Recent eutrophication of coastal waters in southern Finland - A palaeolimnological assessment

Field: Environmental Science (Physical Sciences)

Eutrophication of estuaries and coastal waters has become a significant problem worldwide. In the Baltic Sea, where the nutrient load has strongly increased from its natural level, this has led to marked changes in the coastal ecosystems. The European Water Framework Directive (WFD) requires all surface waters in Europe to fulfill the criterion of “good ecological status”, where the biological and chemical status departs only slightly from undisturbed baseline conditions by 2027. The WFD requires the determination of these baseline conditions against which the extent of anthropogenic eutrophication and the present ecological status of a system can be assessed. To provide a means for water quality assessments in coastal waters of the Baltic Sea area, a near-shore calibration data set of 49 embayments was collected. It consisted of measured environmental data and surface sediment diatom assemblages from the southern coast of Finland. The study sites were small, shallow and sheltered with a mean surface area of 211 ha and a mean depth of 4.3 m. Salinity showed a clear overall increasing trend from east to west ranging from 0.7 to 6.4‰. Higher than average nutrient concentrations occurred in estuaries with significant fresh water input, or close to municipalities, emphasising the significance of local sources to nutrient concentrations; no clear impacts of loading from the Neva River estuary were detected. Only ca. 5 % of all sites had good water quality, while most sites (65 %) fell into the class “moderate water quality” based on a national classification scheme by the Finnish Environment Institute. The collected calibration data set was used to explore the relationship between surface sediment diatom assemblages and 15 environmental variables with special emphasis on nutrients. The main environmental gradients in this data set were represented by nutrients and depth. Total dissolved nitrogen (TDN), total phosphorus (TP), depth and salinity all accounted for significant and independent fractions of variation in the diatom data and together explained 34% of the total variation. There were clear changes in diatom assemblage structures along the nutrient gradients. Although these changes were gradual, it was possible to identify a number of taxa that were more abundant in a particular nutrient environment. Diatom assemblages that were least affected by nutrient enrichment included a variety of benthic species (e.g. Rhoicosphenia curvata, Staurosirella pinnata, Opephora mutabilis and Bacillaria paxillifer) and a relatively high species richness. Several small planktonic taxa such as Cyclotella atomus, Cyclotella meneghiniana and Thalassiosira pseudonana were good indicators of highly elevated nutrient concentrations (>600 μgl-1 TDN and >60 μgl-1 TP) together with low species richness. These taxa could be used as potential indicators of the quality of coastal waters in the Baltic Sea. Their first appearance in regular monitoring could be used as an early warning sign for deteriorating water quality. In order to define baseline or reference conditions and reconstruct a detailed history of eutrophication in the study area, long sediment cores were collected from five of the study sites with varying anthropogenic influence. Two sites (Toolonlahti and Laajalahti) were located in Helsinki, the capital of Finland, with predominantly urban catchment areas, while the remaining three sites (Fasarbyviken, Pieni Pernajanlahti and Hellanlahti) were mostly impacted by agriculture. The cores were dated using 210Pb, 137Cs and, at two sites, spheroidal carbonaceous particles (SCPs). Diatom species richness, % abundance of planktonic taxa, and detrended correspondence analysis (DCA) primary axis scores, which provide a numerical means to summarise the compositional change of diatom assemblages over time, were used to study changes in the trophic status of these sites in addition to the ecological information contained in changes in the abundance of individual species. A diatom-based weighted-averaging partial least squares (WA-PLS) transfer function model for TDN was developed with a prediction accuracy of 0.09 μg L-1 (log10 units). Its performance was assessed using ca. 30 yr records of measured water quality data at four of the sites along with numerical measures such as the modern analogue technique (MAT) at all sites. Overall, the model tracked well the trends in the nutrient record, although it underestimated very high nutrient concentrations during time periods of heavy nutrient loading to the urban embayments. Encouraging, however, is the ability of the model to relatively accurately infer nutrient concentrations, which are in the range of sites included in the used calibration data set. The potential of the multi-proxy approach in a coastal environment was assessed at Laajalahti, where reference conditions and eutrophication history were defined using the stratigraphy of diatoms, sediment geochemistry, stable isotopes, sedimentary pigments, and long-term monitoring results of water quality and nutrient loading. The urban sites showed marked increases in the abundance of planktonic diatoms and diatominferred total dissolved nitrogen (DI-TDN), and a decrease in species richness starting in the 19th – early 20th century with increased urbanisation. At both sites a clear recovery was observed after the cessation of waste water loading by the early 1980s. The presently planktonic diatom assemblages of these embayments, however, show no change back to the pre-disturbance diverse benthic communities. The changes observed in the rural sites were only moderate and occurred later starting in the 1940s. No distinct increase in DI-TDN was seen; however, all sites showed an increase in small planktonic taxa indicating increased nutrient enrichment and turbidity. These small floristic changes could be seen as an early warning signal despite little change in the inferred nutrient concentrations. At four of the sites, significant changes in diatom assemblages and associated DI-TDN occurred after the 1920s, hence background or reference conditions could be set at early 1900s. The multi-proxy study from the urban site Laajalahti defined the mid to late 1800s as realistic reference conditions. In the case of Toolonlahti, a longer sediment core would have to be taken in order to establish pre-urbanisation conditions at the site. This would mean recreating conditions that existed in the embayment over 200 years ago. At that time, however, the embayment did not exist in its present form due to post glacial land uplift and urban development, but was a more open system affected by the open sea. On the basis of these results realistic reference conditions for management purposes could be set at the late 1800s. The reference conditions were defined by generally diverse benthic diatom assemblages (> 80 % benthic taxa) and DI-TDN concentrations of ca. 400 μg l-1. The general productivity as indicated by sedimentary organic phosphorus (OP), chlorophyll a + its degradation products (ChlaD) and diatoxanthin was low and bottom water oxygen conditions were good (Cu:Zn ratio). The present study shows that in urban estuaries major changes in the nutrient status and species assemblages have taken place and that we are still far from a good ecological state of these systems. On the other hand, the results indicate that in rural embayments the effects of eutrophication on the biota have perhaps been less pronounced than expected. Although these sites have crossed an ecologically important threshold, the changes are still only small to moderate. At such sites management efforts may be well rewarded: internal loading from sediments would likely be a minor problem, as external loading has not been heavy (cf. the urban sites) and hence no large nutrient pool has accumulated in the sediments. The results of this thesis have clear implications for the European Water Framework Directive. Diatoms could be applied to water quality classification and monitoring purposes in the coastal waters of the Baltic Sea area using techniques such as weighted-averaging regression and calibration. Analysing dated sediment stratigraphies using diatoms as key indicators or using several biological and geochemical proxies can help to assess ecological changes in coastal waters over longer time scales (ca. 100-200 yr) and provide information on background conditions. These techniques are also a valuable means for detecting early community changes, where water quality problems are not yet evident. Thus, they provide a management tool not only for ecosystem restoration but also for ecosystem protection.