in As(III) concentrations based on the relationship between As(III)/TDIAs ratio and apparent oxygen utilization. If the difference of As(III) profiles (1981 and 2011) actually represents a long-term increase in As(III) concentrations and a shoaling of the As(III) chemocline, these factors could enhance the toxic effects and extend the residence time of arsenic and, hence, potentially have negative impacts on fisheries and ecosystem health in the Baltic Sea.