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The absorption spectral slope, S275–295, is an optical metric frequently employed for characterizing chromophoric dissolved organic matter (CDOM). We collected CDOM absorption (aCDOM) and fluorescence spectra from the oligotrophic offshore South China Sea to identify the major determinant of S275–295 and to explore the potential of S275–295 as physical and biogeochemical tracers. S275–295 linearly decreased with increasing humic‐like fluorescent DOM, revealing the latter as the primary control on S275–295. The variability of S275–295 consistently declined from the surface to deep water, making S275–295 a potential indicator of CDOM molecular heterogeneity. The S275–295 vs. aCDOM plot was found to be a useful tool for characterizing water masses. The 14C content of dissolved organic carbon (Δ14C‐DOC) shows a strong correlation to S275–295, demonstrating S275–295 to be a proxy of the 14C‐age of DOM. A provisional equation, ln(−Δ14C‐DOC) = (−0.85 ± 0.06) × ln(S275–295) + (8.78 ± 0.19), was proposed to estimate Δ14C‐DOC from S275–295 for global oligotrophic oceans. [ABSTRACT FROM AUTHOR]

Dissolved organic matter (DOM) in the transition zone from freshwater to marine systems was analyzed with a new approach for parameterizing the size distribution of organic compounds. We used size‐exclusion chromatography for molecular size analysis and quantified colored DOM (CDOM) on samples from two coastal environments in the Baltic Sea (Roskilde Fjord, Denmark and Gulf of Gdansk, Poland). We applied a Gaussian decomposition method to identify peaks from the chromatograms, providing information beyond bulk size properties. This approach complements methods where DOM is separated into size classes with pre‐defined filtering cutoffs, or methods where chromatograms are used only to infer average molecular weight. With this decomposition method, we extracted between three and five peaks from each chromatogram and clustered these into three size groups. To test the applicability of our method, we linked our decomposed peaks with salinity, a major environmental driver in the freshwater‐marine continuum. Our results show that when moving from freshwater to low‐salinity coastal waters, the observed steep decrease of apparent molecular weight is mostly due to loss of the high‐molecular‐weight fraction (HMW; >2 kDa) of CDOM. Furthermore, most of the CDOM absorbance in freshwater originates from HMW DOM, whereas the absorbing moieties are more equally distributed along the smaller size range (< 2 kDa) in marine samples. [ABSTRACT FROM AUTHOR]

Permafrost thawing mobilizes large quantities of organic carbon that was sequestered in Arctic regions over the last glacial cycle. Processes involved in the oxidation of this carbon need to be further assessed to estimate the fraction to be released into the atmosphere. Shallow tundra ponds are sites of active carbon turnover on the landscape and significant sources of greenhouse gases. Dissolved organic matter (DOM) leached from thawing peat into these ponds is exposed to sunlight, with the potential to accelerate its mineralization directly into CO2 or through the production of more labile molecules. We tested the catalytic effect of sunlight on DOM mineralization in tundra ponds formed on organic‐rich polygonal landscapes originating from syngenetic permafrost, including a pond exposed to active permafrost erosion. Microbial decay rates, measured as the loss of chromophoric DOM, were similar to photodecay rates (1%–3% d−1). Groups of fluorescing molecules were formed through microbial transformation or lost through photolysis at differing rates among studied ponds, with the erosive trough pond presenting a unique response suggesting the involvement of soil microbes. Despite the stimulation of microbial growth under sunlight and the dynamic response of DOM optical properties, the loss of dissolved organic carbon was not significant under any treatment. This suggests that microbial and photochemical mineralization of DOM was slow and potentially substrate‐limited during the dry period when ponds were sampled. The static nature of tundra ponds, with their long water retention time, may thus constrain hot moments when water moves and transports carbon on the landscape. [ABSTRACT FROM AUTHOR]

Arctic photoautotrophic communities must survive through polar night darkness until light returns in spring. We tracked changes in the cellular resource allocations and functional capacities of a polar sea‐ice diatom, Fragilariopsis cylindrus, to understand acclimation processes in both darkness and during the subsequent return to light. We measured parameters at specific time‐points over 3 months of darkness, and then over 6 d after a return to light. Measured parameters included cell number and size, cellular carbon and nitrogen quotas, lipid and pigment contents, concentration of key proteins of the photosynthetic system, photosynthetic parameters based on both variable fluorescence and carbon assimilation, and the level of nonphotochemical quenching. A stable functional state was reached within a few days after the transition to dark and was then maintained throughout 3 months of darkness. The dark period resulted in a decrease of lipid droplet cell quota (−6%), chlorophyll a cell quota (−41%) and the maximum carbon fixation rate per cell (−98%). Return to light after 1.5 months of darkness resulted in a strong induction of nonphotochemical quenching of excitation and a fast recovery of the maximum carbon fixation rate within 1 d, followed by a rapid increase in the cell number. Return to light after 3 months of darkness showed an increase of mortality or a profound downregulation induced over longer periods of darkness. [ABSTRACT FROM AUTHOR]

The article discusses the depth-resolved photochemical lability of dissolved organic matter (DOM) in Western Tropical Pacific Ocean. Also cited are the exposure to solar-simulated radiation and observation of photomineralization of dissolved organic carbon (DOC) and photobleaching of chromophoric DOM (CDOM) of water samples from offshore South China and Philippine Seas, as well as the average radiocarbon age of the DOCs.

The Arctic ice scape is composed by a mosaic of ridges, hummocks, melt ponds, leads, and snow. Under such heterogeneous surfaces, drifting phytoplankton communities are experiencing a wide range of irradiance conditions and intensities that cannot be sampled representatively using single‐location measurements. Combining experimentally derived photosynthetic parameters with transmittance measurements acquired at spatial scales ranging from hundreds of meters (using a remotely operated vehicle, ROV) to thousands of meters (using a surface and underice trawl, SUIT), we assessed the sensitivity of water column primary production estimates to multiscale underice light measurements. Daily primary production calculated from transmittance from both the ROV and the SUIT ranged between 0.004 and 939 mgC·m−2·day−1. Upscaling these estimates at larger spatial scales using satellite‐derived sea ice concentration reduced the variability by 22% (0.004–731 mgC·m−2·day−1). The relative error in primary production estimates was two times lower when combining remote sensing and in situ data compared to ROV‐based estimates alone. These results suggest that spatially extensive in situ measurements must be combined with large‐footprint sea ice coverage sampling (e.g., remote sensing, aerial imagery) to accurately estimate primary production in ice‐covered waters. Also, the results indicated a decreasing error of primary production estimates with increasing sample size and the spatial scale at which in situ measurements are performed. Conversely, existing estimates of spatially integrated phytoplankton primary production in ice‐covered waters derived from single‐location light measurements may be associated with large statistical errors. Considering these implications is important for modeling scenarios and interpretation of existing measurements in a changing Arctic ecosystem. Key Points: Phytoplankton primary production under heterogeneous sea ice is spatially highly variableTransmittance sampled with profiling platforms improves the accuracy of primary production estimatesUpscaling estimates at larger spatial scales using satellite sea ice concentration further reduced the error [ABSTRACT FROM AUTHOR]

Anthropogenic nutrient enrichment is changing the structure and the function of coastal ecosystems. These coastal zones are transitions between freshwater and marine systems where multiple biogeochemical processes remove, produce, and transform organic matter. The extent to which the coastal zone is merely a conduit for terrestrial (allochthonous) organic matter versus a distinct source of autochthonous organic matter fueled by eutrophication is unclear. To address this issue, we characterized the freshwater and marine dissolved organic matter (DOM) pools in a eutrophic estuary with a long water residence time (Roskilde Fjord, Denmark) over an annual cycle. We combined elemental, optical (absorbance and fluorescence), and isotopic analyses to obtain insight about the bulk properties of the DOM pool during this period. We also used sediment traps to analyze the changes related to the exchange of organic matter between the particulate organic matter and DOM fractions. The results showed that labile autochthonous DOM from in situ primary production was rapidly transformed to more recalcitrant DOM that accumulated in the estuary despite continuous exchange with the open sea. Also, parts of the particulate organic matter pool were degraded rapidly (within 24 hr) and transformed into the DOM pool. Accumulated DOM was characterized by relatively low molecular size and stable carbon isotopic value and by high protein‐like fluorescence. These results indicate that autotrophic material can be a major source of specific recalcitrant DOM in eutrophic coastal waters, contributing significantly to the flux of organic carbon to the ocean. Key Points: Nutrient inputs from land are fueling autochthonous DOM and POM production in coastal environmentOrganic matter characterization revealed its rapid transformation in the heterotrophic spiralAccumulated organic matter was characterized by small molecular size and high protein‐like fluorescence [ABSTRACT FROM AUTHOR]

Web-crawling approaches, that is, automated programs data mining the internet to obtain information about a particular process, have recently been proposed for monitoring early signs of ecosystem degradation or for establishing crop calendars. However, lack of a clear conceptual and methodological framework has prevented the development of such approaches within the field of conservation biology. Our objective was to illustrate how Google Trends, a freely accessible web-crawling engine, can be used to track changes in timing of biological processes, spatial distribution of invasive species, and level of public awareness about key conservation issues. Google Trends returns the number of internet searches that were made for a keyword in a given region of the world over a defined period. Using data retrieved online for 13 countries, we exemplify how Google Trends can be used to study the timing of biological processes, such as the seasonal recurrence of pollen release or mosquito outbreaks across a latitudinal gradient. We mapped the spatial extent of results from Google Trends for 5 invasive species in the United States and found geographic patterns in invasions that are consistent with their coarse-grained distribution at state levels. From 2004 through 2012, Google Trends showed that the level of public interest and awareness about conservation issues related to ecosystem services, biodiversity, and climate change increased, decreased, and followed both trends, respectively. Finally, to further the development of research approaches at the interface of conservation biology, collective knowledge, and environmental management, we developed an algorithm that allows the rapid retrieval of Google Trends data. [ABSTRACT FROM AUTHOR]

Five characteristics (intensity or magnitude, duration, frequency, timing, and variability) of drought, defined using the threshold level method (TLM) and recorded in mean annual water levels in Lake Ontario and the St. Lawrence River from 1918 to 2010, were compared. Timing is the only characteristic that is different for the two water bodies. For Lake Ontario, the most intense drought occurred in the 1930s, whereas in the St. Lawrence River, intense droughts took place in the 1960s and 2000s. The Lake Ontario drought produced two shifts in mean before (decrease) and after (increase) the 1930s. The change in variance that took place in the 1960s is thought to be related to the construction of locks during the digging of the seaway. The droughts that affected the St. Lawrence River had no impact on the stationarity (mean and variance) of the annual mean water level series. Analysis of the correlation between drought severity and climate indices revealed that years characterized by very weak to moderate drought are significantly correlated with PDO (Pacific Decadal Oscillation), while those characterized by intense drought are correlated with NAO (North Atlantic Oscillation). Both climate indices are negatively correlated with Lake Ontario water levels, while they are positively correlated with St. Lawrence River levels. The study suggests that NAO may be used to predict the driest years for the two water bodies. [ABSTRACT FROM AUTHOR]

The relationship between the color (measured with Landsat Thematic Mapper and Enhanced Thematic Mapper sensors in bands 1, 2, and 3) of St. Lawrence River (SLR) water and water levels (in SLR and Lake Ontario) was first analyzed, followed by the analysis of the relationship between water levels and four climate indices for the period from 1984 to 2009. Although there is a statistically significant relationship between these first two variables, this link depends on the strength of two factors: the type of color (band) and inflows from tributaries. Bands 1 (blue) and 2 (green) were negatively correlated with water levels. This correlation, however, is spatially influenced by water inflows from tributaries. Band 3 (red) showed no significant correlation with either water level. Over the same period, water levels were negatively correlated with the Atlantic multidecadal oscillation index. Results suggest that the color of waters in the SLR may be used as an indicator to monitor environmental and climate changes in its watershed. [ABSTRACT FROM AUTHOR]

The article presents a study that examines the spatial connectivity in a large river system through the sources and fate of dissolved organic matter (DOM). The study utilizes three-dimensional fluorescence matrices combined with parallel factor analysis (PARAFAC) to characterize the DOM pool in Saint Lawrence River (SLR). It also reveals the importance of spatial connectivity for DOM dynamics.