Henry Vallius

Stable isotope signals of eutrophication in Baltic Sea sediments

Abstract Increasing δ 15 N and δ 13 C values in coastal Baltic marine sediments are evaluated as indicator of changes in the trophic status of the ecosystem. The influence of eutrophication on the δ 15 N values was found to be so dominant that it even overprints the usually observed mixing gradient from terrestrial (low isotope values) to the marine environment (high isotope values). A distinct gradient in stable nitrogen isotope values from eutrophic coastal areas to open more oligotrophic waters in the central Baltic Sea and Gulf of Bothnia was found. Our data show high δ 15 N values in surface sediments: 13‰ in the Oder Lagoon and the Pomeranian Bight, over 9‰ in the Gulf of Riga (Daugava River), 7‰ in the inner Gulf of Finland (Neva River), 6.5‰ in the Curonian Lagoon (Nemunas River), and 5.7‰ in the Gdansk Deep (Vistula River). In the Baltic Sea Proper, significantly lower δ 15 N values of 3–5‰ are found. A decrease in δ 15 N values with depth/age of the sediment was indicated in some cores that were analyzed down to 15–40 cm depth in 1-cm steps. There is a great overall difference between pre-industrial δ 15 N values in coastal sediments and recent ones of 2.3–10‰. As explanations for this increase are suggested, elevated nutrient δ 15 N values of waste water in combination with fractionation processes like nutrient uptake by phytoplankton and denitrification and nitrification processes in rivers discharging into the coastal water. Delta 13 C values in sediment surfaces off the river estuaries primarily indicate differences between the inorganic carbon signatures of the rivers. However, since the δ 13 C values also decrease downcore, we contribute this change to increased primary production caused by the enhanced nutrient load. Since both stable isotope values in sediments ( δ 13 C and δ 15 N) correlate downcore, this strongly suggests that the anthropogenic nutrient loads in the rivers might be the reason for the changes of stable isotope values.

The distribution of some heavy metals and arsenic in recent sediments from the eastern Gulf of Finland

The highest concentrations of heavy metals in the eastern Gulf of Finland are found in the Neva estuary as well as in the open sea for some metals such as Cd. Vertical profiles show lower amounts of anthropogenically-derived heavy metals in the surface sediments compared to those immediately below. This can be attributed to improved sewage water treatment in St. Petersburg during the last decade as well as to a decrease in the output of industrial waste in Russia as a result of the economic depression. The water depth of the gulf increases westwards. This enables the metals to migrate westwards under the influence of bottom currents when they are released from the sediments by strong currents as is predicted to happen when the Kotlin dam is operating.

Heavy metal deposition and variation in sedimentation rate within a sedimentary basin in Central Gulf of Finland.

In studies dealing with the chemical distribution of elements in marine soft sediments, variations in sedimentation rate within a basin can bias the interpretation of the data. A basin in the central part of the Gulf of Finland was sampled at nine sampling sites along a transect, 1.2 nautical miles long. Gravity cores of the topmost recent sediment were analysed for nitric acid leachable concentrations of heavy metals. Almost without exceptions the metal concentrations were lower in the surface sediment, indicating a decrease in pollution load during the last decade. The sedimentation rates within the basin differed substantially, from 2.5 mm a-1 to some 15 mm a-1. Without dating of the sediments, the comparison of different cores is almost impossible. Dating is essential, as is thorough investigation of the basins used in sediment monitoring.

Anthropogenically derived heavy metals in recent sediments of the Gulf of Finland, Baltic Sea

Abstract The anthropogenic loads of selected heavy metals in Recent soft sediments in the Gulf of Finland were studied through gravity corer sampling. Metals derived from silicates was excluded by applying a partial leach. Substraction of background values gave averages for the anthropogenically derived part of the heavy metal concentrations. The horizontal distribution of the metals in the surface sediments shows relatively high concentrations, especially in the eastern Gulf of Finland. The vertical distribution patterns vary from one part of the study area to another. Levels of all metals are relatively high in the upper part of the sediment profiles, usually with a decreasing trend in the topmost part of the profile. Averaged over the years 1993 to 1995 the annual accumulations of anthropogenically derived metals were, chromium 199 t, copper 158 t, cadmium 7.9 t, zinc 728 t, lead 165 t, cobalt 43.7 t and mercury 1.5 t.

Heavy metal concentrations in sediment cores from the northern Baltic Sea: declines over the last two decades.

The Baltic Sea has received considerable loads of pollutants due to industrialization in Eastern Europe. Concern for the Baltics ecological health eventually led to legislation and voluntary measures to limit pollution during the last decades of the 20th century. Heavy metal concentrations in open sea surface sediments reflected these steps to limit contaminant loads almost immediately, suggesting the possibility that the trend would continue in the ensuing years. Recent seafloor samples reveal that the declines have persisted over the past two decades. Currently, almost all heavy metal species have declined in surface sediments to levels approaching the safe limits for humans and the environment. Cadmium and mercury however remain at relatively high concentrations in many areas. Arsenic concentrations, which occur at safe levels within the Gulf of Finland persist at unacceptably high levels in surface sediments of the Bothnian Bay, and thus pose a potential threat to marine life in the area.

A case study of seasonal variation in the chemical composition of accumulating suspended sediments in the central gulf of Finland

Abstract A study was made of suspended sediments from a basin in the eastern part of the Baltic Sea, in the central Gulf of Finland. In this investigation, moored sediment traps were used to collect suspended particulate material during the half-year period from April to October 1996. An equivalent to the primary net sedimentation was estimated through studies of the variations in the content of dry matter, carbon, nitrogen, phosphorus, arsenic, cadmium, cobalt, chromium, copper, nickel, lead, titanium, vanadium, iron, manganese, aluminium and zinc in accumulated particulate matter through the period. Two algal blooms, the spring bloom and the summer bloom, were included in the period. The accumulation rate of total particulate matter (TPM) increased during the blooms by a factor of 2.5 – 3.0. In autumn there was clearly an additional period of increase in the TPM accumulation, not connected with an actual algal bloom. Metal accumulation was lowest during the spring bloom but started rising immediately after that, being highest in the autumn. Calculations show that the metal concentrations accumulated in the trap moored 20 m above the sea bottom correspond to 20–37 % of the concentrations that are deposited in the bottom sediments. According to the TPM accumulation, resuspension seems to correspond to about 77 % of the matter in the trap moored 1 1 m above the sea floor.

Applying sediment quality guidelines on soft sediments of the Gulf of Finland, Baltic Sea

The Gulf of Finland is known to have been rather largely contaminated by heavy metals during the last half of the 20th century, but indications of recovery have been reported. In order to investigate the recent levels of heavy metals and arsenic in the uppermost soft sediments of the off-shore Gulf of Finland coring of altogether 23 sites were performed. The subsamples of the cores are 605 in total and thus give a good picture of heavy metal levels in the surface sediments during the first decade of this century. In order to evaluate methods and predict sediment toxicity the sediment concentrations are compared to American SQG:s. Majority of the subsamples exceeded the threshold levels of both used SQG:s, but some exceeded also the midrange effects quality guidelines. As, Cd, Hg, and Zn concentrations still occur at unacceptably high levels in sediments of the off-shore Gulf of Finland.

Quality of the surface sediments of the northern coast of the Gulf of Finland, Baltic Sea.

The Gulf of Finland is a shallow sea where a multitude of activities compete of space. Many of the activities include reworking of the sea floor. The gulf is known to have been rather largely contaminated by heavy metals during the last century, and although indications of recovery have been reported, it is crucial to know the levels of sea floor sediment contamination before any decisions of activities are taken by the authorities. In order to predict sediment toxicity the sediment concentrations acquired during a study from 2001 to 2004 are compared to Canadian sediment quality guidelines (SQG: s), which reveal that in the majority of the subsamples the metals and arsenic exceed the threshold levels of the used SQG: s, some exceed also the probable effect level. As, Cd, Hg, and especially Zn concentrations occur at unacceptably high levels in the coastal Gulf of Finland sediments.

Seafloor Desertification – A Future Scenario for the Gulf of Finland?

The Gulf of Finland is a shallow semi-enclosed sea area which due to strong anthropogenic pressure and poor water exchange is very sensitive to eutrophication. During its whole postglacial history, the seafloor of the gulf has been periodically anoxic, and anoxia below halocline can thus be seen as a natural phenomenon. During the last decades, however, this has been accompanied by a yearly repeated seasonal anoxia in the shallower basins above halocline. This yearly repeated shallower anoxia is triggered by substantial eutrophication of the sea and is a clear signal of anthropogenic pressure. The seasonal anoxia has during the last decades propagated to basins with water depths less than 20 m. The areal coverage of anoxia has thus expanded substantially. Phosphorus which is bound to oxic seafloor sediments is easily released during episodes of anoxia, which further intensifies eutrophication. It has been estimated that the concretion fields of the eastern Gulf of Finland, only, contain more than 330,000 tons of P2O5 which is equal to some 175,000 tons of elementary phosphorus. In case of shallowing of the area of permanent anoxia, these concretion fields would become anoxic, which would lead to rather rapid dissolution of the concretions and a release of a large amount of phosphorus together with the heavy metals which today are bound to the concretions.