Exploratory study of the Sentinel-3 level 2 product for monitoring chlorophyll-a and assessing ecological status in Danish seas.

In situ Chl-a data were used to perform empirical calibration and validation of Sentinel-3 level 2 product in Danish marine waters. Comparing in situ data with both same-day and ±5 days moving averaged Sentiel-3 Chl-a values yielded two similar positive correlations (p > 0.05) with rpearson values of 0.56 and 0.53, respectively. However, as the moving averaged values resulted in significantly more available data than daily matchups (N = 392 vs. N = 1292) at a similar quality of correlation with similar model parameters (slope (1.53 and 1.7) and intercept (-0.28 and -0.33) respectively), which were not significantly different (p > 0.05), the further analyses were focused on ±5 days moving averaged values. A thorough comparison of seasonal and growing season averages (GSA) also showed a very good agreement, except for a few stations characterized by very shallow depth. Overestimation by the Sentinel-3 occurred in shallow coastal areas and was attributed to the interferences from benthic vegetation and high levels of Colored Dissolved Organic matter (CDOM) interfering with the Chl-a signals. Underestimation observed in the inner estuaries with shallow Chl-a rich waters, however, seen as a result of self-shading at high Chl-a concentrations, reducing effective absorption by phytoplankton. Besides the observed minor disagreements, there was no significant difference when the GSA values from in situ and Sentinel-3 were compared for all three water types (p > 0.05, N = 110). Analyzing Chl-a estimates along a depth gradient showed significant (p < 0.001) non-linear trends of declining concentrations from shallow to deeper waters for both in situ (explaining 15.2 % of the variance (N = 109)) and Sentinel-3 data (explaining 36.3 % of the variance (N = 110)), with higher variability in shallow waters. Furthermore, Sentinel-3 enabled full spatial coverage of all 102 monitored water bodies providing GSA data at much higher spatial and temporal resolutions for good ecological status (GES) assessment compared to only 61 through in situ sampling. This underlines the potential of Sentinel-3 for substantially extending the geographical coverage of monitoring and assessment. However, the systematic over- and underestimation of Chl-a in shallow nutrient rich inner estuaries through Sentinel-3 requires further attention to enable routine application of the Sentinel-3 level 2 standard product in the operational Chl-a monitoring in Danish coastal waters. We provide methodological recommendations on how to improve the Sentinel-3 products' representation of in situ Chl-a conditions. Continued frequent in situ sampling remains important for monitoring as these measurements provide essential data for empirical calibration and validation of satellite based estimates to reduce possible systematic bias.

Pigment signatures of algal communities and their implications for glacier surface darkening.

Blooms of pigmented algae darken the surface of glaciers and ice sheets, thereby enhancing solar energy absorption and amplifying ice and snow melt. The impacts of algal pigment and community composition on surface darkening are still poorly understood. Here, we characterise glacier ice and snow algal pigment signatures on snow and bare ice surfaces and study their role in photophysiology and energy absorption on three glaciers in Southeast Greenland. Purpurogallin and astaxanthin esters dominated the glacier ice and snow algal pigment pools (mass ratios to chlorophyll a of 32 and 56, respectively). Algal biomass and pigments impacted chromophoric dissolved organic matter concentrations. Despite the effective absorption of astaxanthin esters at wavelengths where incoming irradiance peaks, the cellular energy absorption of snow algae was 95% lower than anticipated from their pigmentation, due to pigment packaging. The energy absorption of glacier ice algae was consequently ~ 5 × higher. On bare ice, snow algae may have locally contributed up to 13% to total biological radiative forcing, despite contributing 44% to total biomass. Our results give new insights into the impact of algal community composition on bare ice energy absorption and biomass accumulation during snow melt.

Contribution from a eutrophic temperate estuary to the landscape flux of nitrous oxide.

For mitigation of climate change, all sources and sinks of greenhouse gases from the environment must be quantified and their driving factors identified. Nitrous oxide (N2O) is a strong greenhouse gas, and the contribution of aquatic systems to the global N2O budget remains poorly constrained. In this study, we measured N2O concentrations in a eutrophic coastal system, Roskilde Fjord (Denmark), and combined measurements with statistical modeling to quantify the N2O fluxes and budget in the system over a period of six months. To do so, we collected water at 15 sampling points and measured N2O concentrations along with physico-chemical water quality parameters, e.g. temperature, salinity, dissolved inorganic nitrogen and phosphorus, and silicon. We used mixed-effect regression models to predict N2O concentrations in the water from water quality parameters. We then derived N2O fluxes using well-established equations of N2O solubility and water-atmosphere exchanges. These fluxes were then put in perspective with those measured at the landscape scale by eddy-covariance at a 96 m nearby tall tower, and to those estimated from the agricultural land next to the fjord using Intergovernmental Panel on Climate Change (IPCC) guidelines. N2O concentrations in the Roskilde Fjord ranged between 2.40 and 8.05 nmol l-1. The best fitting model between water parameters and N2O concentrations in water included phosphorus and temperature. We estimated that (i) Roskilde Fjord was a sink of N2O, with a median inward flux of -0.04 nmol m-2 s-1, (ii) while the surrounding median agricultural flux was 0.13-0.18 nmol m-2 s-1, and (iii) the median landscape flux was 0.07 nmol m-2 s-1. All estimates of N2O fluxes were of the same magnitude and consistent with each other. These preliminary results need to be consolidated by further research.

Effects of changes in land use and climate on aquatic ecosystems: Coupling of models and decomposition of uncertainties.

To analyse the potential future ecological state of estuaries located in the temperate climate (here exemplified with the Odense Fjord estuary, Denmark), we combined end-of-the-century climate change projections from four different climate models, four contrasting land use scenarios ("Agriculture for nature", "Extensive agriculture", "High-tech agriculture" and "Market driven agriculture") and two different eco-hydrological models. By decomposing the variance of the model-simulated output from all scenario and model combinations, we identified the key sources of uncertainties of these future projections. There was generally a decline in the ecological state of the estuary in scenarios with a warmer climate. Strikingly, even the most nature-friendly land use scenario, where a proportion of the intensive agricultural area was converted to forest, may not be enough to counteract the negative effects of a future warmer climate on the ecological state of the estuary. The different land use scenarios were the most significant sources of uncertainty in the projections of future ecological state, followed, in order, by eco-hydrological models and climate models, albeit all three sources caused high variability in the simulated outputs. Therefore, when projecting the future state of aquatic ecosystems in a global warming context, one should at the very least consider to evaluate an ensemble of land use scenarios (nutrient loads) but ideally also include multiple eco-hydrological models and climate change projections. Our study may set precedence for future attempts to predict and quantify uncertainties of model and model input ensembles, as this will likely be key elements in future tools for decision-making processes.

Global distribution of dissolved organic matter along the aquatic continuum: Across rivers, lakes and oceans.

Based on an extensive literature survey containing more than 12,000 paired measurements of dissolved organic carbon (DOC) concentrations and absorption of chromophoric dissolved organic matter (CDOM) distributed over four continents and seven oceans, we described the global distribution and transformation of dissolved organic matter (DOM) along the aquatic continuum across rivers and lakes to oceans. A strong log-linear relationship (R2=0.92) between DOC concentration and CDOM absorption at 350nm was observed at a global scale, but was found to be ecosystem-dependent at local and regional scales. Our results reveal that as DOM is transported towards the oceans, the robustness of the observed relation decreases rapidly (R2 from 0.94 to 0.44) indicating a gradual decoupling between DOC and CDOM. This likely reflects the decreased connectivity between the landscape and DOM along the aquatic continuum. To support this hypothesis, we used the DOC-specific UV absorbance (SUVA) to characterize the reactivity of the DOM pool which decreased from 4.9 to 1.7m2 × gC-1 along the aquatic continuum. Across the continuum, a piecewise linear regression showed that the observed decrease of SUVA occurred more rapidly in freshwater ecosystems compared to marine water ecosystems, suggesting that the different degradation processes act preferentially on CDOM rather than carbon content. The observed change in the DOM characteristics along the aquatic continuum also suggests that the terrestrial DOM pool is gradually becoming less reactive, which has profound consequences on cycling of organic carbon in aquatic ecosystems.

Coupling Bacterioplankton Populations and Environment to Community Function in Coastal Temperate Waters.

Bacterioplankton play a key role in marine waters facilitating processes important for carbon cycling. However, the influence of specific bacterial populations and environmental conditions on bacterioplankton community performance remains unclear. The aim of the present study was to identify drivers of bacterioplankton community functions, taking into account the variability in community composition and environmental conditions over seasons, in two contrasting coastal systems. A Least Absolute Shrinkage and Selection Operator (LASSO) analysis of the biological and chemical data obtained from surface waters over a full year indicated that specific bacterial populations were linked to measured functions. Namely, Synechococcus (Cyanobacteria) was strongly correlated with protease activity. Both function and community composition showed seasonal variation. However, the pattern of substrate utilization capacity could not be directly linked to the community dynamics. The overall importance of dissolved organic matter (DOM) parameters in the LASSO models indicate that bacterioplankton respond to the present substrate landscape, with a particular importance of nitrogenous DOM. The identification of common drivers of bacterioplankton community functions in two different systems indicates that the drivers may be of broader relevance in coastal temperate waters.

Phytoplankton Productivity in an Arctic Fjord (West Greenland): Estimating Electron Requirements for Carbon Fixation and Oxygen Production.

Accurate quantification of pelagic primary production is essential for quantifying the marine carbon turnover and the energy supply to the food web. Knowing the electron requirement (Κ) for carbon (C) fixation (ΚC) and oxygen (O2) production (ΚO2), variable fluorescence has the potential to quantify primary production in microalgae, and hereby increasing spatial and temporal resolution of measurements compared to traditional methods. Here we quantify ΚC and ΚO2 through measures of Pulse Amplitude Modulated (PAM) fluorometry, C fixation and O2 production in an Arctic fjord (Godthåbsfjorden, W Greenland). Through short- (2h) and long-term (24h) experiments, rates of electron transfer (ETRPSII), C fixation and/or O2 production were quantified and compared. Absolute rates of ETR were derived by accounting for Photosystem II light absorption and spectral light composition. Two-hour incubations revealed a linear relationship between ETRPSII and gross 14C fixation (R2 = 0.81) during light-limited photosynthesis, giving a ΚC of 7.6 ± 0.6 (mean ± S.E.) mol é (mol C)-1. Diel net rates also demonstrated a linear relationship between ETRPSII and C fixation giving a ΚC of 11.2 ± 1.3 mol é (mol C)-1 (R2 = 0.86). For net O2 production the electron requirement was lower than for net C fixation giving 6.5 ± 0.9 mol é (mol O2)-1 (R2 = 0.94). This, however, still is an electron requirement 1.6 times higher than the theoretical minimum for O2 production [i.e. 4 mol é (mol O2)-1]. The discrepancy is explained by respiratory activity and non-photochemical electron requirements and the variability is discussed. In conclusion, the bio-optical method and derived electron requirement support conversion of ETR to units of C or O2, paving the road for improved spatial and temporal resolution of primary production estimates.

Significant N2 fixation by heterotrophs, photoheterotrophs and heterocystous cyanobacteria in two temperate estuaries.

Nitrogen (N) fixation is fueling planktonic production in a multitude of aquatic environments. In meso- and poly-haline estuaries, however, the contribution of N by pelagic N2 fixation is believed to be insignificant due to the high input of N from land and the presumed absence of active N2-fixing organisms. Here we report N2 fixation rates, nifH gene composition and nifH gene transcript abundance for key diazotrophic groups over 1 year in two contrasting, temperate, estuarine systems: Roskilde Fjord (RF) and the Great Belt (GB) strait. Annual pelagic N2 fixation rates averaged 17 and 61 mmol N m(-2) per year at the two sites, respectively. In RF, N2 fixation was mainly accompanied by transcripts related to heterotrophic (for example, Pseudomonas sp.) and photoheterotrophic bacteria (for example, unicellular diazotrophic cyanobacteria group A). In the GB, the first of two N2 fixation peaks coincided with a similar nifH-expressing community as in RF, whereas the second peak was synchronous with increased nifH expression by an array of diazotrophs, including heterotrophic organisms as well as the heterocystous cyanobacterium Anabaena. Thus, we show for the first time that significant planktonic N2 fixation takes place in mesohaline, temperate estuaries and that the importance of heterotrophic, photoheterotrophic and photosynthetic diazotrophs is clearly variable in space and time.

Bioenergy potential of Ulva lactuca: biomass yield, methane production and combustion.

The biomass production potential at temperate latitudes (56°N), and the quality of the biomass for energy production (anaerobic digestion to methane and direct combustion) were investigated for the green macroalgae, Ulva lactuca. The algae were cultivated in a land based facility demonstrating a production potential of 45T (TS) ha(-1) y(-1). Biogas production from fresh and macerated U. lactuca yielded up to 271 ml CH(4) g(-1) VS, which is in the range of the methane production from cattle manure and land based energy crops, such as grass-clover. Drying of the biomass resulted in a 5-9-fold increase in weight specific methane production compared to wet biomass. Ash and alkali contents are the main challenges in the use of U. lactuca for direct combustion. Application of a bio-refinery concept could increase the economical value of the U. lactuca biomass as well as improve its suitability for production of bioenergy.