The effects of human induced disturbances on the sediment geochemistry of Baltic Sea embayments with a focus on eutrophication history

Historical sediment nutrient concentrations and heavy-metal distributions were studied in five embayments in the Gulf of Finland and an adjacent lake. The main objective of the study was to examine the response of these water bodies to temporal changes in human activities. Sediment cores were collected from the sites and dated using 210Pb and 137Cs. The cores were analyzed for total carbon (TC), total nitrogen (TN), total phosphorus (TP), organic phosphorus (OP), inorganic phosphorus (IP), biogenic silica (BSi), loss on ignition (LOI), grain size, Cu, Zn, Al, Fe, Mn, K, Ca, Mg and Na. Principal component analysis (PCA) was used to summarize the trends in the geochemical variables and to compare trends between the different sites. The links between the catchment land use and sediment geochemical data were studied using a multivariate technique of redundancy analysis (RDA). Human activities produce marked geochemical variations in coastal sediments. These variations and signals are often challenging to interpret due to various sedimentological and post-depositional factors affecting the sediment profiles. In general, the sites studied here show significant upcore increases in sedimentation rates, TP and TN concentrations. Also Cu, which is considered to be a good indicator of anthropogenic influence, showed clear increases from 1850 towards the top part of the cores. Based on the RDA-analysis, in the least disturbed embayments with high forest cover, the sediments are dominated by lithogenic indicators Fe, K, Al and Mg. In embayments close to urban settlement, the sediments have high Cu concentrations and a high sediment Fe/Mn ratio. This study suggests that sediment accumulation rates vary significantly from site to site and that the overall sedimentation can be linked to the geomorphology and basin bathymetry, which appear to be the major factors governing sedimentation rates; i.e. a high sediment accumulation rate is not characteristic either to urban or to rural sites. The geochemical trends are strongly site specific and depend on the local geochemical background, basin characteristics and anthropogenic metal and nutrient loading. Of the studied geochemical indicators, OP shows the least monotonic trends in all studied sites. When compared to other available data, OP seems to be the most reliable geochemical indicator describing the trophic development of the study sites, whereas Cu and Zn appear to be good indicators for anthropogenic influence. As sedimentation environments, estuarine and marine sites are more complex than lacustrine basins with multiple sources of sediment input and more energetic conditions in the former. The crucial differences between lacustrine and estuarine/coastal sedimentation environments are mostly related to Fe. P sedimentation is largely governed by Fe redox-reactions in estuarine environments. In freshwaters, presence of Fe is clearly linked to the sedimentation of other lithogenic metals, and therefore P sedimentation and preservation has a more direct linkage to organic matter sedimentation.

Environmental change and anthropogenic impact on lake sediments during the Holocene in the Finnish - Karelian inland area

This thesis discusses the prehistoric human disturbance during the Holocene by means of case studies using detailed high-resolution pollen analysis from lake sediment. The four lakes studied are situated between 61o 40' and 61o 50' latitudes in the Finnish Karelian inland area and vary between 2.4 and 28.8 ha in size. The existence of Early Metal Age population was one important question. Another study question concerned the development of grazing, and the relationship between slash-and-burn cultivation and permanent field cultivation. The results were presented as pollen percentages and pollen concentrations (grains cm 3). Accumulation values (grains cm 2 yr 1) were calculated for Lake Nautajärvi and Lake Orijärvi sediment, where the sediment accumulation rate was precisely determined. Sediment properties were determined using loss-on-ignition (LOI) and magnetic susceptibility (k). Dating methods used include both conventional and AMS 14C determinations, paleomagnetic dating and varve choronology. The isolation of Lake Kirjavalampi on the northern shore of Lake Ladoga took place ca. 1460 1300 BC. The long sediment cores from Finland, Lake Kirkkolampi and Lake Orijärvi in southeastern Finland and Lake Nautajärvi in south central Finland all extended back to the Early Holocene and were isolated from the Baltic basin ca. 9600 BC, 8600 BC and 7675 BC, respectively. In the long sediment cores, the expansion of Alnus was visible between 7200 - 6840 BC. The spread of Tilia was dated in Lake Kirkkolampi to 6600 BC, in Lake Orijärvi to 5000 BC and at Lake Nautajärvi to 4600 BC. Picea is present locally in Lake Kirkkolampi from 4340 BC, in Lake Orijärvi from 6520 BC and in Lake Nautajärvi from 3500 BC onwards. The first modifications in the pollen data, apparently connected to anthropogenic impacts, were dated to the beginning of the Early Metal Period, 1880 1600 BC. Anthropogenic activity became clear in all the study sites by the end of the Early Metal Period, between 500 BC AD 300. According to Secale pollen, slash-and-burn cultivation was practised around the eastern study lakes from AD 300 600 onwards, and at the study site in central Finland from AD 880 onwards. The overall human impact, however, remained low in the studied sites until the Late Iron Age. Increasing human activity, including an increase in fire frequency was detected from AD 800 900 onwards in the study sites in eastern Finland. In Lake Kirkkolampi, this included cultivation on permanent fields, but in Lake Orijärvi, permanent field cultivation became visible as late as AD 1220, even when the macrofossil data demonstrated the onset of cultivation on permanent fields as early as the 7th century AD. On the northern shore of Lake Ladoga, local activity became visible from ca. AD 1260 onwards and at Lake Nautajärvi, sediment the local occupation was traceable from 1420 AD onwards. The highest values of Secale pollen were recorded both in Lake Orijärvi and Lake Kirjavalampi between ca. AD 1700 1900, and could be associated with the most intensive period of slash-and-burn from AD 1750 to 1850 in eastern Finland.

Sedimentary zooplankton remains as indicators of lake ecological quality and trophic structure

To protect and restore lake ecosystems under threats posed by the increasing human population, information on their ecological quality is needed. Lake sediments provide a data rich archive that allows identification of various biological components present prior to anthropogenic alterations as well as a constant record of changes. By providing a longer dimension of time than any ongoing monitoring programme, palaeolimnological methods can help in understanding natural variability and long-term ecological changes in lakes. As zooplankton have a central role in the lake food web, their remains can potentially provide versatile information on past trophic structure. However, various taphonomic processes operating in the lakes still raise questions concerning how subfossil assemblages reflect living communities. This thesis work aimed at improving the use of sedimentary zooplankton remains in the reconstruction of past zooplankton communities and the trophic structure in lakes. To quantify interspecific differences in the accumulation of remains, the subfossils of nine pelagic zooplankton taxa in annually laminated sediments were compared with monitoring results for live zooplankton in Lake Vesijärvi. This lake has a known history of eutrophication and recovery, which resulted from reduced external loading and effective fishing of plankti-benthivorous fish. The response of zooplankton assemblages to these known changes was resolved using annually laminated sediments. The generality of the responses observed in Lake Vesijärvi were further tested with a set of 31 lakes in Southern Finland, relating subfossils in surface sediments to contemporary water quality and fish density, as well as to lake morphometry. The results demonstrated differential preservation and retention of cladoceran species in the sediment. Daphnia, Diaphanosoma and Ceriodaphnia were clearly underrepresented in the sediment samples in comparison to well-preserved Bosmina species, Chydorus, Limnosida and Leptodora. For well-preserved species, the annual net accumulation rate was similar to or above the expected values, reflecting effective sediment focusing and accumulation in the deepest part of the lake. The decreased fish density and improved water quality led to subtle changes in zooplankton community composition. The abundance of Diaphanosoma and Limnosida increased after the reduction in fish density, while Ceriodaphnia and rotifers decreased. The most sensitive indicator of fish density was the mean size of Daphnia ephippia and Bosmina (E.) crassicornis ephippia and carapaces. The concentration of plant-associated species increased, reflecting expanding littoral vegetation along with increasing transparency. Several of the patterns observed in Lake Vesijärvi could also be found within the set of 31 lakes. According to this thesis work, the most useful cladoceran-based indices for nutrient status and planktivorous fish density in Finnish lakes were the relative abundances of certain pelagic taxa, and the mean size of Bosmina spp. carapaces, especially those of Bosmina (E.) cf. coregoni. The abundance of plant-associated species reflected the potential area for aquatic plants. Lake morphometry and sediment organic content, however, explained a relatively high proportion of the variance in the species data, and more studies are needed to quantify lake-specific differences in the accumulation and preservation of remains. Commonly occurring multicollinearity between environmental variables obstructs the cladoceran-based reconstruction of single environmental variables. As taphonomic factors and several direct and indirect structuring forces in lake ecosystems simultaneously affect zooplankton, the subfossil assemblages should be studied in a holistic way before making final conclusions about the trophic structure and the change in lake ecological quality.