Browsing by Subject "akvaattiset tieteet"

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  • Rintala, Janne-Markus (Helsingin yliopisto, 2009)
    The seasonal occurrence of sea ice that annually covers almost half the Baltic Sea area provides a unique habitat for halo- and cold temperature-tolerant extremophiles. Baltic Sea ice biology has more than 100 years of tradition that began with the floristic observation of species by the early pioneers using light microscopic techniques that were the only thing available at the time. Since the discovery of life within sea ice, more technologies have become available for taxonomy. Electron microscopy and genetic evidence have been used to identify sea ice biota revealing increased numbers of taxa. Meanwhile ecologists have used light microscopic cell enumeration in addition to the chemical and physical properties of sea ice in attempts to explain the food web structure of sea ice and its functions. Thus, during the Baltic winter, the sea ice hosts more abundant and diverse microbial communities than the water column beneath it. These communities are typically dominated by autotrophic diatoms together with a diverse assortment of dinoflagellates, auto- and heterotrophic flagellates, ciliates, metazoan rotifers and bacteria, which are mostly responsible for the recycling of nutrients. This thesis comprises ecological and systematic studies. In addition to the results of the previous studies carried out on landfast ice, the data presented here provide new insight into the spatial distribution of pelagial sea ice, which has remained largely unexplored. The studies reveal spatial heterogeneity in the pelagial sea ice of the Gulf of Bothnia. There were mismatches in chlorophyll-a concentrations and in photosynthetic efficiencies of the communities studied. The temporal succession was followed and experimental studies performed investigating the community responses towards increased or decreased light in landfast ice in the Gulf of Finland. The systematic studies carried out with established dinoflagellate cultures revealed a new resting cyst belonging to common sea ice dinoflagellate, Scrippsiella hangoei (Schiller) Larsen 1995. The cyst can be used to explain the overwintering of this species during prolonged periods of darkness. The dissimilarities and similarities in the material isolated from the sea ice called for description of a new subspecies Heterocapsa arctica ssp. frigida. The cells obtained in the cultured material were unlike those of the previously described species, necessitating description of ssp. frigida. As a result of its own unique habitus, the subspecies had been noted by Finnish taxonomists during the past three decades and thus its annual occurrence and geographical distribution in the Baltic Sea. This illustrates how combining ecology and systematics increases our understanding of organisms.
  • Viitasalo, Satu (Helsingin yliopisto, 2007)
    Benthic-pelagic coupling describes processes that operate across and between the seafloor and open-water ecosystems. In soft-sediment communities, bioturbation by sediment-dwelling and epibenthic organisms may strongly shape habitat characteristics and influence processes, e.g. biogeochemical cycling, which supplies bioavailable nutrients to pelagic primary producers. In addition, benthic fauna may mediate benthic-pelagic coupling by affecting the survival and hatching of zooplankton dormant eggs in the sediment. In the shallow waters and seasonally fluctuating environment of the Baltic Sea, emergence from the seafloor essentially contributes to the dynamics of zooplankton pelagic populations. In this thesis, I examine how benthic organisms with different functional traits affect the link between the benthic and pelagic systems in the northern Baltic Sea. By means of experimental laboratory studies, the effects of sediment-dwelling (Monoporeia affinis, Macoma balthica and Marenzelleria spp.) and nectobenthic (Mysis spp.) taxa on the survival and hatching of zooplankton benthic eggs and on benthic nutrient fluxes and sediment structure were investigated. In the predation studies, the nectobenthic mysids Mysis spp. preyed upon benthic eggs of the cladoceran Bosmina longispina maritima (syn. B. coregoni maritima), both in pelagic and benthic environments. Of the sediment-dwelling species, the amphipod M. affinis and the bivalve M. balthica reduced the number of cladoceran eggs in the sediment, whereas the polychaetes Marenzelleria spp. had no effects on cladoceran eggs. Both M. balthica and M. affinis also increased the mortality rates of benthic eggs of copepods and rotifers. It was estimated that zooplankton eggs provide an additional carbon source for food-limited benthic communities. The results indicate that predation pressure on zooplankton benthic eggs may be strong, but varies widely depending on the season and the functional characteristics of the macrofauna. Macoma balthica buried cladoceran eggs and a fluorescent tracer from the sediment surface to a depth of 3 4 cm, indicating efficient sediment mixing. In contrast, the other taxa had fewer effects on particle distributions. In addition to organic matter mineralization, particle mixing is crucial to the success of benthic recruitment of zooplankton, since only eggs close to the sediment surface may hatch. Macoma balthica and M. affinis altered the patterns of zooplankton emergence from the sediment. In general, the highest emergence rates were observed in the absence of macroscopic fauna, and M. balthica exerted a stronger suppressive effect than M. affinis. Moreover, copepods were less severely affected than cladocerans, while only one species (Temora longicornis) clearly benefited from the presence of the macrofauna. These differences probably result from species-specific differences in the resistance of eggs to disturbances. The results show that benthic fauna may considerably alter the patterns of zooplankton emergence from the seafloor, thereby shaping zooplankton pelagic populations. The semi-motile M. balthica and Marenzelleria spp. increased the fluxes of phosphate and ammonium from the sediment to the water, whereas the motile M. affinis and Mysis mixta had a contrasting effect. In the eutrophied Baltic Sea, efficient internal cycling of bioavailable nutrients forms a strong feedback inhibiting the recovery of the ecosystem. Based on the results, a change in species dominance from the two motile taxa, susceptible to oxygen deficiency, to the more tolerant semi-motile taxa provides additional feedback, strengthening internal nutrient cycling and accelerating eutrophication, with deteriorating near-bottom oxygen conditions and changes in the benthic communities. In shallow-water ecosystems, benthic nutrient regeneration plays a key role in determining the overall productivity of the ecosystem. In addition, the results of this study show that the communities in the benthos may essentially contribute to the structure of those in the plankton.
  • Kallasvuo, Meri (Helsingin yliopisto, 2010)
    Habitat requirements of fish are most strict during the early life stages, and the quality and quantity of reproduction habitats lays the basis for fish production. A considerable number of fish species in the northern Baltic Sea reproduce in the shallow coastal areas, which are also the most heavily exploited parts of the brackish marine area. However, the coastal fish reproduction habitats in the northern Baltic Sea are poorly known. The studies presented in this thesis focused on the influence of environmental conditions on the distribution of coastal reproduction habitats of freshwater fish. They were conducted in vegetated littoral zone along an exposure and salinity gradient extending from the innermost bays to the outer archipelago on the south-western and southern coasts of Finland, in the northern Baltic Sea. Special emphasis was placed on reed-covered Phragmites australis shores, which form a dominant vegetation type in several coastal archipelago areas. The main aims of this research were to (1) develop and test new survey and mapping methods, (2) investigate the environmental requirements that govern the reproduction of freshwater fish in the coastal area and (3) survey, map and model the distribution of the reproduction habitats of pike (Esox lucius) and roach (Rutilus rutilus). The white plate and scoop method with a standardized sampling time and effort was demonstrated to be a functional method for sampling the early life stages of fish in dense vegetation and shallow water. Reed-covered shores were shown to form especially important reproduction habitats for several freshwater fish species, such as pike, roach, other cyprinids and burbot, in the northern Baltic Sea. The reproduction habitats of pike were limited to sheltered reed- and moss-covered shores of the inner and middle archipelago, where suitable zooplankton prey were available and the influence of the open sea was low. The reproduction habitats of roach were even more limited and roach reproduction was successful only in the very sheltered reed-covered shores of the innermost bay areas, where salinity remained low (< 4‰) during the spawning season due to freshwater inflow. After identifying the critical factors restricting the reproduction of pike and roach, the spatial distribution of their reproduction habitats was successfully mapped and modelled along the environmental gradients using only a few environmental predictor variables. Reproduction habitat maps are a valuable tool promoting the sustainable use and management of exploited coastal areas and helping to maintain the sustainability of fish populations. However, the large environmental gradients and the extensiveness of the archipelago zone in the northern Baltic Sea demand an especially high spatial resolution of the coastal predictor variables. Therefore, the current lack of accurate large-scale, high-resolution spatial data gathered at exactly the right time is a considerable limitation for predictive modelling of shallow coastal waters.
  • Nyman, Marjut (Helsingin yliopisto, 2007)
    Climate change contributes directly or indirectly to changes in species distributions, and there is very high confidence that recent climate warming is already affecting ecosystems. The Arctic has already experienced the greatest regional warming in recent decades, and the trend is continuing. However, studies on the northern ecosystems are scarce compared to more southerly regions. Better understanding of the past and present environmental change is needed to be able to forecast the future. Multivariate methods were used to explore the distributional patterns of chironomids in 50 shallow (≤ 10m) lakes in relation to 24 variables determined in northern Fennoscandia at the ecotonal area from the boreal forest in the south to the orohemiarctic zone in the north. Highest taxon richness was noted at middle elevations around 400 m a.s.l. Significantly lower values were observed from cold lakes situated in the tundra zone. Lake water alkalinity had the strongest positive correlation with the taxon richness. Many taxa had preference for lakes either on tundra area or forested area. The variation in the chironomid abundance data was best correlated with sediment organic content (LOI), lake water total organic carbon content, pH and air temperature, with LOI being the strongest variable. Three major lake groups were separated on the basis of their chironomid assemblages: (i) small and shallow organic-rich lakes, (ii) large and base-rich lakes, and (iii) cold and clear oligotrophic tundra lakes. Environmental variables best discriminating the lake groups were LOI, taxon richness, and Mg. When repeated, this kind of an approach could be useful and efficient in monitoring the effects of global change on species ranges. Many species of fast spreading insects, including chironomids, show a remarkable ability to track environmental changes. Based on this ability, past environmental conditions have been reconstructed using their chitinous remains in the lake sediment profiles. In order to study the Holocene environmental history of subarctic aquatic systems, and quantitatively reconstruct the past temperatures at or near the treeline, long sediment cores covering the last 10000 years (the Holocene) were collected from three lakes. Lower temperature values than expected based on the presence of pine in the catchment during the mid-Holocene were reconstructed from a lake with great water volume and depth. The lake provided thermal refuge for profundal, cold adapted taxa during the warm period. In a shallow lake, the decrease in the reconstructed temperatures during the late Holocene may reflect the indirect response of the midges to climate change through, e.g., pH change. The results from three lakes indicated that the response of chironomids to climate have been more or less indirect. However, concurrent shifts in assemblages of chironomids and vegetation in two lakes during the Holocene time period indicated that the midges together with the terrestrial vegetation had responded to the same ultimate cause, which most likely was the Holocene climate change. This was also supported by the similarity in the long-term trends in faunal succession for the chironomid assemblages in several lakes in the area. In northern Finnish Lapland the distribution of chironomids were significantly correlated with physical and limnological factors that are most likely to change as a result of future climate change. The indirect and individualistic response of aquatic systems, as reconstructed using the chironomid assemblages, to the climate change in the past suggests that in the future, the lake ecosystems in the north do not respond in one predictable way to the global climate change. Lakes in the north may respond to global climate change in various ways that are dependent on the initial characters of the catchment area and the lake.
  • Ilus, Erkki (Helsingin yliopisto, 2009)
    During recent decades, thermal and radioactive discharges from nuclear power plants into the aquatic environment have become the subject of lively debate as an ecological concern. The target of this thesis was to summarize the large quantity of results obtained in extensive monitoring programmes and studies carried out in recipient sea areas off the Finnish nuclear power plants at Loviisa and Olkiluoto during more than four decades. The Loviisa NPP is located on the coast of the Gulf of Finland and Olkiluoto NPP on that of the Bothnian Sea. The state of the Gulf of Finland is clearly more eutrophic; the nutrient concentrations in the surface water are about 1½ 2 times higher at Loviisa than at Olkiluoto, and the total phosphorus concentrations still increased in both areas (even doubled at Loviisa) between the early 1970s and 2000. Thus, it is a challenge to distinguish the local effects of thermal discharges from the general eutrophication process of the Gulf of Finland. The salinity is generally low in the brackish-water conditions of the northern Baltic Sea, being however about 1 higher at Olkiluoto than at Loviisa (the salinity of surface water varying at the latter from near to 0 in early spring to 4 6 in late autumn). Thus, many marine and fresh-water organisms live in the Loviisa area close to their limit of existence, which makes the biota sensitive to any additional stress. The characteristics of the discharge areas of the two sites differ from each other in many respects: the discharge area at Loviisa is a semi-enclosed bay in the inner archipelago, where the exchange of water is limited, while the discharge area at Olkiluoto is more open, and the exchange of water with the open Bothnian Sea is more effective. The effects of the cooling water discharged from the power plants on the temperatures in the sea were most obvious in winter. The formation of a permanent ice cover in the discharge areas has been delayed in early winter, and the break-up of the ice occurs earlier in spring. The prolonging of the growing season and the disturbance of the overwintering time, in conditions where the biota has adjusted to a distinct rest period in winter, have been the most significant biological effects of the thermal pollution. The soft-bottom macrofauna at Loviisa has deteriorated to the point of almost total extinction at many sampling stations during the past 40 years. A similar decline has been reported for the whole eastern Gulf of Finland. However, the local eutrophication process seems to have contributed into the decline of the zoobenthos in the discharge area at Loviisa. Thermal discharges have increased the production of organic matter, which again has led to more organic bottom deposits. These have in turn increased the tendency of the isolated deeps to a depletion of oxygen, and this has further caused strong remobilization of phosphorus from the bottom sediments. Phytoplankton primary production and primary production capacity doubled in the whole area between the late 1960s and the late 1990s, but started to decrease a little at the beginning of this century. The focus of the production shifted from spring to mid- and late summer. The general rise in the level of primary production was mainly due to the increase in nutrient concentrations over the whole Gulf of Finland, but the thermal discharge contributed to a stronger increase of production in the discharge area compared to that in the intake area. The eutrophication of littoral vegetation in the discharge area has been the most obvious, unambiguous and significant biological effect of the heated water. Myriophyllum spicatum, Potamogeton perfoliatus and Potamogeton pectinatus, and vigorous growths of numerous filamentous algae as their epiphytes have strongly increased in the vicinity of the cooling water outlet, where they have formed dense populations in the littoral zone in late summer. However, the strongest increase of phytobenthos has extended only to a distance of about 1 km from the outlet, i.e., the changes in vegetation have been largest in those areas that remain ice-free in winter. Similar trends were also discernible at Olkiluoto, but to a clearly smaller extent, which was due to the definitely weaker level of background eutrophy and nutrient concentrations in the Bothnian Sea, and the differing local hydrographical and biological factors prevailing in the Olkiluoto area. The level of primary production has also increased at Olkiluoto, but has remained at a clearly lower level than at Loviisa. In spite of the analogous changes observed in the macrozoobenthos, the benthic fauna has remained strong and diversified in the Olkiluoto area. Small amounts of local discharge nuclides were regularly detected in environmental samples taken from the discharge areas: tritium in seawater samples, and activation products, such as 60Co, 58Co, 54Mn, 110mAg, 51Cr, in suspended particulate matter, bottom sediments and in several indicator organisms (e.g., periphyton and Fucus vesiculosus) that effectively accumulate radioactive substances from the medium. The tritium discharges and the consequent detection frequency and concentrations of tritium in seawater were higher at Loviisa, but the concentrations of the activation products were higher at Olkiluoto, where traces of local discharge nuclides were also observed over a clearly wider area, due to the better exchange of water than at Loviisa, where local discharge nuclides were only detected outside Hästholmsfjärden Bay quite rarely and in smaller amounts. At the farthest, an insignificant trace amount (0.2 Bq kg-1 d.w.) of 60Co originating from Olkiluoto was detected in Fucus at a distance of 137 km from the power plant. Discharge nuclides from the local nuclear power plants were almost exclusively detected at the lower trophic levels of the ecosystems. Traces of local discharge nuclides were very seldom detected in fish, and even then only in very low quantities. As a consequence of the reduced discharges, the concentrations of local discharge nuclides in the environment have decreased noticeably in recent years at both Loviisa and Olkiluoto. Although the concentrations in environmental samples, and above all, the discharge data, are presented as seemingly large numbers, the radiation doses caused by them to the population and to the biota are very low, practically insignificant. The effects of the thermal discharges have been more significant, at least to the wildlife in the discharge areas of the cooling water, although the area of impact has been relatively small. The results show that the nutrient level and the exchange of water in the discharge area of a nuclear power plant are of crucial importance.
  • Estlander, Satu (Helsingin yliopisto, 2011)
    Humic lakes are abundant in the temperate and cold regions of the Boreal Zone. High levels of water colour and strong thermal stratification of humic lakes limit the potential fish habitats and give a special role to the intraspecific and interspecific interactions. Water colour has different effects on species depending on species-specific life-history traits and trophic interactions. Fish species whose success in predation is based on visual cues are more susceptible to suffer in competition. The main aim of the thesis was to demonstrate the effects of water colour on European perch (Perca fluviatilis) in humic lakes. The contribution of water colour to diet, feeding, growth and competitive interactions of fish was studied both in laboratory and in small humic lakes with varying levels of water colour. The main findings of the thesis were that water colour has different effects on species, depending on species-specific life-history traits and trophic interactions. Water colour affected visually-oriented perch feeding and growth negatively, and the prolonged benthic feeding phase of perch resulting from the increased water colour could increase intraspecific competition in perch populations and may result in a partial bottleneck in growth for perch. Moreover, water colour may act as a proximate factor behind the population dependency of sexual growth dimorphism in perch.
  • Sopanen, Sanna (Helsingin yliopisto, 2009)
    The aim of the studies reported in this thesis was to examine the feeding interactions between calanoid copepods and toxic algae in the Baltic Sea. The central questions in this research concerned the feeding, survival and egg production of copepods exposed to toxic algae. Furthermore, the importance of copepods as vectors in toxin transfer was examined. The haptophyte Prymnesium parvum, which produces extracellular toxins, was the only studied species that directly harmed copepods. Beside this, it had allelopathic effects (cell lysis) on non-toxic Rhodomonas salina. Copepods that were exposed to P. parvum filtrates died or became severely impaired, although filtrates were not haemolytic (indicative of toxicity in this study). Monospecific Prymnesium cell suspensions, in turn, were haemolytic and copepods in these treatments became inactive, although no clear effect on mortality was detected. These results suggest that haemolytic activity may not be a good proxy of the harmful effects of P. parvum. In addition, P. parvum deterred feeding, and low egestion and suppressed egg production were consequently observed in monospecific suspensions of Prymnesium. Similarly, ingestion and faecal pellet production rates were suppressed in high concentration P. parvum filtrates and in mixtures of P. parvum and R. salina. These results indicate that the allelopathic effects of P. parvum on other algal species together with lowered viability as well as suppressed production of copepods may contribute to bloom formation and persistence. Furthermore, the availability of food for planktivorous animals may be affected due to reduced copepod productivity. Nodularin produced by Nodularia spumigena was transferred to Eurytemora affinis via grazing on filaments of small N. spumigena and by direct uptake from the dissolved pool. Copepods also acquired nodularin in fractions where N. spumigena filaments were absent. Thus, the importance of microbial food webs in nodularin transfer should be considered. Copepods were able to remove particulate nodularin from the system, but at the same time a large proportion of the nodularin disappeared. This indicates that copepods may possess effective mechanisms to remove toxins from their tissues. The importance of microorganisms, such as bacteria, in the degradation of cyanobacterial toxins could also be substantial. Our results were the first reports of the accumulation of diarrhetic shellfish toxins (DSTs) produced by Dinophysis spp. in copepods. The PTX2 content in copepods after feeding experiments corresponded to the ingestion of <100 Dinophysis spp. cells. However, no DSTs were recorded from field-collected copepods. Dinophysis spp. was not selected by the copepods and consumption remained low. It seems thus likely that copepods are an unimportant link in the transfer of DSTs in the northern Baltic Sea.
  • Majaneva, Markus (Helsingin yliopisto, 2013)
    Protists are unicellular eukaryotes. Some protistan species may be impossible to distinguish under the light or even electron microscope, and a complete balanced study of protistan taxonomy requires molecular analysis and light and electron microscopy. One of the main applications of taxonomic work is the assessment of diversity of organisms in an ecosystem. However, uncertainty in taxonomic precision undermines the diversity measures. DNA sequence data provide assistance since they are easily transformed to numbers that can be compared systematically and in a similar way throughout the eukaryotic domain, using sequence similarity to define operational taxonomic units (OTUs). DNA-based assessment of diversity is called environmental sequencing. The most commonly used gene in the environmental gene sequencing for eukaryotes and also in the protistan taxonomic studies has been the small subunit (18S) ribosomal RNA gene of the ribosomal operon. Also, internal transcribed spacers (ITS) are used. The studies in this thesis were conducted with Baltic Sea protists. The Baltic Sea is a subarctic brackish-water basin that partially freezes over every winter. If the salinity of parent water is higher than 0.6, the forming ice has a semi-solid structure with solid ice crystals and saline water (brine) channels. The brine channels offer habitats for small-sized organisms. Due to the low salinity of the Baltic Sea, the brine channels are small, and therefore, the Baltic Sea ice eukaryotic community is dominated by protists. Studies on Baltic sea-ice biology have been accumulating since the 1980 s, but there are still gaps on knowledge; for example, what protistan species and how many there are associated with sea ice. In this thesis, morphological, molecular and ecological information was combined to delineate species of a Baltic Sea cryptomonad, haptophyte and dinoflagellate. Protistan community composition in Baltic Sea ice was assessed with environmental sequencing, and diversity estimates were compared in different types of ice. The taxonomic and environmental-sequencing studies were linked using the gathered taxonomic information to evaluate the accuracy of the molecular diversity-measurement method. A new cryptomonad species, Rhinomonas nottbecki, was described based on morphological characters distinguished by light- and electron-microscopy together with molecular evidence from 18S rRNA gene and ITS region. The same approach was applied to the identification of the alternate stage Prymnesium polylepis (Haptophyta), which bloomed in the whole Baltic Proper during autumn spring 2007 2008. Also, a novel cold-water and sea-ice associated dinoflagellate subspecies, Heterocapsa arctica subsp. frigida, was described. Environmental 18S-rRNA-gene sequencing revealed that the richest eukaryotic lineages inhabiting the Baltic Sea ice were ciliates, cercozoa, dinoflagellates and diatoms. The different developmental stages and types of ice had different community composition. Protistan richness was higher in ice than water even though water included more divergent lineages. The Baltic fast ice had higher richness than pack and drift ice. The results of this thesis showed that there remains novelty to be described in the Baltic Sea, and what we know about the protistan community in Baltic Sea ice now is very incomplete. Although the environmental sequencing produced data that met the requirements of calculation of comparable diversity indices (all taxa defined at the same level), revealed cryptic taxa, and gave higher protistan richness than basic light microscopy of fixed samples, the lack of taxonomic detail was not restricted to the light-microscopic surveys but was also a result of the environmental-sequencing approach. This was shown when the environmental-sequencing approach was applied on the 18S-rRNA-gene data of the cryptomonad family Pyrenomonadaceae and the haptophyte genus Prymnesium. Only one Pyrenomonadaceae and two Prymnesium OTUs were found although both data sets included 15 distinct taxa. Errors in environmental sequencing and alignment make the use of high similarity levels in the OTU definition questionable, and the variability in the 18S rDNA is not equal within different eukaryotic lineages. Consequently, use of lower similarity level (97 %) is justifiable in the environmental-sequencing, but the approach used gave conservative estimates of the protistan richness in the Baltic Sea ice. The overall conclusion is that we need to apply all available techniques when assessing the diversity of protists, as each technique provides a biased perspective on nature. A labor intensive taxonomic approach that includes the study of live cells by light microscopy, detailed morphological description based on electron microscopy and phylogenetic analysis of suitable genetic markers gives us the best chance of finding out how many different species of protists live within the Baltic Sea ice or any other environment, and what they do.
  • Holmroos, Heidi (Helsingin yliopisto, 2014)
    The dynamics of phosphorus (P) and nitrogen (N) were examined in two (shallow and deep) eutrophic lakes. The effects of resuspension and macrophytes as well as oxygen availability were determined, as was the role of natural nitrogen (N) removal (denitrification). The effect of resuspension on the availability of N and P was examined in experimental columns in a shallow eutrophic lake. The release of soluble reactive P (SRP) from resuspended matter varied both seasonally and interannually, depending on the conditions in the lake. SRP release was most significant during periods of high primary production, probably due to pH-induced ligand-exchange reactions. The effect of resuspension on dissolved N was less clear, although resuspension also increased the concentrations of ammonium, nitrite and nitrate during some of the experiments. The studies also showed that strong resuspension decreases the total N to total P ratio in the water independently of the phase of the growing season. The nutrient dynamics were further studied among two different macrophyte stands (submerged and floating-leaved macrophytes) and in the open water. The concentration of combined nitrate and nitrite N (NOx N) decreased to the detection limit (PIENEMPI 2 µg l 1) at all of the stations during summer. Among the submerged macrophytes, the NOx N was depleted almost 2 months earlier than at the other stations, which restricted denitrification. The rate of denitrification was measured with the isotope-pairing technique in both lakes. In the shallow lake, denitrification was measured at the location of the resuspension experiments as well as at the macrophyte stations and adjacent open-water area. Denitrification was mainly dependent on the availability of NOx N and temperature, and also contributed to the changes observed in the total N to total P ratio in water during summer by removing substantial amounts of NOx N from the water. The effects of oxygen (O2) availability were studied in a deeper, stratifying lake, where the deeps were treated with continuous or pulsed aeration, or with no aeration. The SRP release in the deeps was linked with the O2 in the sediment and the SRP accumulated in the hypolimnion during the periods of anoxia. During the aeration pauses and pulses, the concentration of NOx N decreased and increased respectively and the concentration of ammonium varied contrastingly. Continuous aeration prevented the ammonium from accumulating and provided NOx N for denitrification to occur. Accordingly, denitrification showed higher rates during aeration. However, aeration also increased the temperature of the hypolimnion and thereby also the O2 consumption.
  • Nikula, Raisa (Helsingin yliopisto, 2008)
    This study addressed the large-scale molecular zoogeography in two brackish water bivalve molluscs, Macoma balthica and Cerastoderma glaucum, and genetic signatures of the postglacial colonization of Northern Europe by them. The traditional view poses that M. balthica in the Baltic, White and Barents seas (i.e. marginal seas) represent direct postglacial descendants of the adjacent Northeast Atlantic populations, but this has recently been challenged by observations of close genetic affinities between these marginal populations and those of the Northeast Pacific. The primary aim of the thesis was to verify, quantify and characterize the Pacific genetic contribution across North European populations of M. balthica and to resolve the phylogeographic histories of the two bivalve taxa in range-wide studies using information from mitochondrial DNA (mtDNA) and nuclear allozyme polymorphisms. The presence of recent Pacific genetic influence in M. balthica of the Baltic, White and Barents seas, along with an Atlantic element, was confirmed by mtDNA sequence data. On a broader temporal and geographical scale, altogether four independent trans-Arctic invasions of Macoma from the Pacific since the Miocene seem to have been involved in generating the current North Atlantic lineage diversity. The latest trans-Arctic invasion that affected the current Baltic, White and Barents Sea populations probably took place in the early post-glacial. The nuclear genetic compositions of these marginal sea populations are intermediate between those of pure Pacific and Atlantic subspecies. In the marginal sea populations of mixed ancestry (Barents, White and Northern Baltic seas), the Pacific and Atlantic components are now randomly associated in the genomes of individual clams, which indicates both pervasive historical interbreeding between the previously long-isolated lineages (subspecies), and current isolation of these populations from the adjacent pure Atlantic populations. These mixed populations can be characterized as self-supporting hybrid swarms, and they arguably represent the most extensive marine animal hybrid swarms so far documented. Each of the three swarms still has a distinct genetic composition, and the relative Pacific contributions vary from 30 to 90 % in local populations. This diversity highlights the potential of introgressive hybridization to rapidly give rise to new evolutionarily and ecologically significant units in the marine realm. In the south of the Danish straits and in the Southern Baltic Sea, a broad genetic transition zone links the pure North Sea subspecies M. balthica rubra to the inner Baltic hybrid swarm, which has about 60 % of Pacific contribution in its genome. This transition zone has no regular smooth clinal structure, but its populations show strong genotypic disequilibria typical of a hybrid zone maintained by the interplay of selection and gene flow by dispersing pelagic larvae. The structure of the genetic transition is partly in line with features of Baltic water circulation and salinity stratification, with greater penetration of Atlantic genes on the Baltic south coast and in deeper water populations. In all, the scenarios of historical isolation and secondary contact that arise from the phylogeographic studies of both Macoma and Cerastoderma shed light to the more general but enigmatic patterns seen in marine phylogeography, where deep genetic breaks are often seen in species with high dispersal potential.
  • Peltomaa, Elina (Helsingin yliopisto, 2013)
    Phytoplankton constitute the autotrophic, photosynthesizing component of the plankton community in freshwaters as well as in oceans. Today, phytoplankton account for about half of Earth s primary production (PP). Carbon and energy fixed by phytoplankton are transported further in the aquatic food web to heterotrophic zooplankton and finally to fish or, alternatively, are decomposed by heterotrophic bacteria that also act as food for higher trophic-level organisms. Since phytoplankton fix inorganic carbon (IC), they are highly important in lake carbon cycling and balance. Many of the lakes in the boreal area are characterized by heavy loadings of brown-coloured humic matter, mostly dissolved organic carbon (DOC), that diminishes light penetration in the water column. This is problematic for phytoplankton which, as photosynthetic organisms, are dependent on solar radiation. The phytoplanktonic life in boreal humic lakes is also hampered by strong thermal stratification patterns that due to nutrient uptake, lead to inorganic nutrient limitation in the illuminated epilimnion. However, nutrients are often plentiful in the dark hypolimnion. Since phytoplankton are ubiquitous in aquatic ecosystems, they must have several adaptations to help them survive in various environments, including boreal humic lakes. The present study focused on the traits of motility and cell size, both of which affect phytoplankton capability to not only obtain nutrients and light, but also to avoid zooplankton grazing. Special attention was given to the group of autotrophic picoplankton (APP), which are nonmotile, small (cell size 0.2 2 µm) and less studied than the larger phytoplankton. The seasonal dynamics of APP and larger phytoplankton were associated with changes in the abiotic environment, especially parameters prone to the ongoing climate change. In addition, the associations between phytoplankton and their competitors and grazers in the microbial food web (MFW), as well as the possible top-down effects of fish on the MFW, phytoplankton and surface water carbon dioxide (CO2) concentrations were studied in more detail. Four of the five studies were undertaken in situ in the small, strongly stratified, humic headwater Lake Valkea-Kotinen. The fifth study was a fish biomanipulation experiment conducted in enclosures in the humic Lake Pääjärvi and the clearwater Lake Vesijärvi. The most successful phytoplankton taxa in Lake Valkea-Kotinen in terms of PP as well as biomass were flagellated. However, motility was really advantageous only when combined with large cell size (> 20 µm): Peridinium dinoflagellates dominated in PP and the biomass in spring and autumn, whereas in summer Gonyostomum semen (Ehr.) Diesing took over. This was probably because only the large cells were able to migrate long distances between the illuminated epilimnion and nutrient-rich hypolimnion. Interestingly, the most abundant phytoplankton taxa in Lake Valkea-Kotinen were the nonmotile and tiny (~ 2 µm) Choricystis (Skuja) Fott-like eukaryotic APP. The strength of the APP was in isopycny, i.e. the capability to remain at the boundary layer between the epi- and hypolimnion, where they obtained access to light and nutrients. Both G. semen and APP correlated positively with high water column stability, which also indicates that they benefitted from strong stratification patterns. There were changes in the water quality in Lake Valkea-Kotinen during the study period of 1990 2006, most importantly, as increases in DOC and water colour, whereas phosphorus, which was the limiting nutrient, decreased. This was problematic for the large flagellates (studied in 1990 2003) and prokaryotic APP (Merismopedia warmingiana Lagerheim; in 2002 2006). However, the eukaryotic APP (in 2002 2006) were favoured by the increased water colour. APP abundance correlated negatively with heterotrophic bacteria in the epilimnion of Lake Valkea-Kotinen, which indicates nutrient competition between these two groups. The bacteria correlated positively with large phytoplankton (measured as chlorophyll a), and probably were partly sustained by G. semen, which was associated with high extracellular organic carbon (EOC) release. However, both the APP and bacterial numbers were in general low in Lake Valkea-Kotinen, which was explained by the high nanoflagellate (NF) and ciliate abundance. Nevertheless, the NFs did not graze on the APP, and the APP as well as the larger phytoplankton were able to avoid ciliate grazing during the strongest stagnation by remaining in the anoxic parts of the water column, where algivorous ciliates were less abundant. The enclosure experiment in lakes Pääjärvi and Vesijärvi showed no top-down effects of fish on APP or any other components of the MFW. This was probably due to the low abundance of cladocerans, especially the large daphnids. However, in the humic Lake Pääjärvi, fish influenced the food web via nutrient enrichment, i.e. through bottom-up effects. The total phytoplankton biomass did not change, but the PP increased and led to increments in bacterial production (BP) and ciliates, which took advantage of the enhanced phytoplankton production. Therefore, although unexpected, the higher PP did not translate into lower water CO2 concentration, but the BP and ciliate algivory increased concurrently and produced more CO2. Thus, the net ecosystem production (NEP) remained stable.
  • Korhonen, Jenni (Helsingin yliopisto, 2014)
    The variation in biodiversity has intrigued ecologists for centuries. Currently, studying biodiversity is increasingly important because of its seminal role for maintaining ecosystem functions. Thus, one of the central questions in modern ecology is how species richness and composition can affect ecosystem functioning. Besides spatial variation in diversity, scientists are increasingly interested in the temporal patterns in diversity and community structure. The aim of the PhD thesis was to study spatial and temporal turnover in aquatic communities. I investigated productivity-diversity relationships in three planktonic groups and at two spatial scales. Also, spatial patterns in community composition were compared among the three taxon groups and two spatial scales. Further, I studied the relationships between resource availability, species richness, biomass and resource ratio in phytoplankton communities. Temporal turnover in aquatic assemblages was studied in relation to several ecological, physical and geographical factors. Finally, within and between year variation in lotic diatom communities was investigated in Finnish streams showing wide variability in trophic status and size. The results show that the relationships between ecosystem productivity and plankton diversity are highly variable, ranging from linear negative to linear positive and unimodal. Both alpha and beta diversity showed scale-dependency, highlighting that community patterns may be weaker at smaller scales covering shorter environmental gradients. I also found several key drivers affecting temporal variation in aquatic communities, such as study duration, latitude and organism body size. For example, turnover was faster in low latitude environments than at high latitudes at short time scales, but slower at long time scales. Ecosystem size seems also to be of high importance for turnover rate in many kinds of aquatic ecosystems. This study revealed the suite of factors affecting aquatic species richness and composition both locally and regionally in several types of aquatic ecosystems. The results indicate how different types of communities and ecosystems change and are able to adapt to changing environmental conditions, such as increasing water temperatures or nutrient input due to global climate change. The factors affecting spatial and temporal components of diversity have an effect not only on the diversity and the identity of the biological organisms, but also on socio-economic well-being of humankind as we benefit from many resources and processes that are supplied by natural ecosystems, i.e. ecosystem services.
  • Juntunen, Teppo (Helsingin yliopisto, 2013)
    A typical decision problem in an environmental field includes a complex system with countless uncertain factors of both nature and human behavior. There are many stakeholders with conflicting objectives and a lot of decision alternatives, and results need to be communicated clearly to decision makers and stakeholders. Organized analysis is needed to tackle these challenges. In an ideal situation, we should analyze the objectives of every stakeholder and the responses from different parts of the ecosystem within one framework, which integrates the expertise and efforts of many different disciplines. Bayesian inference, especially the influence diagram, is a perfect tool to be used in such decision problems. The main contribution of this thesis is in developing methods for the modeling of uncertainties in environmental decision problems. The focus is on having more complete decision analyses where more uncertainties are realistically modeled. By including more stochastic variables in the analysis, the decision makers get a more realistic picture of the uncertainties involved and can account for them in the decision making. The thesis consists of five separate research articles, which all contribute to the different parts of the Bayesian decision process presented in this summary. The process is divided into four steps: (1.) building a decision model, (2.) data gathering and processing, (3.) using the model, and (4.) post analysis. The summary presents the research articles and their contributions and critically reviews the tools and methods needed in the process. The articles include a model for oil spill management, a spatial multispecies stock assessment model, a model for the stock assessment of data-poor species, a model to estimate uncertainties in environmental valuation and an influence diagram for value of information analysis. The methods used cover many aspects of the Bayesian decision process, outlining the problem, different ways to define prior distributions, utility functions, and finding maximum utility policies and value of information analysis. Hence, the tools used are diverse, too. In the models, I have used graphical Bayesian networks, numerical MCMC estimation, and Gaussian processes. In conclusion, the results found in this thesis are small but important steps toward better and more comprehensive Bayesian decision analyses in environmental and fisheries management. They show that significant uncertainties exist in many parts of the system. Another important factor was the cooperation of scientists from many different disciplines with a variety of backgrounds, which is needed in the modeling of complex environmental problems.
  • Salonen, Maiju (Helsingin yliopisto, 2012)
    Organisms have environmental requirements in order to grow and reproduce. These restrictions concerning environmental requirements lead to the overall distribution of organisms. Alterations in the environment leads to acclimatisation, changes in distribution or even extinction of a species in the specific environment. Eutrophication, increase of nutrients and consequently the general productivity level, is a global phenomenon in the aquatic environment. In the Baltic Sea, the progress of eutrophication has been observed since the 1960s. The central issue in my thesis is the shift in an environmental condition, i.e. the increase of water turbidity due to increased primary production and the effects of turbidity on predatory fish, i.e. larval pike. In general, phytoplankton-induced turbidity deteriorates the visual environment by absorbing visible light. Depending on the fish species in question, this can have positive, negative or neutral effects on predator-prey interactions. In addition to effects of water turbidity, my thesis focuses on the ramifications of the quality and quantity of prey on first-feeding larval pike in the nursery grounds of the northern Baltic Sea archipelago areas. The amount of suitable prey is important for effective growth. Rapid growth is essential for larvae, as growing larvae becomes more aware of their surroundings, and are more able to avoid contact with predators and escape attacks. The quality of zooplankton as prey is affected by community species composition, phytoplankton community composition, temperature and eutrophication for example. The quality, in turn, has a direct impact on the growth and development of planktivorous fish. Pike populations have diminished in some coastal areas of the Baltic Sea since the 1970s, and the catches have collapsed in areas where environmental changes have been considerable. My thesis included two field studies, where environmental factors (salinity, turbidity, temperature, oxygen concentration, wave exposure) and important resources (zooplankton quantity and community structure) are related to the distribution and condition of pike larvae. I also conducted two sets of experiments that focused on the effects of turbidity and food quality on prey capture and larval growth. According to my results, phytoplankton-induced turbidity has a negative effect on larval pike condition and growth. Turbidity also impacts prey capture negatively by diminishing the number of copepods captured. In my experiment the quality of prey, measured as fatty acids, was higher in the outer site than in the inner site. Both increased water turbidity and food quality affected the growth of larval pike when food quantity was kept constant. However, more sites from both inner and outer archipelago zones should be surveyed in the future to find out whether this is a trend or an outcome of this experiment. There were also other factors that affected the larval pike populations in the two areas; the density of zooplankton is higher in the inner archipelago sites compared with the outer archipelago sites while the density and condition of larval pike were decreasing towards the outer archipelago sites simultaneously with increasing exposition to pelagic circumstances. The occurrence and density of important prey species help explain the distribution of larval pike in the archipelago areas, but as larval pike do not disperse much from the spawning grounds; this is also related to adult pike spawning behaviour. The countries surrounding the Baltic Sea have made actions to diminish the amount of allochtonous nutrients entering the sea. Even though many projects have led to a decrease of these substances, the nutrient load is still quite high as the internal loading of nutrients sustains the eutrophicated and turbid conditions of the sea for a long period of time.
  • Härkönen, Laura (Helsingin yliopisto, 2014)
    Visibility conditions of lakes in the Northern Hemisphere have been predicted to decline, due to climate change- induced variations in vegetation of the surrounding catchment areas, precipitation, soil erosion, as well as sediment resuspension. At the same time, climate models predict increasing wind and storm activities, resulting in increasing turbulent velocities in lakes. The aim here was to experimentally clarify how these changes in abiotic factors may affect planktivorous predation in lake ecosystems. We studied how turbulence affects the ability of pelagic invertebrates (Chaoborus flavicans) to avoid fish predation by altering their distribution, the feeding efficiency of C. flavicans and planktivorous perch (Perca fluviatilis) under varying turbidity conditions, and the response of the zooplankton community to various predators, i.e. pelagic invertebrates (Chaoborus), and fish (perch and roach (Rutilus rutilus) in highly colored water. Increasing turbulence negatively affected the ability of Chaoborus larvae to exploit their vertical refuge and also to determine their horizontal position, which in turn was assumed to affect their predator escape efficiency. Indeed, a positive interaction of turbulence and turbidity with planktivorous perch feeding was discovered. Our novel findings challenged the previous assumption that fish larger than a few centimeters in body length are unaffected by turbulence. This was attributed to increased encounter rates between predators and prey, as well as difficulties of chaoborids in escaping predators under high turbulence; the time lost in searching for the prey was compensated. Additionally, intermediate turbulence combined with humic water altered the selective feeding of planktivorous fish on zooplankton compared with calm conditions. Under turbulent conditions, planktivorous fish preferred copepods over cladocerans, whereas under calm conditions the contrasting situation prevailed. Turbulence-mediated changes in the selective feeding of planktivorous fish under low visibility conditions may result in drastic changes in the lower trophic levels in fish-dominated systems. The studies also revealed that intermediate turbulence benefits the feeding of C. flavicans, but only when introduced to a natural, versatile zooplankton community. In contrast to the effect of fish predation, the dark-water experiments in mesocosms showed that the combined effect of turbulence and Chaoborus predation was strongest on cladocerans. High turbulence, on the other hand, caused decreases in Chaoborus feeding. We suggest that turbulence together with varying visibility conditions can have crucial implications for planktivorous predation and should thus be treated as a significant factor in food web studies. Furthermore, intermediate turbulence together with contemporaneous increases in water color may possibly result in cascading effects on primary producers. Depending on the dominant planktivores present, these changes in abiotic factors can have significant consequences on the lower trophic levels.
  • Kallio, Kari (Helsingin yliopisto, 2012)
    In lake-rich regions, the gathering of information about water quality is challenging because only a small proportion of the lakes can be assessed each year by conventional methods. One of the techniques for improving the spatial and temporal representativeness of lake monitoring is remote sensing from satellites and aircrafts. The experimental material included detailed optical measurements in 11 lakes, air- and spaceborne remote sensing measurements with concurrent field sampling, automatic raft measurements and a national dataset of routine water quality measurements from over 1100 lakes. The analyses of the spatially high-resolution airborne remote sensing data from eutrophic and mesotrophic lakes showed that one or a few discrete water quality observations using conventional monitoring can yield a clear over- or underestimation of the overall water quality in a lake. The use of TM-type satellite instruments in addition to routine monitoring results substantially increases the number of lakes for which water quality information can be obtained. The preliminary results indicated that coloured dissolved organic matter (CDOM) can be estimated with TM-type satellite instruments, which could possibly be utilised as an aid in estimating the role of lakes in global carbon budgets. Based on the results of reflectance modelling and experimental data, MERIS satellite instrument has optimal or near-optimal channels for the estimation of turbidity, chlorophyll a and CDOM in Finnish lakes. MERIS images with a 300 m spatial resolution can provide water quality information in different parts of large and medium-sized lakes, and in filling in the gaps resulting from conventional monitoring. Algorithms that would not require simultaneous field data for algorithm training would increase the amount of remote sensing-based information available for lake monitoring. The MERIS Boreal Lakes processor, trained with the optical data and concentration ranges provided by this study, enabled turbidity estimations with good accuracy without the need for algorithm correction with field measurements, while chlorophyll a and CDOM estimations require further development of the processor. The accuracy of interpreting chlorophyll a via semi empirical algorithms can be improved by classifying lakes prior to interpretation according to their CDOM level and trophic status. Optical modelling indicated that the spectral diffuse attenuation coefficient can be estimated with reasonable accuracy from the measured water quality concentrations. This provides more detailed information on light attenuation from routine monitoring measurements than is available through the Secchi disk transparency. The results of this study improve the interpretation of lake water quality by remote sensing and encourage the use of remote sensing in lake monitoring.