37 research outputs found

    Denitrification in an oligotrophic estuary: a delayed sink for riverine nitrate

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    Estuaries are often seen as natural filters of riverine nitrate, but knowledge of this nitrogen sink in oligotrophic systems is limited. We measured spring and summer dinitrogen production (denitrification, anammox) in muddy and non-permeable sandy sediments of an oligotrophic estuary in the northern Baltic Sea, to estimate its function in mitigating the riverine nitrate load. Both sediment types had similar denitrification rates, and no anammox was detected. In spring at high nitrate loading, denitrification was limited by likely low availability of labile organic carbon. In summer, the average denitrification rate was similar to 138 mu mol N m(-2) d(-1). The corresponding estuarine nitrogen removal for August was similar to 1.2 t, of which similar to 93% was removed by coupled nitrification-denitrification. Particulate matter in the estuary was mainly phytoplankton derived (> 70% in surface waters) and likely based on the riverine nitrate which was not removed by direct denitrification due to water column stratification. Subsequently settling particles served as a link be tween the otherwise uncoupled nitrate in surface waters and benthic nitrogen removal. We suggest that the riverine nitrate brought into the oligotrophic estuary during the spring flood is gradually, and with a time delay, removed by benthic denitrification after being temporarily ` trapped' in phytoplankton particulate matter. The oligotrophic system is not likely to face eutrophication from increasing nitrogen loading due to phosphorus limitation. In response, coupled nitrification-denitrification rates are likely to stay constant, which might increase the future export of nitrate to the open sea and decrease the estuary's function as a nitrogen sink relative to the load.Peer reviewe

    Particulate organic matter controls benthic microbial N retention and N removal in contrasting estuaries of the Baltic Sea

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    Estuaries worldwide act as "filters" of landderived nitrogen (N) loads, yet differences in coastal environmental settings can affect the N filter function. We investigated microbial N retention (nitrification, ammonium assimilation) and N removal (denitrification, anammox) processes in the aphotic benthic system (bottom boundary layer (BBL) and sediment) of two Baltic Sea estuaries differing in riverine N loads, trophic state, geomorphology, and sediment type. In the BBL, rates of nitrification (5-227 nmol N L-1 d(-1)) and ammonium assimilation (9-704 nmol N L-1 d(-1)) were not enhanced in the eutrophied Vistula Estuary compared to the oligotrophic Ore Estuary. No anammox was detected in the sediment of either estuary, while denitrification rates were twice as high in the eutrophied (352 +/- 123 mu mol N m(-2) d(-1)) as in the oligotrophic estuary. Particulate organic matter (POM) was mainly of phytoplankton origin in the benthic systems of both estuaries. It seemed to control heterotrophic denitrification and ammonium assimilation as well as autotrophic nitrification by functioning as a substrate source of N and organic carbon. Our data suggest that in stratified estuaries, POM is an essential link between riverine N loads and benthic N turnover and may furthermore function as a temporary N reservoir. During long particle residence times or alongshore transport pathways, increased time is available for the recycling of N until its eventual removal, allowing effective coastal filtering even at low process rates. Understanding the key controls and microbial N processes in the coastal N filter therefore requires to also consider the effects of geomorphological and hydrological features.Peer reviewe

    Linking process rates with modelling data and ecosystem characteristics

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    This report is related to the BONUS project “Nutrient Cocktails in COAstal zones of the Baltic Sea” alias COCOA. The aim of BONUS COCOA is to investigate physical, biogeochemical and biological processes in a combined and coordinated fashion to improve the understanding of the interaction of these processes on the removal of nutrients along the land-sea interface. The report is especially related to BONUS COCOA WP 6 in which the main objective is extrapolation of results from the BONUS COCOA learning sites to coastal sites around the Baltic Sea in general. Specific objectives of this deliverable (D6.4) were to connect observed process rates with modelling data and ecosystem characteristics. In the report we made statistical analyses of observations from BONUS COCOA study sites together with results from the Swedish Coastal zone Model (SCM). Eight structural variables (water depth, temperature, salinity, bottom water concentrations of oxygen, ammonium, nitrate and phosphate, as well as nitrogen content in sediment) were found common to both the experimentally determined and the model data sets. The observed process rate evaluated in this report was denitrification. In addition regressions were tested between observed denitrification rates and several structural variables (latitude, longitude, depth, light, temperature, salinity, grain class, porosity, loss of ignition, sediment organic carbon, total nitrogen content in the sediment,  sediment carbon/nitrogen-ratio, sediment chlorphyll-a as well as bottom water concentrations of oxygen, ammonium, nitrate, and dissolved inorganic  phosphorus and silicate) for pooled data from all learning sites. The statistical results showed that experimentally determined multivariate data set from the shallow, illuminated stations was mainly found to be similar to the multivariate data set produced by the SCM model. Generally, no strong correlations of simple relations between observed denitrification and available structural variables were found for data collected from all the learning sites. We found some non-significant correlation between denitrification rates and bottom water dissolved inorganic phosphorous and dissolved silica but the reason behind the correlations is not clear. We also developed and evaluated a theory to relate process rates to monitoring data and nutrient retention. The theoretical analysis included nutrient retention due to denitrification as well as burial of phosphorus and nitrogen. The theory of nutrient retention showed good correlations with model results. It was found that area-specific nitrogen and phosphorus retention capacity in a sub-basin depend much on mean water depth, water residence time, basin area and the mean nutrient concentrations in the active sediment layer and in the water column

    Effects of CO2 perturbation on phosphorus pool sizes and uptake in a mesocosm experiment during a low productive summer season in the northern Baltic Sea

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    Studies investigating the effect of increasing CO2 levels on the phosphorus cycle in natural waters are lacking although phosphorus often controls phytoplankton development in many aquatic systems. The aim of our study was to analyse effects of elevated CO2 levels on phosphorus pool sizes and uptake. The phosphorus dynamic was followed in a CO2-manipulation mesocosm experiment in the Storfjarden (western Gulf of Finland, Baltic Sea) in summer 2012 and was also studied in the surrounding fjord water. In all mesocosms as well as in surface waters of Storfjarden, dissolved organic phosphorus (DOP) concentrations of 0.26aEuro-+/- aEuro-0.03 and 0.23aEuro-+/- aEuro-0.04aEuro-A mu molaEuro-L-1, respectively, formed the main fraction of the total P-pool (TP), whereas phosphate (PO4) constituted the lowest fraction with mean concentration of 0.15aEuro-A +/- aEuro-0.02 in the mesocosms and 0.17aEuro-A +/- aEuro-0.07aEuro-A mu molaEuro-L-1 in the fjord. Transformation of PO4 into DOP appeared to be the main pathway of PO4 turnover. About 82aEuro-% of PO4 was converted into DOP whereby only 18aEuro-% of PO4 was transformed into particulate phosphorus (PP). PO4 uptake rates measured in the mesocosms ranged between 0.6 and 3.9aEuro-nmolaEuro-L(-1)aEuro-h(-1). About 86aEuro-% of them was realized by the size fraction aEuro-1000aEuro-A mu atm during periods when phytoplankton biomass increased. In addition, we found significant relationships (e.g., between PP and Chl a) in the untreated mesocosms which were not observed under high fCO(2) conditions. Consequently, it can be hypothesized that the relationship between PP formation and phytoplankton growth changed with CO2 elevation. It can be deduced from the results, that visible effects of CO2 on P pools are coupled to phytoplankton growth when the transformation of PO4 into POP was stimulated. The transformation of PO4 into DOP on the other hand does not seem to be affected. Additionally, there were some indications that cellular mechanisms of P regulation might be modified under CO2 elevation changing the relationship between cellular constituents.Peer reviewe

    Enhanced transfer of organic matter to higher trophic levels caused by ocean acidification and its implications for export production : A mass balance approach

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    Ongoing acidification of the ocean through uptake of anthropogenic CO2 is known to affect marine biota and ecosystems with largely unknown consequences for marine food webs. Changes in food web structure have the potential to alter trophic transfer, partitioning, and biogeochemical cycling of elements in the ocean. Here we investigated the impact of realistic end-of-the-century CO2 concentrations on the development and partitioning of the carbon, nitrogen, phosphorus, and silica pools in a coastal pelagic ecosystem (Gullmar Fjord, Sweden). We covered the entire winter-to-summer plankton succession (100 days) in two sets of five pelagic mesocosms, with one set being CO2 enriched (similar to 760 mu atm pCO(2)) and the other one left at ambient CO2 concentrations. Elemental mass balances were calculated and we highlight important challenges and uncertainties we have faced in the closed mesocosm system. Our key observations under high CO2 were: (1) A significantly amplified transfer of carbon, nitrogen, and phosphorus from primary producers to higher trophic levels, during times of regenerated primary production. (2) A prolonged retention of all three elements in the pelagic food web that significantly reduced nitrogen and phosphorus sedimentation by about 11 and 9%, respectively. (3) A positive trend in carbon fixation (relative to nitrogen) that appeared in the particulate matter pool as well as the downward particle flux. This excess carbon counteracted a potential reduction in carbon sedimentation that could have been expected from patterns of nitrogen and phosphorus fluxes. Our findings highlight the potential for ocean acidification to alter partitioning and cycling of carbon and nutrients in the surface ocean but also show that impacts are temporarily variable and likely depending upon the structure of the plankton food web.Peer reviewe

    Influence of Ocean Acidification on a Natural Winter-to-Summer Plankton Succession : First Insights from a Long-Term Mesocosm Study Draw Attention to Periods of Low Nutrient Concentrations

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    Every year, the oceans absorb about 30% of anthropogenic carbon dioxide (CO2) leading to a re-equilibration of the marine carbonate system and decreasing seawater pH. Today, there is increasing awareness that these changes-summarized by the term ocean acidification (OA)-could differentially affect the competitive ability of marine organisms, thereby provoking a restructuring of marine ecosystems and biogeochemical element cycles. In winter 2013, we deployed ten pelagic mesocosms in the Gullmar Fjord at the Swedish west coast in order to study the effect of OA on plankton ecology and biogeochemistry under close to natural conditions. Five of the ten mesocosms were left unperturbed and served as controls (similar to 380 mu atm pCO(2)), whereas the others were enriched with CO2-saturated water to simulate realistic end-of-the-century carbonate chemistry conditions (mu 760 mu atm pCO(2)). We ran the experiment for 113 days which allowed us to study the influence of high CO2 on an entire winter-to-summer plankton succession and to investigate the potential of some plankton organisms for evolutionary adaptation to OA in their natural environment. This paper is the first in a PLOS collection and provides a detailed overview on the experimental design, important events, and the key complexities of such a "long-term mesocosm" approach. Furthermore, we analyzed whether simulated end-of-the-century carbonate chemistry conditions could lead to a significant restructuring of the plankton community in the course of the succession. At the level of detail analyzed in this overview paper we found that CO2-induced differences in plankton community composition were non-detectable during most of the succession except for a period where a phytoplankton bloom was fueled by remineralized nutrients. These results indicate: (1) Long-term studies with pelagic ecosystems are necessary to uncover OA-sensitive stages of succession. (2) Plankton communities fueled by regenerated nutrients may be more responsive to changing carbonate chemistry than those having access to high inorganic nutrient concentrations and may deserve particular attention in future studies.Peer reviewe

    Nitrogen cycling in aphotic coastal sandy sediments of the Baltic Sea

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    Benthic nitrogen (N) cycling in sandy sediments in the stratified aphotic coastal zone (> 15 m) of the Baltic Sea was investigated along a north–south environmental gradient of N loading, trophic status, coastal geomorphology and sediment permeability. The aim was to establish a more comprehensive view of the Baltic Sea coastal N filter, where N transformation processes remove (via denitrification and anaerobic ammonium oxidation) and retain (via dissimilatory nitrate reduction to ammonium) land-derived N and thereby reduce its availability to the open sea; so far these processes have not been quantified in the deeper, aphotic sandy sediments. The main results are that a) not all sandy sediments were permeable enough to experience advective pore-water flow – mass transport in non-permeable sands functions via diffusion and fauna-mediated fluxes only, which simplifies biogeochemical measurement design; b) N removal rates were affected by the availability of labile particulate organic matter as a source of labile organic carbon and N, resulting in higher removal rates in eutrophic than in oligotrophic conditions, as well as similar removal rates in non-permeable sands and muds when also the substrate availability was similar; c) seasonal N removal in the stratified aphotic coastal zone is largely driven by the hydrography-controlled development of bottom water temperature, and differs from the seasonal pattern observed in the mixed photic coastal zone; and d) the role of dissimilatory nitrate reduction to ammonium in the aphotic coastal sandy sediments of the Baltic Sea is presumably more important than previously anticipated. These results indicate that the sandy sediments in the aphotic coastal zone of the Baltic Sea have an important role in N removal and retention, and are thus an integral component of the Baltic coastal N filter. The results further show the strong influence of the local environment on N cycling rates, emphasizing the need for context dependent data analysis, particularly in a diverse coastal setting such as in the Baltic Sea

    Benthic N2 production rates (denitrification, anammox) in sand and mud sediments of the TvÀrminne Archipelago, southern Finland, and accompanying environmental characteristics

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    Sediment samples for measuring N2 production rates and for the characterization of the sediment system were taken every month (early spring to late autumn) over 2 years from a sand and a mud sediment in the TvĂ€rminne Archipelago at the Finnish coast of the Gulf of Finland, Northern Baltic Sea. Sediment was sampled with a HAPS bottom corer (sand) and a GEMAX twin corer (mud). Bottom water dissolved oxygen (O2) and nutrients (nitrate [NO3-], nitrate + nitrite [NO3- + NO2-], ammonium [NH4+], phosphate [PO43-]) were analysed from a sample withdrawn ~ 5 cm above the sediment surface. O2 concentrations were measured with Winkler titration; nutrient samples were 0.2 ”m filtered and kept dark and cool until measurement with an autosampler (Thermo Scientific Aquakem 250). Photosynthetic active radiation (PAR) was measured close to the bottom and at the water surface with a spherical light sensor (Li-COR). The grain size of the sand sediment was analyzed with a particle detector (CILAS 1180 Naß) and the sediment type was classified after Wentworth (1922). Porosity and water content were analyzed both from sediment slices and from entire core subsamples, which means that sediment in a sampling core (heights 20 cm, iD 2.3 cm) was mixed and sub sampled, assuming vertical homogeneity; sediment was dried overnight at 105°C and calculations followed Burdige (2006). Sediment organic matter content was analyzed as loss on ignition (LOI), for which dried sediment was combusted at 550°C for 4h. Sediment permeability was measured from pooled surface sediments (~1-2 cm homogeneous surface layer) with a permeameter cell following the constant head method for laminar flow of water through granular soil; calculations were derived from Darcy's Law. The oxygen penetration depth (OPD) was determined by automated O2 profiling (except for May, October 2015: manual profiling) at bottom water temperature, electrode tip 100 ”m (except for May 2016, StorfjĂ€rden: 250 ”m). Benthic denitrification rates were measured with the revised isotope pairing technique (r-IPT; Risgaard-Petersen et al. 2003, 2004) that accounts for the potential contribution of anammox (anaerobic ammonium oxidation) to total N2 production. Incubations were done in acrylic cores (heights 15-20 cm, iD 2.3 cm) in a concentration series of 30, 60, 90, 120 ”M 15NO3- (n=3; 2015), and 40 (n=4), 80 (n=4), 120 (n=8) ”M 15NO3- (2016) for 4h at in situ bottom water temperature and darkness. Incubations were done in diffusive cores in both mud and sand, as sand permeability was <2.5*10-12 m2 and thus significant effects of advective pore water flow on sediment biogeochemistry could be neglected (2.5*10-12 m2 was used as threshold for the onset of effects of advection in Baltic Sea coastal sediments according to Forster et al. 2003). Dw gives denitrification of nitrate from the water column; Dn gives denitrification of nitrate from sediment nitrification (coupled nitrification-denitrification). If no contribution of anammox to total N2 production was found, columns hold a zero (0)

    Benthic N2 production rates (denitrification, anammox), sediment characteristics and silicate in coastal sediments of the Vistula Estuary and the Bay of Gdansk during 3 seasons (winter, sping, summer)

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    Sediment samples for measuring N2 production rates and for the characterization of the sediment system were taken in three seasons (winter, spring, summer) from sand and mud sediments of the Vistula Estuary and the open Bay of Gdansk at the Polish coast, Southern Baltic Sea. Sediment was sampled with a HAPS bottom corer (coarse sand), a Multicorer and a Boxcorer (fine sand, mud). Porosity was analyzed both from sediment slices and from entire core subsamples, which means that sediment in a sampling core (heights 15-20 cm, iD 2.3 cm) was mixed and sub sampled, assuming vertical homogeneity; sediment was dried overnight at 105°C and calculations followed Burdige (2006). Sediment organic matter content was analyzed as loss on ignition (LOI), for which dried sediment was combusted at 550°C for 4h. Sediment permeability was measured from pooled surface sediments (~1-2 cm homogeneous surface layer) with a permeameter cell following the constant head method for laminar flow of water through granular soil; calculations were derived from Darcy's Law. The oxygen penetration depth (OPD) was determined by manual (EMB 77, AL 449) and automated (EMB 123) profiling at bottom water temperature, electrode tip 100 ”m: EMB 123, AL 449 (mud) and 250 ”m: EMB 77, AL 449 (sand), EMB 123 (VE05, VE49). Denitrification rates were measured with the revised isotope pairing technique (r-IPT; Risgaard-Petersen et al. 2003, 2004) that accounts for the potential contribution of anammox (anaerobic ammonium oxidation) to total N2 production. Incubations were done in acrylic cores (heights 15-20 cm, iD 2.3 cm) in a concentration series of 30, 60, 90, 120 ”M 15NO3- (n=3, EMB 77, AL 449) and 40 (n=4), 80 (n=4), 120 (n=12) ”M 15NO3- for 3-5h at in situ bottom water temperature and darkness. In the presence of significant advective pore water flow, an advective incubation design was used. Dw gives denitrification of nitrate from the water column; Dn gives denitrification of nitrate from sediment nitrification (coupled nitrification-denitrification). If no contribution of anammox to total N2 production was found, columns hold a zero (0). The sediment silicate content (ASi =amorphous, biogenic Si (Na2CO3-extractable), Ca-Si = easily available Si (CaCl2-extractable), Ox-Si = oxide-bound Si (extractable by acid oxalate)) was analyzed from the top sediment layer
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