North Sea Water Research Articles

Going with the flow: Tidal influence on the occurrence of the harbour porpoise (Phocoena phocoena) in the Marsdiep area, The Netherlands

One of the most important factors explaining the distribution and behaviour of coastal marine mammals are tides. Tidal forces drive a large number of primary and secondary processes, such as changes in water depth, salinity, temperature, current velocity and direction. Unravelling which tidal process is the most influential for a certain species is often challenging, due to a lack of observations of all tide related covariates, strong correlation between them, and the elusive nature of most marine organisms which often hampers their detection.In the Marsdiep area, a tidal inlet between the North Sea and the Dutch Wadden Sea, the presence of harbour porpoises (Phocoena phocoena) was studied as a function of tide related covariates. Observations were carried out in early spring from a ferry crossing the inlet on a half hourly basis. Environmental and sightings data were collected by one observer, while an on-board Acoustic Doppler Current Profiler (ADCP) and temperature sensor continuously recorded current velocity profiles and temperature, respectively. Sea surface temperature and salinity were measured at a nearby jetty. Sightings (n=134) were linked to tidal elevation, geographical position, local depth-averaged current velocity, water temperature (with and without trend correction) and salinity.Variation in sighting rate was best described by salinity, with highest sighting rate at high levels of salinity (>30gkg−1), indicating that porpoises enter the area in bodies of (more saline) North Sea water. Second best variable was time of day, with the highest sighting rate early morning, and decreasing during the day. However, surveys in the morning happened to coincide more often with high water and hence, the apparent time of day effect could be due to collinearity. Most porpoises were present in the northern part of the Marsdiep, particularly during high tide.Tide dependent sighting rates confirmed that porpoises reside in the North Sea, and enter the western Wadden Sea during the flood and leave during ebb. This tidal influx is most likely related to prey availability, which corresponds to other recent studies in this area showing higher fish abundance during high tide. Documenting information on tide related patterns could be used in practice, when e.g. planning anthropogenic activities or assessing critical habitats for this species.

Stratified and nonstratified areas in the N orth S ea: Long‐term variability and biological and policy implications

The European Unions' Marine Strategy Framework Directive aims to limit anthropogenic influences in the marine environment. But marine ecosystems are characterized by high variability, and it is not trivial to define its natural state. Here, we use the physical environment as a basis for marine classification, as it determines the conditions in which organisms must operate to survive and thrive locally. We present a delineation of the North Sea into five distinct regimes, based on multidecadal stratification characteristics. Results are based on a 51 year simulation of the region using the coupled hydrobiogeochemical model GETM-ERSEM-BFM. The five identified regimes are: permanently stratified, seasonally stratified, intermittently stratified, permanently mixed, and Region Of Freshwater Influence (ROFI). The areas characterized by these regimes show some interannual variation in geographical coverage, but are overall remarkable stable features within the North Sea. Results also show that 29% of North Sea waters fail to classify as one of the defined stratification regimes, due to high interannual variability. Biological characteristics of these regimes differ from diatom-based food webs in areas with prolonged stratification to Phaeocystis-dominated food webs in areas experiencing short-lived or no stratification. The spatial stability of the identified regimes indicates that carefully selected monitoring locations can be used to represent a substantive area of the North Sea.

Spatial and temporal variability of sea surface temperature in the Baltic Sea based on 32-years (1982–2013) of satellite data

Summary Satellite measurements provide synoptic view of sea surface temperature (SST) and can be used to trace global and regional climate trends. In this study we have examined the multiyear trends and variability of the Baltic Sea SST using 32-years (1982–2013) of satellite data. Our results indicate that there is a statistically significant trend of increasing SST in the entire Baltic Sea, with values ranging from 0.03 to 0.06°C year−1, depending on the location. SSTs averaged over the entire Baltic Sea increase at the rate of 0.05°C year−1. Higher values of SST trend are generally present in the summer months, while trend is not statistically significant in the winter months. The seasonal cycle of SST in the Baltic Sea is characterized by well-defined winter and summer seasons. The average amplitude (16–18°C) of this cycle is significantly larger than in the North Sea waters located at the same latitudes as the Baltic Sea. The analyzed data set also highlights considerable interannual SST variability, which is coherent in different regions of the Baltic Sea and significantly correlated with interannual variability of the air temperature. SST variability in the Baltic Sea in winter can be linked to the North Atlantic Oscillation index.

Status of 236U analyses at ETH Zurich and the distribution of 236U and 129I in the North Sea in 2009

Compact, low energy accelerator mass spectrometry (AMS) has evolved over the past years as one of the most sensitive, selective, and robust techniques for the analysis of heavy and long lived radionuclides. In this study, we will first focus on the analytical capabilities of the compact AMS system TANDY, mainly for 236U analyses, and then present a new dual tracer approach, that combines 129I and 236U. The measured 129I/236U ratios of samples collected in the North Sea in 2009 are in reasonable agreement with the expectations from documented or estimated releases from the two major nuclear reprocessing plants located at Sellafield (GB) and La Hague (F), suggesting that the 129I/236U ratio can be used as a water mass tag in the North Atlantic region. However, our results indicate that, in contrast to 129I, additional contributions of bomb produced 236U cannot be neglected in the North Sea region. This complicates the simple and straight forward use of the 129I/236U ratio as a quantitative tool for the calculation of transport times of North Sea water in the Arctic Ocean.

Determination of Ti, Zr, Nb, V, W and Mo in seawater by a new online-preconcentration method and subsequent ICP–MS analysis

We present a new method for the determination of Ti, Zr, Nb, V, W and Mo in seawater by adapting the online-preconcentration procedure with the system SeaFAST and subsequent analysis by ICP–MS to these metals. The trace elements Ti, Zr, Nb and W are present in seawater in ultratrace concentrations in the range of pmolkg−1, whereas Mo and V, which are biologically essential elements, are present in the concentration range of nmolkg−1. The online-preconcentration system we used consists of an autosampler, a sample loop, a preconcentration column and two valves controlling the sample loop and the preconcentration processes. The pre-packed preconcentration column contains a chelating resin with two functional groups, ethylenediamine triacetic acid (EDTriA) and iminodiacetic acid (IDA), immobilized on a polymer backbone. The preconcentration process was optimized for loading the sample and subsequent rinsing to remove residues of seawater matrix prior to elution with the optimized elution acid (0.5M HNO3–0.002M HF). We used acidified North Sea seawater (0.02M HCl–0.002M HF) for the method development. Samples and calibration standards were loaded onto the preconcentration column and after elution directly transferred to the quadrupole ICP–MS and measured immediately. Best results were achieved with matrix-matched calibration standards (0.6M NaCl–0.02M HCl–0.002M HF) simulating acidified seawater samples. Titanium, Zr, Nb and W are measured simultaneously in one run, whereas V and Mo are determined simultaneously in a separate run. Low procedure blanks were calculated for Ti, Zr, Nb, W (635, 14.5, 1.35, 10.2pmolkg−1) and for V and Mo, 9.79 and 5.61nmolkg−1, respectively. Very good spike recoveries achieved with spiked North Sea water demonstrate the applicability for all six elements. Analysis of the seawater standard NASS-6 gave recoveries of 97–99% (0.9–2.2% standard deviation, SD) for Mo and V. We also provide Zr, Nb and W data for this reference standard. The applicability of the on-line preconcentration method was demonstrated for samples collected during a GEOTRACES cruise in the Atlantic. Due to the low limit of quantification, LOQ, and SD, this method enables the determination of ultratrace concentration levels for Ti, Zr, Nb and W as well as the detection of small variations in concentration for all six elements. The required sample volume of only 50mL, the fast determination within 1h for all elements and the principle of a closed system are a clear advantage of the analytical procedure compared to many other methods and facilitates sample logistics for the determination of Ti, Zr, Nb, V, W and Mo in seawater.

Occurrence of flow parallel and flow transverse bedforms in Fehmarn Belt (SW Baltic Sea) related to the local palaeomorphology

This study describes the spatial distribution of flow-parallel sand ribbons and flow-transverse large and very large subaqueous dunes in the south-western Baltic Sea offshore Fehmarn Island between 13m and 20m water depth, based on hydroacoustic and grain size data. The system of sand ribbons and dunes is intermittently active due to currents induced during major inflows of the North Sea water into the Baltic Sea. The sand ribbons are located on a lag deposit on top of glacial till, while the dunes rest on top of drowned Holocene nearshore deposits. The sand ribbons reach heights between 0.4m and 0.6m, with widths varying between 60m and several hundreds of metres. The observed dunes have heights between 0.09m and 2.35m, while their wavelengths range from 17m to 120m. Offshore Fehmarn Island, the transition from sand ribbons to dunes is most likely linked to a contrast in sediment supply, as reworked drowned nearshore deposits provide sediment available for transport in significantly larger amounts than glacial till. Similar to an earlier approach for river bed states, the dimensionless thickness of sediment available for transport is able to differentiate between the bed states.

External total alkalinity loads versus internal generation: The influence of nonriverine alkalinity sources in the Baltic Sea

AbstractIn this study we first present updated riverine total alkalinity (TA) loads to the various Baltic Sea sub‐basins, based on monthly measurements in 82 of the major rivers that represent 85% of the total runoff. Simulations in the coupled physical‐biogeochemical BALTSEM (BAltic sea Long‐Term large Scale Eutrophication Model) model show that these river loads together with North Sea water inflows are not sufficient to reproduce observed TA concentrations in the system, demonstrating the large influence from internal sources. Budget calculations indicate that the required internal TA generation must be similar to river loads in magnitude. The nonriverine source in the system amounts to about 2.4 mmol m−2 d−1 on average. We argue here that the majority of this source is related to denitrification together with unresolved sediment processes such as burial of reduced sulfur and/or silicate weathering. This hypothesis is supported by studies on sediment processes on a global scale and also by data from sediment cores in the Baltic Sea. In a model simulation with all internal TA sources and sinks switched on, the net absorption of atmospheric CO2 increased by 0.78 mol C m−2 yr−1 compared to a simulation where TA was treated as a passive tracer. Our results clearly illustrate how pelagic TA sources together with anaerobic mineralization in coastal sediments generate a significant carbon sink along the aquatic continuum, mitigating CO2 evasions from coastal and estuarine systems.

Jellyfish and Ctenophores in Limfjorden (Denmark)—Mini-Review, with Recent New Observations

Limfjorden is a major Danish water system that connects the North Sea via Thyborøn Kanal in the west and to the Kattegat in the east. Limfjorden is heavily eutrophicated and certain areas suffer from oxygen depletion each summer. Bottom-dwelling fish have disappeared as the number of jellyfish and ctenophores has increased. The abundance and predation impact of jellyfish and ctenophores in Limfjorden have been described in a number of studies conducted during the last decade, and a mini-review of this literature is given here in Section 2. The common jellyfish Aurelia aurita may, in a few years, be very abundant and exert a considerable predatory impact on zooplankton and fish larvae. Abundance, species composition, and population dynamics of A. aurita may, at irregular time intervals, be strongly influenced not only by the water brought into Limfjorden from the North Sea, but also by competition with the invasive ctenophore Mnemiopsis leidyi that occurred for the first time in extremely high numbers in 2007, brought into the fjord system with North Sea water. It has been suggested that Limfjorden may function as an incubator for M. leidyi with the potential to further seed M. leidyi into the Kattegat and adjacent Danish waters, and recent observations seem to support this hypothesis. In Section 3, we report on two bloom events of ctenophores, Pleurobrachia pileus and M. leidyi, along with their predators (Beroe spp.) in Limfjorden in the autumns of 2012 and 2013, when the usually dominating A. aurita was absent. However, the present observation of B. ovata being M. leidyi’s native predator and a new species in Limfjorden may in the future reduce the abundance of M. leidyi.

A single algorithm to retrieve turbidity from remotely-sensed data in all coastal and estuarine waters

Ocean color remote sensing has been shown to be a useful tool to map turbidity (T) and suspended particulate matter (SPM) concentration in turbid coastal waters. Different algorithms to retrieve T and/or SPM from water reflectance already exist, however there are important questions as to whether these algorithms need to be calibrated specifically for different regions. In the present work the potential generality of a semi-empirical single band turbidity retrieval algorithm using the near infrared (NIR) band at 859nm in highly turbid waters is assessed. For completeness the use of 645nm in medium to low turbidity waters is also proposed. Radiative transfer simulations and in situ measurements from various European and South American coastal and shallow estuarine environments characterized by high concentrations of suspended sediments are analyzed. Reflectance and turbidity measurements were performed in the southern North Sea (SNS) and French Guyana (FG) coastal waters, and Scheldt (SC), Gironde (GIR) and Río de la Plata (RdP) estuaries. Simulations showed that uncertainty for turbidity estimation associated with different particle types and bidirectional effects is typically less than 6%. When applied to field data from the five different sites, the semi-analytical algorithm performed well: turbidity estimates were within 12% and 22% of in situ values. A good performance was also found when the entire database was analyzed (n=106) with a mean relative error of 13.7% and bias of 4.8%. The good performance of the algorithm for all these regions, despite differences in sediment characteristics, and the results of the radiative transfer simulations suggest the global applicability of the algorithm to map turbidity up to 1000FNU. Consequently regional algorithms to retrieve SPM concentration from reflectance can be designed by combining this global algorithm to retrieve T from water reflectance with a regional relationship to convert T to SPM. This has the very practical advantage that the measurements needed to calibrate the latter T/SPM conversion for any new region are much easier and cheaper than in situ reflectance measurements.

Occurrence of the antidiabetic drug Metformin and its ultimate transformation product Guanylurea in several compartments of the aquatic cycle

In 2030, the World Health Organization estimates that more than 350million people will be diagnosed with diabetes. Consequently, Metformin – the biguanide drug of choice orally administered for diabetes type II – is anticipated to see a spike in production.Unlike many pharmaceutical drugs, Metformin (Met) is not metabolized by humans but passes through the body unchanged. Entering aquatic compartments, such as in sewage, it can be bacterially transformed to the ultimate transformation product Guanylurea (Gua).Sampling over one week (n=5) from a Southern German sewage treatment plant revealed very high average (AV) concentrations in influent (AVMet=111,800ng/L, AVGua=1300ng/L) and effluent samples (AVMet=4800ng/L, AVGua=44,000ng/L).To provide a more complete picture of the distribution and potential persistence of these compounds in the German water cycle, a new, efficient and highly sensitive liquid chromatography mass spectrometric method with direct injection was used for the measurement of Metformin and Guanylurea in drinking, surface, sewage and seawater. Limits of quantification (LOQ) ranging from 2–10ng/L allowed the detection of Metformin and Guanylurea in different locations such as: Lake Constance (n=11: AVMet=102ng/L, AVGua=16ng/L), river Elbe (n=12: AVMet=472ng/L, AVGua=9ng/L), river Weser (n=6: AVMet=349ng/L, AVGua=137ng/L) and for the first time in marine North Sea water (n=14: AVMet=13ng/L, AVGua=11ng/L). Based on daily water discharges, Metformin loads of 15.2kg/d (Elbe) and 6.4kg/d (Weser) into the North Sea were calculated. Lake Constance is used to abstract potable water which is further purified to be used as drinking water. A first screening of two tap water samples contained 2ng/L and 61ng/L of Metformin, respectively. The results of this study suggest that Metformin and Guanylurea could be distributed over a large fraction of the world's potable water sources and oceans. With no natural degradation processes, these compounds can be easily reintroduced to humans as they enter the food chain.

Genetic differentiation of brackish water populations of cod Gadus morhua in the southern Baltic, inferred from genotyping using SNP-arrays

The Baltic is a semi-enclosed sea characterised by decreasing salinity in the eastern and northern direction with only the deeper parts of the southern Baltic suitable as spawning grounds for marine species like cod. Baltic cod exhibits various adaptations to brackish water conditions, yet the inflow of salty North Sea water near the bottom remains an influence on the spawning success of the Baltic cod. The eastern Baltic population has been very weakly studied in comparison with the western population.The aim of this study is to demonstrate for the first time genetic differentiation by the use of a large number of SNPs between eastern and western Baltic populations existing in differentiated salinity conditions. Two cod samples were collected from the Bay of Gdańsk, Poland and one from the Kiel Bight, Germany. Samples were genotyped using a cod derived SNP-array (Illumina) with 10 913 SNPs. A selection of diagnostic SNPs was performed. A set of 7944 validated SNPs were analysed to assess the differentiation of three samples of cod. Results indicated a clear distinctness of the Kiel Bight from the populations of the eastern Baltic. FST comparison between both eastern samples was non-significant. Clustering analysis, principal coordinates analysis and assignment test clearly indicated that the eastern samples should be considered as one subpopulation, well differentiated from the western subpopulation. With the SNP approach, no differentiation between groups containing ‘healthy’ and ‘non-healthy’ cod individuals was observed.

DOC and POC in the water column of the southern Baltic. Part I. Evaluation of factors influencing sources, distribution and concentration dynamics of organic matter

Organic substances are important components of the marine environment as they determine the properties of seawater and the key biogeochemical processes taking place in it. Organic carbon (OC) is a measure of organic matter. For practical purposes, OC is divided into dissolved organic carbon (DOC) and particulate organic carbon (POC). Both DOC and POC play a major role in the carbon cycle, especially in shelf seas like the Baltic, where their concentrations are substantial. In a three-year study (2009–2011) seawater samples for DOC and POC measurements were collected from stations located in the Gdańsk Deep, the Gotland Deep and the Bornholm Deep. The accuracy and precision of analysis were satisfactory; the recovery was better than 95%, and the relative standard deviation was 4% (n =5). Concentrations of chlorophyll a, phaeopigment a, salinity, pH and temperature were also measured in the same samples. These parameters were selected as proxies of processes contributing to DOC and POC abundance.The aim of the study was to address questions regarding the vertical, horizontal and seasonal dynamics of both DOC and POC in the Baltic Sea and the factors influencing carbon concentrations. In general, the highest concentrations of both DOC and POC were recorded in the surface water layer (DOC~4.7mgdm−3, POC~0.6mgdm−3) as a consequence of intensive phytoplankton activity, and in the halocline layer (DOC~5.1mgdm−3, POC~0.4mgdm−3). The lowest DOC and POC concentrations were measured in the sub-halocline water layer, where the values did not exceed 3.5mgdm−3 (DOC) and 0.1mgdm−3 (POC). Seasonally, the highest DOC and POC concentrations were measured during the growing season: surface DOC~5.0mgdm−3; sub-halocline DOC~4.1mgdm−3 and surface POC~∼0.9mgdm−3, sub-halocline POC~∼0.2mgdm−3. The ANOVA Kruskal-Wallis test results indicate statistically significant differences among the three study sites regarding average concentrations, and concentrations in particular water layers and seasons. It shows that concentrations of DOC and POC differ in sub-basins of the Baltic Sea. The differences were attributed to the varying distances from river mouths to study sites or the different starting times and/or durations of the spring algal blooms. Statistically significant dependences were found between both DOC and POC concentrations and Chl a (phytoplankton biomass), pH (phytoplankton photosynthetic rate), pheo (zooplankton sloppy feeding), salinity (river run-off and North Sea water inflows) and water temperature (season). This was taken as proof that these factors influence DOC and POC in the study areas.

Chlorophyll in North Sea coastal and offshore waters does not reflect long term trends of phytoplankton biomass

This study analyses long-term and seasonal changes of phytoplankton community Carbon: Chlorophyll a ratio (θ) during the period 1991 to 2010 in North Sea waters and its relationship to environmental drivers. Based on the data from the Dutch water monitoring programme covering 20years, major trends in phytoplankton abundance, community structure and chlorophyll were identified. Overall C:Chla increased during the study period, particularly in coastal areas. This increase was related to an increase in average underwater photosynthetically active radiation (PAR) and a decrease in nutrient concentrations. A mismatch was detected between chlorophyll a and biomass carbon trends caused by both a decrease in chlorophyll a values and an increase in biomass. In coastal waters, physiological adaptation to higher light and lower nutrient levels may have enhanced the θ, increasing from yearly averages of 12gC∗gChla−1 in 1990 to 69gC∗gChla−1 in 2010. Offshore, the increased stratification coincided with a shift towards dinoflagellate dominance. This dominance of dinoflagellates co-occurred with an increased θ yearly averages from 62gC∗gChla−1 in 1991 to 119gC∗gChla−1 in 2010. Because of these changes detected in C:Chl a ratio of multispecies phytoplankton communities, we question the validity of chlorophyll a as a proxy for phytoplankton biomass and argue its possible misrepresentation of phytoplankton dynamics.

A reliable DNA barcode reference library for the identification of the North European shelf fish fauna

Valid fish species identification is an essential step both for fundamental science and fisheries management. The traditional identification is mainly based on external morphological diagnostic characters, leading to inconsistent results in many cases. Here, we provide a sequence reference library based on mitochondrial cytochrome c oxidase subunit I (COI) for a valid identification of 93 North Atlantic fish species originating from the North Sea and adjacent waters, including many commercially exploited species. Neighbour-joining analysis based on K2P genetic distances formed nonoverlapping clusters for all species with a ≥99% bootstrap support each. Identification was successful for 100% of the species as the minimum genetic distance to the nearest neighbour always exceeded the maximum intraspecific distance. A barcoding gap was apparent for the whole data set. Within-species distances ranged from 0 to 2.35%, while interspecific distances varied between 3.15 and 28.09%. Distances between congeners were on average 51-fold higher than those within species. The validation of the sequence library by applying BOLDs barcode index number (BIN) analysis tool and a ranking system demonstrated high taxonomic reliability of the DNA barcodes for 85% of the investigated fish species. Thus, the sequence library presented here can be confidently used as a benchmark for identification of at least two-thirds of the typical fish species recorded for the North Sea.

On the resonance-wave mechanism of major Baltic inflows

The features of the spatiotemporal variability of the sea level in the North and Baltic seas during the periods of formation of major Baltic inflows are investigated using the analysis of satellite altimetry data. It is demonstrated that dramatic drops in the sea level between the Baltic and North seas are observed during a few weeks before major inflows. A process of intensive inflows of the North Sea water to the Baltic Sea is accompanied not only by horizontal motions but also by vertical ones manifested in the increase in convergent flows in the North Sea and divergent flows in the Baltic Sea. A pronounced feature of the low-frequency dynamics of water of the North and Baltic seas is its wave structure. In both seas, low-frequency waves with the periods of 14–36 days propagate with the eastern component of the phase velocity along the isobaths and are identified as barotropic topographic Rossby waves. Phase velocities and lengths of low-frequency waves in the Baltic Sea are smaller by several times than those in the North Sea. Using the data of the analysis of meteorological information, a resonance-wave mechanism of generation of major Baltic inflows is studied.

Estuarine suspended particulate matter concentrations from sun-synchronous satellite remote sensing: Tidal and meteorological effects and biases

Optical data from a sun-synchronous satellite were used to investigate how large-scale estuarine suspended particulate matter (SPM) concentrations were affected by tidal and bulk meteorological drivers, and how retrieved SPM is biased by tidal aliasing and sampling under clear sky conditions. Local absorption and scattering properties were used to derive surface SPM maps from 84 cloud-free ENVISAT MERIS FR reflectance images of the Westerschelde estuary (51° 30′ N, 3° 30′ E) for the period 2006–2008, and validated with in situ SPM at fixed stations (r=0.89 for geometric means). The distinctly different SPM maps were categorized for different tidal and seasonal conditions. Resulting composites reveal spatial patterns in SPM as a function of semi-diurnal tidal phase, fortnightly tidal phase, or season. For the estuary proper, tidal and seasonal effects on the variation of SPM are similar in magnitude. Observed controls for surface SPM are distance to shallow source area, tidal current velocity, and advection of North Sea and estuarine surface waters. Turbidity maxima appear only during favourable tidal and meteorological conditions. For the Westerschelde, the bias introduced by sun-synchronous sampling causes low water image acquisitions to uniquely coincide with spring tides, and high water images with neap tides. Cloud-free images were associated with low wind velocities. Simulations from a mud transport model confirmed the overestimation of geometric mean SPM from the tidal aliasing, and underestimation from fair weather. This resulted in a net relative error of −8% at the wave-exposed mouth, but biases cancelled out in the upper estuary. We argue that local biases should be considered when interpreting water quality products for estuaries and coasts around the world.

Redox-dependent changes in manganese speciation in Baltic Sea sediments from the Holocene Thermal Maximum: An EXAFS, XANES and LA-ICP-MS study

Manganese (Mn) enrichments in sediments of the deep basins of the Baltic Sea are believed to consist of Ca–Mn–carbonates that form from Mn oxides following periodic inflows of oxygen-rich North Sea waters. However, a range of Mn-bearing mineral phases, that besides Mn-carbonates (e.g. Ca-rhodochrosite), can include Mn-sulfides are known to be present in marine sediments, with formation mechanisms that are sensitive to redox conditions. In this study, we use high resolution synchrotron EXAFS and XANES combined with LA-ICP-MS and micro-XRF, to investigate the nature of the Mn enrichments in sediments from the Holocene Thermal Maximum (approx. 8000–4000cal.yrBP) at a site in the northern Gotland Basin. Analyses were performed on epoxy-embedded sediment sequences. We specifically address the role of changes in redox conditions in bottom waters, as inferred from sediment molybdenum (Mo) contents, for Mn sequestration. We find that an up-core increase in Mo, indicating a transition into more anoxic and sulfidic (euxinic) bottom water conditions, is accompanied by a decline in total sediment Mn contents. While Mn-carbonates dominate the Mn mineral fraction in the low-Mo interval, in the more sulfidic, high Mo interval, Mn is associated with framboidal pyrite. Mn/Fe ratios in the sulfidic interval vary between the investigated sequences but reach values of up to 7.7mol%. This exceeds ratios previously reported for sedimentary pyrite and EXAFS spectra indicate that sulfide bound Mn is predominately tetrahedrally coordinated when Mn/Fe ratios are high. This suggests that Mn is incorporated in other Fe–S phases such as mackinawite besides pyrite. There is also evidence for the presence of a low concentration of Mn associated with detrital aluminosilicates throughout the investigated intervals. Our results suggest that increased bottom water euxinia inhibited the formation of Mn-carbonate at this site during the Holocene Thermal Maximum, possibly due to a more rapid reduction of Mn oxides in more sulfidic bottom waters.

Annual and spatial variability in endo- and ectoparasite infections of North Sea cod (Gadus morhua Linnaeus, 1758) larvae, post-larvae and juveniles.

A parasitological investigation was performed on a total of 5380 Atlantic cod larvae, post-larvae and small juveniles sampled from the North Sea during a period of five years. The copepod Caligus elongatus (Von Nordmann, 1832) and the nematode Hysterothylacium aduncum (Rudolphi, 1802) were found at a relatively high prevalence of infection (4.6% and 5.2%, respectively). The infection by both parasites showed annual and spatial variability. C. elongatus showed a higher prevalence in 1992 compared to the following years, whereas the prevalence of H. aduncum increased from 1992 to 2001.We observed a relation between parasite distribution and parameters such as latitude and water depth. Adult digeneans (Lecithaster gibbosus and Derogenes varicus) and larval cestodes were also found with lower infection rates. Since changes of infection levels coincided with increasing North Sea water temperature in the studied period, it is hypothesized that temperature may affect parasite population levels. However, it is likely that other environmental factors may contribute to the observed variations. Absence of infection intensities higher than one nematode per fish in small larvae and post-larvae suggests that host survival may be affected by a high infection pressure. The relatively high levels of infection in the younger stages of cod, and the annual/spatial variability of these infections should be considered in the understanding of the early life dynamics of the species.

Variability of North Sea pH and CO2 in response to North Atlantic Oscillation forcing

AbstractHigh biological activity causes a distinct seasonality of surface water pH in the North Sea, which is a strong sink for atmospheric CO2 via an effective shelf pump. The intimate connection between the North Sea and the North Atlantic Ocean suggests that the variability of the CO2 system of the North Atlantic Ocean may, in part, be responsible for the observed variability of pH and CO2 in the North Sea. In this work, we demonstrate the role of the North Atlantic Oscillation (NAO), the dominant climate mode for the North Atlantic, in governing this variability. Based on three extensive observational records covering the relevant levels of the NAO index, we provide evidence that the North Sea pH and CO2 system strongly responds to external and internal expressions of the NAO. Under positive NAO, the higher rates of inflow of water from the North Atlantic Ocean and the Baltic outflow lead to a strengthened north‐south biogeochemical divide. The limited mixing between the north and south leads to a steeper gradient in pH and partial pressure of CO2 (pCO2) between the two regions in the productive period. This is exacerbated further when coinciding with higher sea surface temperature, which concentrates the net community production in the north through shallower stratification. These effects can be obscured by changing properties of the constituent North Sea water masses, which are also influenced by NAO. Our results highlight the importance of examining interannual trends in the North Sea CO2 system with consideration of the NAO state.

Seawater electrodialysis with preferential removal of divalent ions

In this work desalination of a ternary salt mixture and artificial North Sea water is studied with a lab scale electrodialysis stack, which was used in a recycling batch mode. During desalination samples were taken and the ionic composition of the dilute stream was determined. The effect of applied current density (10–300A/m2) on this composition was investigated.A clear effect of applied current density was observed. A lower applied current density leads to a more complete reduction in concentration of divalent ions, in an earlier extent of desalination. This influence of the applied current density could be related to the concentration polarization effects that occur in the diffusional boundary layer and are explained with a model based on the Nernst–Planck flux equation. It was found that the lower initial ion concentration of Ca2+, Mg2+, but also of K+, and SO42− compared to respectively Na+ and Cl−, leads to stronger depletion of these ions in the transport layer adjacent the membrane. These boundary layer effects are more pronounced at higher applied current densities, resulting in reduced transport of ions with a low initial concentration. High monovalent over divalent ion ratios can be found at low applied current.