Baltic Sea Surface Water Research Articles

Multidrug-resistant high-risk clonal Escherichia coli lineages occur along an antibiotic residue gradient in the Baltic Sea

The global spread of multidrug-resistant Escherichia coli threatens human, animal, and environmental health. Despite the recognition of water bodies as potential reservoirs, research on antimicrobial resistance (AMR) and antibiotic residues in the Baltic Sea is limited. In this study, we examined the presence of ESBL-producing E. coli in surface water in northeastern Germany. We then conducted phenotypic and genotypic analyses of the strains, which encompassed AMR and heavy metal/metalloid tolerance, biofilm formation, and detailed plasmid examination. In addition, we used solid-phase extraction with ultra-high performance liquid chromatography and mass spectrometry to quantify antibiotic residues. Our analysis identified 30 ESBL-producing E. coli isolates, some of which were associated with clinically relevant high-risk clonal lineages. We detected antibiotic residues in the environmental samples, but the antibiotic concentrations were significantly lower compared to wastewater samples. These results suggest that the Baltic Sea surface water could serve as a reservoir for ESBL-producing E. coli and clinically relevant antibiotics.

Climate change induces shifts in coastal Baltic Sea surface water microorganism stress and photosynthesis gene expression.

The world's oceans are challenged by climate change linked warming with typically highly populated coastal areas being particularly susceptible to these effects. Many studies of climate change on the marine environment use large, short-term temperature manipulations that neglect factors such as long-term adaptation and seasonal cycles. In this study, a Baltic Sea 'heated' bay influenced by thermal discharge since the 1970s from a nuclear reactor (in relation to an unaffected nearby 'control' bay) was used to investigate how elevated temperature impacts surface water microbial communities and activities. 16S rRNA gene amplicon based microbial diversity and population structure showed no difference in alpha diversity in surface water microbial communities, while the beta diversity showed a dissimilarity between the bays. Amplicon sequencing variant relative abundances between the bays showed statistically higher values for, e.g., Ilumatobacteraceae and Burkholderiaceae in the heated and control bays, respectively. RNA transcript-derived activities followed a similar pattern in alpha and beta diversity with no effect on Shannon's H diversity but a significant difference in the beta diversity between the bays. The RNA data further showed more elevated transcript counts assigned to stress related genes in the heated bay that included heat shock protein genes dnaKJ, the co-chaperonin groS, and the nucleotide exchange factor heat shock protein grpE. The RNA data also showed elevated oxidative phosphorylation transcripts in the heated (e.g., atpHG) compared to control (e.g., atpAEFB) bay. Furthermore, genes related to photosynthesis had generally higher transcript numbers in the control bay, such as photosystem I (psaAC) and II genes (psbABCEH). These increased stress gene responses in the heated bay will likely have additional cascading effects on marine carbon cycling and ecosystem services.

Validation of measurements of pine pollen grain concentrations in Baltic Sea waters

Each spring, pine pollen coats considerable expanses of Baltic Sea surface waters. Measurements have shown that there are areas where its concentrations in this surface layer are so high that they are the dominant constituent of the suspended particulate matter (SPM) (Pawlik and Ficek, 2016). It then determines to a large extent the optical properties of the water surface, inter alia by modifying the sea colour. To date, however, the concentration of this constituent in the marine environment has rarely been studied, and its presence is not accounted for in the satellite algorithms used to define the composition and properties of sea water. This may well be the source of substantial errors in the remote sensing of the optical properties of the water and the measurement of concentrations of the optically important constituents it contains (chlorophyll a, TSM, CDOM). Measuring the concentration of pollen suspensions in Baltic Sea water, which often contains prodigious amounts of other SPM, is a daunting experimental challenge. Firstly, we characterized the pollen from pine trees growing near the southern shores of the Baltic Sea (northern Poland) using a microscope and two instruments routinely used in oceanography for measuring SPM size distributions: the LISST-100X and the Coulter counter. The measurements and analyses showed that a correct interpretation of the LISST-100X and Coulter measurements, is sufficient to count the number of pollen grains in distilled water alone. Furthermore, our laboratory analysis of the particle size distribution spectra enabled the fraction due only to pine pollen grains to be separated from the overall SPM. We then tested our method of analysing the SPM composition, which showed that the LISST-100x instrument is both a useful and an effective means for the in situ detection of the pine pollen that one sees in spring in Baltic waters.

Parameterization of phytoplankton spectral absorption coefficients in the Baltic Sea: general, monthly and two-component variants of approximation formulas

Abstract. This paper presents approximate formulas (empirical equations) for parameterizing the coefficient of light absorption by phytoplankton aph(λ) in Baltic Sea surface waters. Over a thousand absorption spectra (in the 350–750 nm range), recorded during 9 years of research carried out in different months of the year and in various regions of the southern and central Baltic, were used to derive these parameterizations. The empirical material was characterized by a wide range of variability: the total chlorophyll a concentration (Tchl a) varied between 0.31 and 142 mg m−3, the ratio of the sum of all accessory pigment concentrations to chlorophyll a (∑Ci/Tchla) ranged between 0.21 and 1.5, and the absorption coefficients aph(λ) at individual light wavelengths varied over almost 3 orders of magnitude. Different versions of the parameterization formulas were derived on the basis of these data: a one-component parameterization in the “classic” form of a power function with Tchl a as the only variable and a two-component formula – the product of the power and exponential functions – with Tchl a and ∑Ci/Tchla as variables. We found distinct differences between the general version of the one-component parameterization and its variants derived for individual months of the year. In contrast to the general variant of parameterization, the new two-component variant takes account of the variability of pigment composition occurring throughout the year in Baltic phytoplankton populations.

Dissection of Microbial Community Functions during a Cyanobacterial Bloom in the Baltic Sea via Metatranscriptomics

Marine and brackish surface waters are highly dynamic habitats that undergo repeated seasonal variations in microbial community composition and function throughout time. While succession of the various microbial groups has been well investigated, little is known about the underlying gene-expression of the microbial community. We investigated microbial interactions via metatranscriptomics over a spring to fall seasonal cycle in the brackish Baltic Sea surface waters, a temperate brackish water ecosystem periodically promoting massive cyanobacterial blooms, which have implications for e.g. primary production, nutrient cycling, and expansion of hypoxic zones. Network analysis of the gene expression of all microbes from 0.22 - 200 µm in size and of the major taxonomic groups dissected the seasonal cycle into four components that comprised genes peaking during different periods of the bloom. Photoautotrophic nitrogen-fixing Cyanobacteria displayed the highest connectivity among the microbes, in contrast to chemoautotrophic ammonia-oxidizing Thaumarchaeota, while heterotrophs dominated connectivity among pre- and post-bloom peaking genes. The network was also composed of distinct functional connectivities, with an early season balance between carbon metabolism and ATP synthesis shifting to a dominance of ATP synthesis during the bloom, while carbon degradation, specifically through the glyoxylate shunt, characterized the post-bloom period, driven by Alphaproteobacteria as well as by Gammaproteobacteria of the SAR86 and SAR92 clusters. Our study stresses the exceptionally strong biotic driving force executed by cyanobacterial blooms on associated microbial communities in the Baltic Sea and highlights the impact cyanobacterial blooms have on functional microbial community composition.

Occurrence of diverse alkane hydroxylase alkB genes in indigenous oil-degrading bacteria of Baltic Sea surface water

Formation of specific oil degrading bacterial communities in diesel fuel, crude oil, heptane and hexadecane supplemented microcosms of the Baltic Sea surface water samples was revealed. The 475 sequences from constructed alkane hydroxylase alkB gene clone libraries were grouped into 30 OPFs. The two largest groups were most similar to Pedobacter sp. (245 from 475) and Limnobacter sp. (112 from 475) alkB gene sequences. From 56 alkane-degrading bacterial strains 41 belonged to the Pseudomonas spp. and 8 to the Rhodococcus spp. having redundant alkB genes. Together 68 alkB gene sequences were identified. These genes grouped into 20 OPFs, half of them being specific only to the isolated strains. Altogether 543 diverse alkB genes were characterized in the brackish Baltic Sea water; some of them representing novel lineages having very low sequence identities with corresponding genes of the reference strains.

Genomics and Ecophysiology of Heterotrophic Nitrogen-Fixing Bacteria Isolated from Estuarine Surface Water.

ABSTRACTThe ability to reduce atmospheric nitrogen (N2) to ammonia, known as N2 fixation, is a widely distributed trait among prokaryotes that accounts for an essential input of new N to a multitude of environments. Nitrogenase reductase gene (nifH) composition suggests that putative N2-fixing heterotrophic organisms are widespread in marine bacterioplankton, but their autecology and ecological significance are unknown. Here, we report genomic and ecophysiology data in relation to N2 fixation by three environmentally relevant heterotrophic bacteria isolated from Baltic Sea surface water: Pseudomonas stutzeri strain BAL361 and Raoultella ornithinolytica strain BAL286, which are gammaproteobacteria, and Rhodopseudomonas palustris strain BAL398, an alphaproteobacterium. Genome sequencing revealed that all were metabolically versatile and that the gene clusters encoding the N2 fixation complex varied in length and complexity between isolates. All three isolates could sustain growth by N2 fixation in the absence of reactive N, and this fixation was stimulated by low concentrations of oxygen in all three organisms (≈4 to 40 µmol O2 liter−1). P. stutzeri BAL361 did, however, fix N at up to 165 µmol O2 liter−1, presumably accommodated through aggregate formation. Glucose stimulated N2 fixation in general, and reactive N repressed N2 fixation, except that ammonium (NH4+) stimulated N2 fixation in R. palustris BAL398, indicating the use of nitrogenase as an electron sink. The lack of correlations between nitrogenase reductase gene expression and ethylene (C2H4) production indicated tight posttranscriptional-level control. The N2 fixation rates obtained suggested that, given the right conditions, these heterotrophic diazotrophs could contribute significantly to in situ rates.

A novel modeling tool with multi-stressor functionality for organic contaminant transport and fate in the Baltic Sea

The coupled physical–biogeochemical model BALTSEM, previously used to assess nutrient/carbon cycles and eutrophication in the Baltic Sea, has been expanded to include algorithms for calculations of organic contaminant environmental transport and fate. This novel model version (BALTSEM-POP) is evaluated for polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins/dibenzofurans (PCDD/Fs) and hexachlorobenzene (HCB) in Baltic Sea surface water and sediment. Modeled dissolved concentrations are usually within a factor of 2–4 of observed concentrations, however with larger deviations for furans. Calculated concentrations in particulate organic matter are less accurate (within factors of 1–700), likely due to errors in estimated pelagic biomass, particulate matter–water partitioning, and large natural variability in field data. Concentrations in sediments are usually predicted within a factor of 6. The good performance of the model illustrates its usefulness for exploration of contaminant fate in response to variations in nutrient input and climatic conditions in the Baltic Sea marine environment.

Isolation of heterotrophic diazotrophic bacteria from estuarine surface waters

The wide distribution of diverse nitrogenase (nifH) genes affiliated with those of heterotrophic bacteria in marine and estuarine waters indicates ubiquity and an ecologically relevant role for heterotrophic N2 -fixers (diazotrophs) in aquatic nitrogen (N) cycling. However, the lack of cultivated representatives currently precludes an evaluation of their N2 -fixing capacity. In this study, microoxic or anoxic N-free media were inoculated with estuarine Baltic Sea surface water to select for N2 -fixers. After visible growth and isolation of single colonies on oxic plates or in anoxic agar tubes, nifH gene amplicons were obtained from 64 strains and nitrogenase activity, applying the acetylene reduction assay, was confirmed for 40 strains. Two strains, one Gammaproteobacterium affiliated with Pseudomonas and one Alphaproteobacterium affiliated with Rhodopseudomonas were shown to represent established members of the indigenous diazotrophic community in the Baltic Sea, with abundances of up to 7.9 × 10(4) and 4.7 × 10(4) nifH copies l(-1) respectively. This study reports media for successful isolation of heterotrophic diazotrophs. The applied methodology and the obtained strains will facilitate future identification of factors controlling heterotrophic diazotrophic activity in aquatic environments, which is a prerequisite for understanding and evaluating their ecology and contribution to N cycling at local and regional scales.

Contrasting geochemical cycling of hafnium and neodymium in the central Baltic Sea

The central Baltic Sea is a marginal brackish basin which comprises anoxic bottom waters and is surrounded by geological source terrains with a wide variety of compositions and ages. This allows the investigation of water mass mixing using radiogenic isotope compositions of Nd and Hf as well as their geochemical cycling across varying redox conditions in the water column. In this study, we present the distribution of Nd and Hf concentrations and their isotopic compositions for 6 depth profiles and 3 surface water sites obtained during a cruise in the central Baltic Sea onboard the RV Oceania as a part of the international GEOTRACES program.The results obtained indicate that Nd isotopes effectively trace the mixing between more radiogenic saline waters from the south and unradiogenic fresh waters from the north, which helps to understand the reliability of Nd isotopes as water mass tracer in the open ocean. In surface waters, Nd shows higher concentrations and less radiogenic isotope compositions at the northern stations, which are progressively diluted and become more radiogenic to the south, consistent with the counterclockwise circulation pattern of central Baltic Sea surface waters. In contrast to the variable Nd concentrations, Hf shows much less variability. At the Gotland Deep station, the Nd concentrations of the euxinic waters are higher by a factor >10 than those of the overlying oxygen-depleted waters, whereas Hf only shows small concentration variations. This indicates faster removal of Hf from the water column than Nd. Moreover, the dissolved Hf isotope signatures document great variability but no consistent mixing trends. Our explanation is that Hf has a lower residence time than Nd, and also that the Hf isotope signatures of the sources are highly heterogeneous, which is attributed to their differing magmatic and tectonic histories as well as incongruent post-glacial weathering around the central Baltic Sea.

Limnobacter spp. as newly detected phenol-degraders among Baltic Sea surface water bacteria characterised by comparative analysis of catabolic genes

A set of phenol-degrading strains of a collection of bacteria isolated from Baltic Sea surface water was screened for the presence of two key catabolic genes coding for phenol hydroxylases and catechol 2,3-dioxygenases. The multicomponent phenol hydroxylase (LmPH) gene was detected in 70 out of 92 strains studied, and 41 strains among these LmPH+ phenol-degraders were found to exhibit catechol 2,3-dioxygenase (C23O) activity. Comparative phylogenetic analyses of LmPH and C23O sequences from 56 representative strains were performed. The studied strains were mostly affiliated to the genera Pseudomonas and Acinetobacter. However, the study also widened the range of phenol-degraders by including the genus Limnobacter. Furthermore, using a next generation sequencing approach, the LmPH genes of Limnobacter strains were found to be the most prevalent ones in the microbial community of the Baltic Sea surface water. Four different Limnobacter strains having almost identical 16S rRNA gene sequences (99%) and similar physiological properties formed separate phylogenetic clusters of LmPH and C23O genes in the respective phylogenetic trees.

A model of life cycle, connectivity and population stability of benthic macro-invertebrates in the North Sea/Baltic Sea transition zone

The life cycle of many benthic macro-invertebrates is characterised by a planktonic life stage where larvae are transported by ocean currents and mixing and after some time they reach a stage where they settle onto the ocean bottom and, provided a suitable habitat is available, initiate their benthic life stage as juvenile organisms and complete their life cycle when they mature and spawn as adults. Such age-specific behaviour is in general difficult to include in large scale ocean models unless dispersal is considered on an individual basis, i.e. individual based model coupled with a Lagrangian description of the flow field. However, here we address this issue in an Eulerian framework and develop and apply a new life cycle and population dynamical model of benthic fauna and implement the model in a high-resolution three-dimensional circulation model of the North Sea/Baltic Sea transition zone. The model explicitly describes specific life stages of the population and considers the different processes affecting the organisms during their life cycle, e.g. spawning, dispersion and settling. The model considers different life stages of an idealised marine organism, representing a typical benthic macro-invertebrate species in the area. Populations of juvenile and adult benthic organisms are maintained by spawning, occurring regularly every spring, and subsequent settling of larvae. The pelagic larval stages are simulated by a larval concentration distribution function, i.e. discrete age-classes of the total larval concentration, and age-specific physiological processes, as the onset of their settling behaviour, is explicitly accounted for in the model. Model simulations show, in general, a large connectivity between habitats in the northern and southern part of the area but also that self-recruitment is sufficient to sustain the two populations independently. A sensitivity study were carried out with the spawning rate as a control parameter and two non-trivial quasi-stationary steady states of benthic biomass distributions were identified, characterised by a high and low distribution of organisms in the area, respectively. A stability diagram identifies a bifurcation point when the spawning rate is reduced by 65% and where lower spawning rates implies two different stable equilibria. The existence of multiple quasi-stationary steady states can be explained by the general circulation in the area: when spawning into the surface layer takes place from recruitment areas close to the North Sea where the out-flowing Baltic Sea surface water hinder the southward transport of pelagic larvae. The existence of multiple equilibria support the hypothesis of regime shifts in coupled physical-biological systems where modest changes in critical processes causes rapid and extensive structural changes in the ecosystem. Such changes could occur if a system is close to a bifurcation point such that small changes in critical internal biotic dynamics or in environmental conditions force the system into a new equilibria, for example due to hypoxia or changes in temperature or salinity. Finally, it is shown that model simulations of periods with hypoxic bottom water masses reduces the total benthic biomass distribution in the area significantly.

Observation of eddy structures in the Baltic Sea with the use of radiolocation and radiometric satellite data

The manifestations of mesoscale and submesoscale eddy elements in surface currents of the Baltic Sea in satellite images differing in physical nature and spatial resolution are considered. The investigation is based on the Envisat ASAR and ERS-2 SAR high-resolution radiolocation images (RLIs) obtained in 2008–2009 for different sites of the Baltic Sea water area and the use of the Envisat MERIS and Landsat ETM+ radiometric images of the visible spectrum. Possible mechanisms of the appearance of eddy structures in RLIs of the Baltic Sea water area are considered. Joint analysis of the mentioned data revealed specific features of the appearance of eddy structures in satellite images, taking into account the variability of optical characteristics of Baltic Sea surface waters during the summer blooming of cyanobacteria and the spring blooming of diatomic algae.

Perfluorinated compounds in marine surface waters: data from the Baltic Sea and methodological challenges for future studies

Environmental context.Perfluorinated compounds are man-made chemicals of emerging environmental concern because of their global distribution in water, air and biota. We investigate the distribution of these chemicals in surface water of the Baltic Sea, a unique ecosystem and the world’s largest body of brackish water. The observed contamination was of the same order of magnitude as classical persistent organic pollutants in the world’s oceans, and decreased with lower population density in the Baltic Sea catchments. Abstract.Poly- and perfluorinated compounds (PFCs) are chemicals of emerging environmental concern. Except for very few coastal sites, PFC contamination of the Baltic Sea has not been investigated. In order to assess the PFC contamination of Baltic Sea water and evaluate the spatial distribution of PFCs, 74 surface water samples from the entire Baltic Sea were taken during two sampling campaigns in the summer of 2008 and analysed for PFCs. Of 40 analysed PFCs, 13 were detected at concentrations below 1 ng L–1, which indicates a rather low PFC contamination of Baltic Sea surface water. Usually, PFOA was the analyte observed in highest concentrations followed by PFNA, PFBS, and PFOS. PFC concentrations decreased from the Kattegat to the Bothnian Bay and the Gulf of Finland, reflecting the decreasing population density and thus the potential contamination in corresponding catchments or the decreasing influence of potentially contaminated North Sea water.

Dissolved iron (II) in the Baltic Sea surface water and implications for cyanobacterial bloom development

Abstract. Iron chemistry measurements were conducted during summer 2007 at two distinct locations in the Baltic Sea (Gotland Deep and Landsort Deep) to evaluate the role of iron for cyanobacterial bloom development in these estuarine waters. Depth profiles of Fe(II) were measured by chemiluminescent flow injection analysis (CL-FIA). Up to 0.9 nmol Fe(II) L−1 were detected in light penetrated surface waters, which constitutes up to 20% to the dissolved Fe pool. This bioavailable iron source is a major contributor to the Fe requirements of Baltic Sea phytoplankton and apparently plays a major role for cyanobacterial bloom development during our study. Measured Fe(II) half life times in oxygenated water exceed predicted values and indicate organic Fe(II) complexation. Potential sources for Fe(II) ligands, including rainwater, are discussed. Fe(II) concentrations of up to 1.44 nmol L−1 were detected at water depths below the euphotic zone, but above the oxic anoxic interface. Mixed layer depths after strong wind events are not deep enough in summer time to penetrate the oxic-anoxic boundary layer. However, Fe(II) from anoxic bottom water may enter the sub-oxic zone via diapycnal mixing and diffusion.

Modelling the uptake and release of carbon dioxide in the Baltic Sea surface water

We present and analyse a fully coupled physical–biogeochemical model of the uptake and release of carbon dioxide in the Baltic Sea. The modelling includes the interaction between physical (stratification, temperature, salinity, penetration of solar radiation, and ice), chemical (total alkalinity, pH, dissolved inorganic carbon, oxygen, and nutrients), and biological processes (plankton and dissolved organic carbon (DOC)). These processes have been built into an advanced process-oriented coupled basin ocean model that has been extensively explored and validated for the Baltic Sea. The model captures major physical–chemical and biological response patterns, as evaluated based on observations from the central Baltic Sea, and illustrate the need to include fractional nutrient release in the photic zone for consistency with CO 2 observations. The study indicates that long-term values of the water partial pressure of carbon dioxide were above atmospheric values before industrialization, with a net release of CO 2 to the atmosphere. Seasonal variability increased in the modern industrialization era with the inclusion of eutrophication, making the Baltic Sea both a sink and source of CO 2 to the atmosphere. Modelling long-term variations in pH indicates the existence of stable conditions before industrialization and slight decrease due to increased atmospheric carbon dioxide concentrations during industrialization. Eutrophication effects may have damped acidification, but have caused increased seasonal pH variability with low values occurring during winter season.

The process of self-purification of the Baltic Sea waters from artificial radionuclides

An analysis of the variations in the concentrations of 137Cs and 90Sr radionuclides in the Baltic Sea surface waters after the accident at the Chernobyl nuclear power plant was performed. An instability of the 137Cs concentration values during the short-term observations was found, when these values were differed 2-to 3-fold. The concentrations of 90Sr appeared to be more stable; meanwhile, their deviations sometimes exceeded the ranges of the experimental errors. By the variations in the monthly average values of the radionuclide concentrations in the surface waters of the Baltic Sea in 1989–1995, no trend of the water self-purification was observed. The theoretical results obtained confirmed the potential of the formation and propagation of patches with increased concentrations of 137Cs in the southeastern part of the Baltic Sea. The most reliable factor that controlled the process of self-purification of the Baltic Sea water appeared to be the mean annual value of the concentration of radionuclides. Pronounced divergences were obtained between the measured and calculated mean annual concentrations of 137Cs and 90Sr radionuclides in the surface waters of the Baltic Sea in 1989–2001. These divergences are explained by the potential influence of the waters from the Gulf of Bothnia and by other additional supplies of radionuclides to the marine environment, which were not included into the mathematical models.

Bioconcentration of persistent organic pollutants in four species of marine phytoplankton

The uptake of polychlorinated biphenyls (PCBs) was studied in four species of marine algae. A novel experimental system to establish and maintain constant dissolved concentrations of PCBs was employed. Headspace sampling was used to verify that the freely dissolved concentrations remained constant with time. The headspace analysis also allowed sorption to dissolved organic carbon (DOC) to be quantified for all but the most lipophilic PCB congeners. Equilibration with the dissolved phase was rapid for three of the four algae species (<1 d for the majority of congeners). Organic carbon-normalized algae/water partition coefficients (KAlgW) were similar for three of the four species, but were lower by a factor of 10 to 20 for Phaeodactylum tricornutum. The KAlgW values of the first three species were similar to the octanol/water partition coefficient (Kow) for those PCB congeners for which DOC sorption could be quantified. These KAlgW values also agreed well with organic carbon-normalized bioconcentration factors for PCBs in suspended particulate matter (BCF(SPM)) sampled in Baltic Sea surface water during the summer.

Size distribution of colloidal trace metals and organic carbon during a coastal bloom in the Baltic Sea

The physico-chemical speciation of organic carbon and selected metals was measured during a coastal bloom in Ekhagen Bay, Baltic Sea, using ultrafiltration. One important objective with the study was to see if any depletion of trace metals could be measured in the directly bioavailable fraction (<1000 Da, the soluble low molecular weight fraction, LMW) during a plankton bloom. Filters with five different cut-offs were used (1 kD (1000 Da), 5 kD, 10 kD, 100 kD and 0.22 μm) in order to delineate the size distribution of colloidal organic carbon (COC) and trace metals. During the bloom in May, LMW Al, Co, Cu, Mn and Ni concentrations decreased although the colloidal and particulate concentrations were relatively high. Data show that desorption of colloidal and particulate bound trace metals to the LMW fraction was slower than the process depleting the LMW fraction. Estimates of the maximum active uptake of Cu, Ni and Mn by the phytoplankton, and the loss of non-bioactive Al from the LMW fraction, indicate that processes other than active uptake by phytoplankton must contribute to the observed depletion of trace metals in the LMW fraction. Hence, in order to estimate the bioavailable pool of trace metals for plankton during bloom conditions, these other processes must be understood and quantified. Transparent Exopolymeric Particles (TEP, reflecting sugar-rich phytoplankton exudates) increased around eight times during the plankton bloom. We hypothesize that the formation of TEP is a process that might be important for the transfer of trace metals from the LMW to the particulate fraction during the phytoplankton bloom, but the significance of TEP for this depletion in Baltic Sea surface water remains to be shown.

Further Evidence for a Drastic Decline of Potentially Hazardous Trace Metals in Baltic Sea Surface Waters

We present results of a Baltic Sea surface water study on selected dissolved trace metals (< 0.4 μm ) in which we compare our ‘historical’ data of 1982 with measurements obtained from samples collected in 1995 in the same area. The methods of collection, storage and analysis followed the same procedures with the exception of Zn. Between 1982 and 1995, significant concentration decreases have been observed for Pb (≈73%), Cd (≈66%), Zn (≈23%), Cu (≈19%) and Ni (≈18%) in the Baltic proper. Concentration decreases of 5–6% yr −1 for Cd and Pb, and 1.4–1.8% yr −1 for Ni, Cu and Zn have been observed in the Bornhom and Gotland Seas. We attribute these changes to reduced input from river, waste waters, and atmospheric deposition. The study also reveals that short-term or seasonal variations in the concentrations of these elements are small in Baltic surface waters. In contrast, strong seasonal variations of Mn are observed in open Baltic surface waters.