Kiertoa sulkemassa : typen ja fosforin virrat Suomen merialueiden kalankasvatuksessa

Rehevöityminen on Itämeren näkyvin ympäristöongelma. Itämereen päätyvät typpi- ja fosforivirrat lisäävät perustuotantoa, jonka seurauksena lajisto yksipuolistuu ja pohjalle vajoavan biomassan määrä kasvaa. Pohjan hajotustoiminta kuluttaa laajoilla alueilla vedestä kaiken käytössä olevan hapen, jonka seurauksena vain anaerobinen eliöstö pystyy näillä alueilla selviytymään. Kalan merkitys ihmisravintona kasvaa jatkuvasti. Muun eläintuotannon eettiset kysymykset ja kalaruoan terveellisyys ovat etenkin länsimaissa nostaneet kalan suosiota muiden eläintuotteiden kustannuksella. Yhä suurempi osa kulutetusta kalasta on kasvatettua, myös Suomessa. Kalankasvatuksen kuormitus Suomessa on pienentynyt viimeisen 15 vuoden ajan, pääasiassa rehujen kehityksen myötä. Kalankasvatuksesta aiheutuu kuitenkin edelleen paikallisesti merkittäviä rehevöittäviä ravinnepäästöjä, tuotannon tehokkuudesta huolimatta. Teolliseen ekologiaan sisältyvän teollisen metabolian periaatteiden mukaan teollisten prosessien ainevirtoja tulisi sulkea. Tässä tutkimuksessa tarkasteltiin ainevirta-analyysin avulla Suomen merialueiden kalankasvatusjärjestelmän keskimääräisiä vuotuisia typpi- ja fosforivirtoja vuosien 2004-2006 tuotantotietojen perusteella. Kalankasvatusjärjestelmään tulee typpeä 849 t ja fosforia 118 t kalojen rehusta. Ravinteet sitoutuvat kasvatettavaan kalaan tai kulkeutuvat ympäröivään vesistöön. Kaksi kolmasosaa kalankasvatusjärjestelmän ravinnepäästöistä päätyy kasvatusaltaasta suoraan veteen, kolmasosan sitoutuessa kasvuun. ImPACT-analyysillä todettiin vuosien 1980-2006 välillä kulutustottumusten ja teknologian vaikuttaneen merkittävästi kalankasvatuksen typen ja fosforin aiheuttamaan vesistökuormitukseen, sen sijaan väestön ja varallisuuden muutoksilla ei ollut merkitystä. Tutkimuksessa tarkasteltiin myös mahdollisia muutoksia ravinnevirroissa, jos rehun sisältämä Itämeren valuma-alueen ulkopuolelta peräisin olevasta kalasta tehty kalajauho korvattaisiin Itämeren kalasta tehdyllä kalajauholla. Menetelmällä saavutettaisiin huomattavat vähennykset Itämeren altaaseen päätyvissä ravinnevirroissa: typen osalta vähennys olisi 420 t, fosforin osalta nettokuormitus muuttuisi negatiiviseksi, -10 t. Ravinteet konsentroituisivat intensiivisen kasvatuksen alueille, mutta kyseessä on kustannustehokas keino Suomessa Itämereen kohdistuvan ravinnekuormituksen pienentämiseksi

Ubiquitous patchiness in chlorophyll a concentration in coastal archipelago of Baltic Sea

Productivity and trophic status of aquatic systems is traditionally quantified by chlorophyll a measurements. Environmental conditions and ecological interactions cause variability in chlorophyll a abundance. In coastal ecosystems, shallow and complex bathymetry reduces vertical heterogeneity, but promotes horizontal heterogeneity. However, coastal monitoring programs and scientific surveys are primarily focused on the vertical dimension. Here we demonstrate the spatial patchiness of chlorophyll a in coastal waters. We collected horizontally detailed and extensive in situ chlorophyll a data from the coastal Baltic Sea (SW Finland), covering the ice-free season of an annual cycle. Altogether, more than 200,000 observations were logged by an automated underway measurement system equipped with an optical sensor connected to a flow-through system. We analyzed the spatial heterogeneity of calibrated chlorophyll a data by using multiple statistical approaches, and quantified the chlorophyll a patches using a rolling average filter. We were able to identify patches and quantify their abundance and size for each of the 11 sampling campaigns. On average, 285 patches, ranging from 0.6 to 3142 m in size, were observed on the 830 km sampling transect. The average size of the patches was 237 (95% CI 226-248) m, most patches being between 10 and 1000 m. Our results show that patches of chlorophyll a can be effectively identified and quantified by modern in situ optical instrumentation. Such information is both theoretically and practically relevant. First, these results increase our understanding of the overall heterogeneity of the coastal environment. Further, they demonstrate the value of knowing the magnitude and occurrence of chlorophyll a patchiness in accurate detection of changes in coastal ecosystems caused by increased inputs of nutrients.Peer reviewe

Transformation and removal of riverine dissolved organic matter in Baltic Sea estuaries

The pool of riverine dissolved organic matter (DOM) results from integration of complex catchment processes, and links the terrestrial and coastal systems by transporting organic matter from watersheds to estuaries. In this thesis, field samplings and laboratory experiments were combined to assess the spatio-temporal variation in riverine DOM quantity and quality in three Finnish estuaries discharging to the Baltic Sea. Also, the biogeochemical transformation and removal processes influencing the composition of the DOM pool were studied. Large-scale catchment characteristics were linked to the properties of the riverine DOM. Throughout the work the DOM quality was assessed using multiple analytical approaches: C/N stoichiometry, colored DOM (CDOM) absorption, CDOM fluorescence, molecular weight and iron content. Estuarine DOM was subjected to heterotrophic degradation in factorial experiments to quantify the role of salinity, inorganic nutrients and predegradation to DOM bioavailability. Additionally salt-induced flocculation of DOM was studied by combining field samplings, laboratory experiments and modeling. The selected three study catchments differed markedly in their land-use, and these differences were reflected on the riverine DOM quantity and quality. The experiments provided evidence that increasing proportion of forests and peatlands were linked to the increase of carbon loading from the catchment, and to decreases in the subsequent quantities of bioavailable dissolved organic carbon (DOC) and bacterial growth efficiencies (BGE). A higher proportion of agricultural land in the catchment indicated an increase of the amount and bioavailability of dissolved organic nitrogen (DON) in the DOM pool. A larger proportion of lakes in the catchments were related to decreased bioavailable DON. Replete inorganic nutrients did not influence the DOM bioavailability, although did increase BGE on average from 11 to 40%. Increasing predegradation, i.e. DOM subjected to heterotrophic degradation for varying times before the actual bioassays, decreased BGE from 65 to 25% on average. Flocculation caused deviations from conservative mixing of DOM variables in the study estuaries, and the quantity and quality of flocculated DOM was studied in a laboratory experiment. The maximum deviation from conservative mixing of DOC in estuaries was -16% at salinities between 1 and 2, indicating significant flocculation within a relatively narrow salinity range. Both processes, biodegradation and flocculation, removed riverine DOM before reaching the open sea (so-called marginal filter), but also changed the properties of the remaining DOM pool. Also, both processes increased the humic-like fluorescence and DOC-specific absorbance of the DOM pool, which suggests that the refractory DOM pool reaching the sea is a result of multiple, interacting processes along the hydrological path. All in all, both biodegradation and flocculation remove riverine DOM in estuaries, and also transform the remaining DOM pool that ultimately reaches the open sea. Findings from this thesis show that DOM quality has a pivotal role in the functioning of both of these essential and ubiquitous mechanisms

Multiple anthropogenic drivers behind upward trends in organic carbon concentrations in boreal rivers

Increases of riverine organic carbon concentrations have been observed across the northern hemisphere over the past few decades. These increases are the result of multiple environmental drivers, but the relative importance of the drivers is still unclear. We analyzed a dataset of >10 000 observations of riverine total organic carbon (TOC) concentrations and associated water chemistry and hydrological observations from 1993 to 2017. The observations span a ~600 km north–south gradient from 30 individual river systems in Finland. Our data show significantly increasing TOC concentrations in 25 out of 30 systems, with an average increase from 12.0 to 15.1 mg l−1. The observed increase in riverine TOC concentrations led to an increase of 0.28 Mt in annual TOC load to the Baltic Sea from 1993 level to 2017 level. We analyzed the role of three putative environmental drivers of the observed TOC trends. Multiple regression analysis revealed that the most common driver was discharge, which alone explained TOC increases in 13 rivers, whereas pH and temperature were less important drivers (sole predictor in one and zero rivers, respectively). Different permutations of these three drivers were also found to be significant; the combination of discharge and pH being the most common (4 rivers). Land use was not in general linked with trends in TOC, except for the proportion of ditched land in the catchment, which was significantly correlated with increases in TOC concentration. Land use showed significant relationships with trends in discharge and pH. We also found that catchment characteristics are regulating the extent of these regional or global environmental changes causing the upward trends of riverine organic carbon.peerReviewe

Composition of natural phytoplankton community has minor effects on autochthonous dissolved organic matter characteristics

Dissolved organic matter (DOM) is an important component of nutrient cycling, but the role of different organisms controlling the processing of autochthonous DOM remains poorly understood. Aiming to characterize phytoplankton-derived DOM and the effects of complex pelagic communities on its dynamics, we incubated natural plankton communities from a temperate mesohaline estuary under controlled conditions for 18 days. The incubations were carried out in contrasting seasons (spring and autumn) and changes in the planktonic community (phytoplankton, bacteria and microzooplankton), nutrients and DOM were assessed. Our results highlight the complexity of DOM production and fate in natural planktonic communities. Small changes in DOM composition were observed in the experiments relative to the orders-of-magnitude variations experienced in the phytoplankton assembly. We argue that the tight coupling between microbial processing and DOM production by phytoplankton and grazers stabilizes variations in quantity and characteristics of autochthonous DOM, resulting in apparently homogeneous semi-labile DOM pool throughout the experiments. However, seasonal differences in the production and processing of DOM were observed, reflecting differences in the nutrient regimes and initial DOM characteristics in each experiment, but also likely influenced by changes in the successional status of the pelagic community. Acknowledging that characteristics of the DOM derived from phytoplankton growth can vary broadly, heterotrophic processing and successional status of the community are synergistically important factors for shaping those characteristics, and thus affecting the seasonal signature of the semi-labile autochthonous DOM pool.peerReviewe

Autochthonous organic matter promotes DNRA and suppresses N2O production in sediments of the coastal Baltic Sea

Coastal environments are nitrogen (N) removal hot spots, which regulate the amount of land-derived N reaching the open sea. However, mixing between freshwater and seawater creates gradients of inorganic N and bioavailable organic matter, which affect N cycling. In this study, we compare nitrate reduction processes between estuary and offshore archipelago environments in the coastal Baltic Sea. Denitrification rates were similar in both environments, despite lower nitrate and carbon concentrations in the offshore archipelago. However, DNRA (dissimilatory nitrate reduction to ammonium) rates were higher at the offshore archipelago stations, with a higher proportion of autochthonous carbon. The production rate and concentrations of the greenhouse gas nitrous oxide (N2O) were higher in the estuary, where nitrate concentrations and allochthonous carbon inputs are higher. These results indicate that the ratio between nitrate and autochthonous organic carbon governs the balance between N-removing denitrification and N-recycling DNRA, as well as the end-product of denitrification. As a result, a significant amount of the N removed in the estuary is released as N2O, while the offshore archipelago areas are characterized by efficient internal recycling of N. Our results challenge the current understanding of the role of these regions as filters of land-to-sea transfer of N.Peer reviewe

Contrasting patterns of carbon cycling and dissolved organic matter processing in two phytoplankton-bacteria communities

Microbial consumption of phytoplankton-derived organic carbon in the pelagic food web is an important component of the global C cycle. We studied C cycling in two phytoplankton-bacteria systems (non-axenic cultures of a dinoflagellate Apocalathium malmogiense and a cryptophyte Rhodomonas marina) in two complementary experiments. In the first experiment we grew phytoplankton and bacteria in nutrient-replete conditions and followed C processing at early exponential growth phase and twice later when the community had grown denser. Cell-specific primary production and total community respiration were up to 4 and 7 times higher, respectively, in the A. malmogiense treatments. Based on the optical signals, accumulating dissolved organic C (DOC) was degraded more in the R. marina treatments, and the rate of bacterial production to primary production was higher. Thus, the flow of C from phytoplankton to bacteria was relatively higher in R. marina treatments than in A. malmogiense treatments, which was further supported by faster C-14 transfer from phytoplankton to bacterial biomass. In the second experiment we investigated consumption of the phytoplankton-derived DOC by bacteria. DOC consumption and transformation, bacterial production, and bacterial respiration were all higher in R. marina treatments. In both experiments A. malmogiense supported a bacterial community predominated by bacteria specialized in the utilization of less labile DOC (class Bacteroidia), whereas R. marina supported a community predominated by copiotrophic Alphaand Gammaproteobacteria. Our findings suggest that large dinoflagellates cycle relatively more C between phytoplankton biomass and the inorganic C pool, whereas small cryptophytes direct relatively more C to the microbial loop.Peer reviewe

Elevated organic carbon pulses persist in estuarine environment after major storm events

Estuaries regulate transport of dissolved organic carbon (DOC) from land to ocean. Export of terrestrial DOC from coastal watersheds is exacerbated by increasing major rainfall and storm events and human activities, leading to pulses of DOC that are shunted through rivers downstream to estuaries. Despite an upward trend of extreme events, the fate of the pulsed terrestrial DOC in estuaries remains unclear. We analyzed the effects of seven major tropical cyclones (TC) from 1999 to 2017 on the quantity and fate of DOC in the Neuse River Estuary (NC, USA). Significant TC-induced increases in DOC were observed throughout the estuary; the increase lasting from around 50 d at head-of-tide to over 6 months in lower estuary. Our results suggest that pulsed terrestrial DOC associated with TCs temporarily overwhelms the estuarine filter's abiotic and biotic degradation capacity under such high flow events, enhancing the shunt of terrestrial carbon to the coastal ocean.Non peer reviewe

Role of particle dynamics in processing of terrestrial nitrogen and phosphorus in the estuarine mixing zone

Multiple biogeochemical processes in estuaries modulate the flux of nutrients from land to sea, thus contributing to the coastal filter. The role of particle dynamics in regulating the fate of terrestrial nutrients in estuaries is poorly constrained. To address this issue, we resolved the particle size distribution of suspended material, and quantified size-fractionated particulate nitrogen (PN) and phosphorus (PP), in a stratified mesotrophic estuary (Pojoviken, Finland). We also carried out a mixing experiment where the effects of salt-induced flocculation on particle size distribution and concentrations of PN and PP were examined. The experimental results showed that salt-induced flocculation at already very low salinities increases the total particle concentration and mean particle size, indicating transfer of dissolved material into particulates. Correspondingly, a significant increase in PP and particulate iron (Fe) was observed in the experiment results, suggesting coupled flocculation of P-containing organic matter (OM) and ferrihydrite. Particle dynamics in the field data were dominated by processes occurring downstream of the flocculation zone. Primary production created a downward flux of autochthonous OM particles, promoting passive aggregation by random collisions with terrestrial material in the water column. Maximum particle concentrations were observed at and below the halocline. The highest PN and PP concentrations were observed in the subhalocline layer, 3.5 and 0.14 mu mol L-1, respectively. Molar ratios of N:P in this material were >40, consistent with typical marine snow in the early stages of microbial processing. Our study provides a mechanistic overview of the biogeochemical drivers of particulate nutrient dynamics in stratified estuarine environments.Peer reviewe