Dissolved Organic Matter Dynamics in the Oligo/Meso-Haline Zone of Wetland-Influenced Coastal Rivers

Wetlands are key components in the global carbon cycle and export significant amounts of terrestrial carbon to the coastal oceans in the form of dissolved organic carbon (DOC). Conservative behavior along the salinity gradient of DOC and chromophoric dissolved organic matter (CDOM) has often been observed in estuaries from their freshwater end-member (salinity = 0) to the ocean (salinity = 35). While the oligo/meso-haline (salinity \u3c 10) tidal zone of upper estuaries has been suggested to be more complex and locally influenced by geomorphological and hydrological features, the environmental dynamics of dissolved organic matter (DOM) and the environmental drivers controlling its source, transport, and fate have scarcely been evaluated. Here, we investigated the distribution patterns of DOC and CDOM optical properties determined by UV absorbance at 254 nm (A254) and excitation–emission matrix (EEM) fluorescence coupled with parallel factor analysis (PARAFAC) along the lower salinity range (salinity \u3c 10) of the oligo/meso-haline zone for three distinct wetland-influenced rivers; namely the Bekanbeushi River, a cool-temperate river with estuarine lake in Hokkaido, Japan, the Harney River, a subtropical river with tidally-submerged mangrove fringe in Florida, USA, and the Judan River, a small, acidic, tropical rainforest river in Borneo, Malaysia. For the first two rivers, a clear decoupling between DOC and A254 was observed, while these parameters showed similar conservative behavior for the third. Three distinct EEM-PARAFAC models established for each of the rivers provided similar spectroscopic characteristics except for some unique fluorescence features observed for the Judan River. The distribution patterns of PARAFAC components suggested that the inputs from plankton and/or submerged aquatic vegetation can be important in the Bekanbeushi River. Further, DOM photo-products formed in the estuarine lake were also found to be transported upstream. In the Harney River, whereas upriver-derived terrestrial humic-like components were mostly distributed conservatively, some of these components were also derived from mangrove inputs in the oligo/meso-haline zone. Interestingly, fluorescence intensities of some terrestrial humic-like components increased with salinity for the Judan River possibly due to changes in the dissociation state of acidic functional groups and/or increase in the fluorescence quantum yield along the salinity gradient. The protein-like and microbial humic-like components were distributed differently between three wetland rivers, implying that interplay between loss to microbial degradation and inputs from diverse sources are different for the three wetland-influenced rivers. The results presented here indicate that upper estuarine oligo/meso-haline regions of coastal wetland rivers are highly dynamic with regard to the biogeochemical behavior of DOM

Comparison of Microbial Communities in the Sediments and Water Columns of Frozen Cryoconite Holes in the McMurdo Dry Valleys, Antarctica

Although cryoconite holes, sediment-filled melt holes on glacier surfaces, appear small and homogenous, their microbial inhabitants may be spatially partitioned. This partitioning could be particularly important for maintaining biodiversity in holes that remain isolated for many years, such as in Antarctica. We hypothesized that cryoconite holes with greater species richness and biomass should exhibit greater partitioning between the sediments and water, promoting greater biodiversity through spatial niche partitioning. We tested this hypothesis by sampling frozen cryoconite holes along a gradient of biomass and biodiversity in the Taylor Valley, Antarctica, where ice-lidded cryoconite holes are a ubiquitous feature of glaciers. We extracted DNA and chlorophyll a from the sediments and water of these samples to describe biodiversity and quantify proxies for biomass. Contrary to our expectation, we found that cryoconite holes with greater richness and biomass showed less partitioning of phylotypes by the sediments versus the water, perhaps indicating that the probability of sediment microbes being mixed into the water is higher from richer sediments. Another explanation may be that organisms from the water were compressed by freezing down to the sediment layer, leaving primarily relic DNA of dead cells to be detected higher in the frozen water. Further evidence of this explanation is that the dominant sequences unique to water closely matched organisms that do not live in cryoconite holes or the Dry Valleys (e.g., vertebrates); so this cryptic biodiversity could represent unknown microbial animals or DNA from atmospheric deposition of dead biomass in the otherwise low-biomass water. Although we cannot rule out spatial niche partitioning occurring at finer scales or in melted cryoconite holes, we found no evidence of partitioning between the sediments and water in frozen holes. Future work should include more sampling of cryoconite holes at a finer spatial scale, and characterizing the communities of the sediments and water when cryoconite holes are melted and active

Evaluation of Forest Disturbance Legacy Effects on Dissolved Organic Matter Characteristics in Streams at the Hubbard Brook Experimental Forest, New Hampshire

Dissolved organic matter (DOM) source and composition are critical drivers of its reactivity, impact microbial food webs and influence ecosystem functions. It is believed that DOM composition and abundance represent an integrated signal derived from the surrounding watershed. Recent studies have shown that land-use may have a long-term effect on DOM composition. Methods for characterizing DOM, such as those that measure the optical properties and size of the molecules, are increasingly recognized as valuable tools for assessing DOM sources, cycling, and reactivity. In this study we measured DOM optical properties and molecular weight determinations to evaluate whether the legacy of forest disturbance alters the amount and composition of stream DOM. Differences in DOM quantity and composition due to vegetation type and to a greater extent, wetland influence, were more pronounced than effects due to disturbance. Our results suggest that excitation-emission matrix fluorescence with parallel factor analysis is a more sensitive metric of disturbance than the other methods evaluated. Analyses showed that streams draining watersheds that have been clearcut had lower dissolved organic carbon (DOC) concentrations and higher microbially-derived and protein-like fluorescence features compared to reference streams. DOM optical properties in a watershed amended with calcium, were not significantly different than reference watersheds, but had higher concentrations of DOC. Collectively these results improve our understanding of how the legacy of forest disturbances and natural landscape characteristics affect the quantity and chemical composition of DOM in headwater streams, having implications for stream water quality and carbon cycling

Using Optical Properties to Quantify Fringe Mangrove Inputs to the Dissolved Organic Matter (DOM) Pool in a Subtropical Estuary

Dissolved organic carbon (DOC) concentration and dissolved organic matter (DOM) optical properties were analyzed along two estuarine river transects during the wet and dry seasons to better understand DOM dynamics and quantify mangrove inputs. A tidal study was performed to assess the impacts of tidal pumping on DOM transport. DOM in the estuaries showed non-conservative mixing indicative of mangrove-derived inputs. Similarly, fluorescence data suggest that some terrestrial humic-like components showed non-conservative behavior. An Everglades freshwater-derived fluorescent component, which is associated with soil inputs from the Northern Everglades, behaved conservatively. During the dry season, a protein-like component behaved conservatively until the mid-salinity range when non-conservative behavior due to degradation and/or loss was observed. The tidal study data suggests mangrove porewater inputs to the rivers following low tide. The differences in quantity of DOM exported by the Shark and Harney Rivers imply that geomorphology and tidal hydrology may be a dominant factor controlling the amount of DOM exported from the mangrove ecotone, where up to 21 % of the DOC is mangrove-derived. Additionally, nutrient concentrations and other temporal factors may control DOM export from the mangroves, particularly for the microbially derived fluorescent components, contributing to the seasonal differences. The wet and dry season fluxes of mangrove DOM from the Shark River is estimated as 0.27 × 109 mg C d−1 and 0.075 × 109 mg C d−1, respectively, and the Harney River is estimated as 1.9 × 109 mg C d−1 and 0.20 × 109 mg C d−1

Environmental Dynamics of Dissolved Black Carbon in Wetlands

Wetlands are ecosystems commonly characterized by elevated levels of dissolved organic carbon (DOC), and although they cover a surface area less than 2 % worldwide, they are an important carbon source representing an estimated 15 % of global annual DOC flux to the oceans. Because of their unique hydrological characteristics, fire can be an important ecological driver in pulsed wetland systems. Consequently, wetlands may be important sources not only of DOC but also of products derived from biomass burning, such as dissolved black carbon (DBC). However, the biogeochemistry of DBC in wetlands has not been studied in detail. The objective of this study is to determine the environmental dynamics of DBC in different fire-impacted wetlands. An intensive, 2-year spatial and temporal dynamics study of DBC in a coastal wetland, the Everglades (Florida) system, as well as one-time sampling surveys for the other two inland wetlands, Okavango Delta (Botswana) and the Pantanal (Brazil), were reported. Our data reveal that DBC dynamics are strongly coupled with the DOC dynamics regardless of location, season or recent fire history. The statistically significant linear regression between DOC and DBC was applied to estimate DBC fluxes to the coastal zone through two main riverine DOC export routes in the Everglades ecosystem. The presence of significant amounts of DBC in these three fire-impacted ecosystems suggests that sub-tropical wetlands could represent an important continental-ocean carrier of combustion products from biomass burning. The discrimination of DBC molecular structure (i.e. aromaticity) between coastal and terrestrial samples, and between samples collected in wet and dry season, suggests that spatially-significant variation in DBC source strength and/or degree of degradation may also influence DBC dynamics

Utilization of PARAFAC-Modeled Excitation-Emission Matrix (EEM) Fluorescence Spectroscopy to Identify Biogeochemical Processing of Dissolved Organic Matter in a Northern Peatland

In this study, we contrast the fluorescent properties of dissolved organic matter (DOM) in fens and bogs in a Northern Minnesota peatland using excitation emission matrix fluorescence spectroscopy with parallel factor analysis (EEM-PARAFAC). EEM-PARAFAC identified four humic-like components and one protein-like component and the dynamics of each were evaluated based on their distribution with depth as well as across sites differing in hydrology and major biological species. The PARAFAC-EEM experiments were supported by dissolved organic carbon measurements (DOC), optical spectroscopy (UV-Vis), and compositional characterization by ultrahigh resolution Fourier transform ion cyclotron resonance mass spectroscopy (FT-ICR MS). The FT-ICR MS data indicate that metabolism in peatlands reduces the molecular weights of individual components of DOM, and oxygen-rich less aromatic molecules are selectively biodegraded. Our data suggest that different hydrologic and biological conditions within the larger peat ecosystem drive molecular changes in DOM, resulting in distinctly different chemical compositions and unique fluorescent fingerprints. PARAFAC modeling of EEM data coupled with ultrahigh resolution FT-ICR MS has the potential to provide significant molecular-based information on DOM composition that will support efforts to better understand the composition, sources, and diagenetic status of DOM from different terrestrial and aquatic systems

Characterization of dissolved organic material in the interstitial brine of Lake Vida, Antarctica

© 2016 Elsevier Ltd. Lake Vida (LV) is located in the McMurdo Dry Valleys (Victoria Valley, East Antarctica) and has no inflows, outflows, or connectivity to the atmosphere due to a thick (16 m), turbid ice surface and cold (\u3c-20 °C) subsurface alluvium surrounding the lake. The liquid portion of LV has a salinity about seven times that of seawater and is entrained in ice and sediment below the ice cap. This subzero (-13.4 °C), anoxic brine supports a microbial community, which has low levels of activity and has been isolated from the atmosphere for at least 2800 14C years before present. The brine has high dissolved organic carbon concentration (DOC; 580 mg-C L-1 or greater); the study of which provides a unique opportunity to better understand biological and/or abiotic processes taking place in an isolated saline ecosystem with no external inputs. We isolated two sub-fractions of LV dissolved organic matter (DOM) by chemical separation using XAD-8 and XAD-4 resins in series. This separation was followed by physical separation using ultrafiltration to isolate a higher molecular weight (HMW) fraction that was retained by the membrane and a salty, dilute low molecular weight fraction. This analytical path resulted in three, low salt sub-fractions and allowed comparison to other Antarctic lake DOM samples isolated using similar procedures. Compared to other Antarctic lakes, a lower portion of the DOC was retained by XAD-8 (~10% vs. 16-24%) resin, while the portions retained by XAD-4 (~8%) resin and the 1 kDa ultrafiltration membrane (~50%) were similar. The 14C radiocarbon ages of the XAD-8 (mean 3940 ybp), XAD-4 (mean 4048 ybp) and HMW (mean 3270 ybp) fractions are all older than the apparent age of ice-cover formation (2800 ybp). Ultrahigh resolution mass spectrometry showed that compounds with two and three nitrogen atoms in the molecular formulas were common in both the LV-XAD8 and LV-XAD4 fractions, consistent with microbial production and processing. The long-term oxidation of LVBr DOM by abiotic oxidants including perchlorate and chlorate may explain the low portion in the XAD8 fraction and the lack of aromatic carbon, as measured by 13C NMR spectroscopy, found for all but the most hydrophobic fraction, LV-XAD8. Overall, the chemical characteristics of Lake Vida brine DOM suggest that legacy DOM sealed and concentrated within the brine has been altered due to a combination of both biological and abiotic chemical reactions

Comparison of Microbial Communities in the Sediments and Water Columns of Frozen Cryoconite Holes in the McMurdo Dry Valleys, Antarctica

Although cryoconite holes, sediment-filled melt holes on glacier surfaces, appear small and homogenous, their microbial inhabitants may be spatially partitioned. This partitioning could be particularly important for maintaining biodiversity in holes that remain isolated for many years, such as in Antarctica. We hypothesized that cryoconite holes with greater species richness and biomass should exhibit greater partitioning between the sediments and water, promoting greater biodiversity through spatial niche partitioning. We tested this hypothesis by sampling frozen cryoconite holes along a gradient of biomass and biodiversity in the Taylor Valley, Antarctica, where ice-lidded cryoconite holes are a ubiquitous feature of glaciers. We extracted DNA and chlorophyll a from the sediments and water of these samples to describe biodiversity and quantify proxies for biomass. Contrary to our expectation, we found that cryoconite holes with greater richness and biomass showed less partitioning of phylotypes by the sediments versus the water, perhaps indicating that the probability of sediment microbes being mixed into the water is higher from richer sediments. Another explanation may be that organisms from the water were compressed by freezing down to the sediment layer, leaving primarily relic DNA of dead cells to be detected higher in the frozen water. Further evidence of this explanation is that the dominant sequences unique to water closely matched organisms that do not live in cryoconite holes or the Dry Valleys (e.g., vertebrates); so this cryptic biodiversity could represent unknown microbial animals or DNA from atmospheric deposition of dead biomass in the otherwise low-biomass water. Although we cannot rule out spatial niche partitioning occurring at finer scales or in melted cryoconite holes, we found no evidence of partitioning between the sediments and water in frozen holes. Future work should include more sampling of cryoconite holes at a finer spatial scale, and characterizing the communities of the sediments and water when cryoconite holes are melted and active

Characterization of fulvic acid fractions of dissolved organic matter during ice-out in a hyper-eutrophic, coastal pond in Antarctica

Dissolved humic material (HDOM) is ubiquitous to all natural waters and its source material influences its chemical structure, reactivity, and bioavailability. While terrestrially derived HDOM reference materials distributed by the International Humic Substances Society (IHSS) have been readily available to engineering and scientific communities, a microbially derived reference HDOM was not, despite the well-characterized differences in the chemistry and reactivity of HDOM derived from terrestrial versus microbial sources. To address this gap, we collected a microbial reference fulvic acid from Pony Lake (PLFA) for distribution through the IHSS. Pony Lake is a saline coastal pond on Ross Island, Antarctica, where the landscape is devoid of terrestrial plants. Sample collection occurred over a 17-day period in the summer season at Pony Lake. During this time, the dissolved organic carbon (DOC) concentrations increased nearly two-fold, and the fulvic acid fraction (collected using the XAD-8 method) accounted for 14.6% of the DOC. During the re-concentration and desalting procedures we isolated two other chemically distinct fulvic acid fractions: (1) PLFA-2, which was high in carbohydrates and (2) PLFA-CER, which was high in nitrogen. The chemical characteristics (elemental analysis, optical characterization with UV-vis and fluorescence spectroscopy, and C-13 NMR spectroscopy) of the three fulvic acid fractions helped to explain their behavior during isolation