Transformations and Fates of Terrigenous Dissolved Organic Matter in River-Influenced Ocean Margins

Rivers contribute about 0.25 Pg of terrigenous dissolved organic carbon (tDOC) to the ocean each year. The fate and transformations of this material have important ramifications for the metabolic state of the ocean, air-sea CO2 exchange, and the global carbon cycle. Stable isotopic compositions and terrestrial biomarkers suggest tDOC must be efficiently mineralized in ocean margins. Nonetheless, the extent of tDOC mineralization in these environments remains unknown, as no quantitative estimate is available. The complex interplay of biogeochemical and physical processes in these systems compounded by the limited practicality of chemical proxies (organic biomarkers, isotopic compositions) make the quantification of tDOC mineralization in these dynamic systems particularly challenging. In this dissertation, new optical proxies were developed (Chapters 1 and 2) and facilitated the first quantitative assessment of tDOC mineralization in a dynamic river-influenced ocean margin (Chapter 3) and the monitoring of continental runoff distributions in the coastal ocean using remote sensing (Chapter 4). The optical properties of chromophoric dissolved organic matter (CDOM) were used as optical proxies for dissolved organic carbon concentration ([DOC]) and %tDOC. In both proxies, the CDOM spectral slope coefficient (S275-295) was exploited for its informative properties on the chemical nature and composition of dissolved organic matter. In the first proxy, a strong relationship between S275-295 and the ratio of CDOM absorption to [DOC] facilitated accurate retrieval (±4%) of [DOC] from CDOM. In the second proxy, the existence of a strong relationship between S275-295 and the DOC-normalized lignin yield facilitated the estimation of the %tDOC from S275-295. Using the proxies, the tDOC concentration can be retrieved solely from CDOM absorption coefficients (&lambda = 275-295 nm) in river-influenced ocean margins. The practicality of optical proxies facilitated the calculation of tDOC mineralization rates on the Louisiana shelf. Seasonal tDOC mass balances for the shelf revealed that between 26% (winter) and 71% (summer) of the mixed layer tDOC is mineralized during its residence on the shelf. Independent approaches further indicated biomineralization accounts for 60% of the tDOC mineralization whereas photomineralization contributes only 8%. The remaining 32% was attributed to the coupled photo-biomineralization. On an annual basis, our results indicated ~40% of the tDOC discharged by the Mississippi and Atchafalaya rivers to the Louisiana shelf (~1 Tg tDOC) is mineralized within 2 to 3 months. This extensive mineralization on the shelf is direct evidence ocean margins act as efficient filters of tDOC between the land and ocean. Finally, the amenability of S275-295 to ocean color remote sensing was demonstrated, and facilitates the real-time, synoptic monitoring of tDOC and freshwater runoff in coastal waters. Implementation of this approach provided the first pan-Arctic distributions of tDOC and continental runoff in surface polar waters, and will help understand the manifestations of climate change in this remote region

Biological hot spots and the accumulation of marine dissolved organic matter in a highly productive ocean margin

Concentrations of dissolved organic carbon (DOC) and major biochemicals (amino acids and carbohydrates) were measured during five cruises (2009–2010) to the Louisiana margin in the northern Gulf of Mexico. Concentrations of amino acids and carbohydrates were elevated at mid-salinities and were indicative of plankton production of dissolved organic matter (DOM) in surface waters. Hot spots of two compositionally distinct types of labile DOM were identified based on the relative abundances of amino acids and carbohydrates. Amino acid-rich hot spots occurred sporadically in regions of high phytoplankton biomass and were mostly observed between dusk and dawn, reflecting a grazing source. In contrast, carbohydrate-rich hot spots were more widespread and were often found in nutrient-poor waters, indicating the production of carbon-rich DOM associated with nutrient limitation. Major biochemical indicators and bioassay experiments indicated labile DOM comprised a relatively small fraction of the DOC. Most DOM was degraded and had a semi-labile nature. Substantial accumulations of marine (plankton-derived) DOC were observed in surface waters, particularly at mid-salinities during the summer. Microbial alteration of marine DOC and nutrient limitation of microbial utilization of carbon-rich DOM appeared largely responsible for the accumulation of DOC. The reservoir of accumulated marine DOC in the shelf surface mixed layer ranged from 0.11 Tg C to 0.23 Tg C, with the lowest and highest values occurring during winter and summer. Substantial cross-shelf export of semi-labile marine DOM occurred during the summer and provided a major carbon and energy subsidy to microbial food webs in offshore waters.We are grateful to Steven E. Lohrenz and Wei-Jun Cai for providing the opportunity to participate in the GulfCarbon cruises. We appreciate the sampling assistance by Leanne Powers and the crews of the R/V Cape Hatteras and the R/V Hugh Sharp. We thank the anonymous reviewers for their comments and suggestions. This research was funded by a grant from the U.S. National Science Foundation (0850653 to RB) and by the 111 Project of China (B13030 to SKL). (0850653 - U.S. National Science Foundation; B13030 - 111 Project of China

Pan-Arctic distribution of bioavailable dissolved organic matter and linkages with productivity in ocean margins

Rapid environmental changes in the Arctic Ocean affect plankton productivity and the bioavailability of dissolved organic matter (DOM) that supports microbial food webs. We report concentrations of dissolved organic carbon (DOC) and yields of amino acids (indicators of labile DOM) in surface waters across major Arctic margins. Concentrations of DOC and bioavailability of DOM showed large pan-Arctic variability that corresponded to varying hydrological conditions and ecosystem productivity, respectively. Widespread hot spots of labile DOM were observed over productive inflow shelves (Chukchi and Barents Seas), in contrast to oligotrophic interior margins (Kara, Laptev, East Siberian, and Beaufort Seas). Amino acid yields in outflow gateways (Canadian Archipelago and Baffin Bay) indicated the prevalence of semilabile DOM in sea ice covered regions and sporadic production of labile DOM in ice-free waters. Comparing these observations with surface circulation patterns indicated varying shelf subsidies of bioavailable DOM to Arctic deep basins.Published version2019-07-3

Sources and transformations of dissolved lignin phenols and chromophoric dissolved organic matter in Otsuchi Bay, Japan

Dissolved lignin phenols and optical properties of dissolved organic matter (DOM) were measured to investigate the sources and transformations of terrigenous DOM (tDOM) in Otsuchi Bay, Japan. Three rivers discharge into the bay, and relatively high values of syringyl:vanillyl phenols (0.73 ± 0.07) and cinnamyl:vanillyl phenols (0.33 ± 0.10) indicated large contributions of non-woody angiosperm tissues to lignin and tDOM. The physical mixing of river and seawater played an important role in controlling the concentrations and distributions of lignin phenols and chromophoric DOM (CDOM) optical properties in the bay. Lignin phenol concentrations and the CDOM absorption coefficient at 350 nm, a(350), were strongly correlated in river and bay waters. Measurements of lignin phenols and CDOM in bay waters indicated a variety of photochemical and biological transformations of tDOM, including oxidation reactions, photobleaching and a decrease in molecular weight. Photodegradation and biodegradation of lignin and CDOM were investigated in decomposition experiments with river water and native microbial assemblages exposed to natural sunlight or kept in the dark. There was a rapid and substantial removal of lignin phenols and CDOM during the first few days in the light treatment, indicating transformations of tDOM and CDOM can occur soon after discharge of buoyant river water into the bay. The removal of lignin phenols was slightly greater in the dark (34%) than in the light (30%) during the remaining 59 days of the incubation. Comparison of the light and dark treatments indicated biodegradation was responsible for 67% of total lignin phenols removal during the 62-day incubation exposed to natural sunlight, indicating biodegradation is a dominant removal process in Otsuchi Bay.Ministry of Education, Culture, Sports, Science, and Technology; 1504137 - National Science Foundatio

Dissolved Organic Matter Composition and Bioavailability Reflect Ecosystem Productivity in the Western Arctic Ocean

Dissolved organic carbon (DOC) and total dissolved amino acids (TDAA) were measured in high (Chukchi Sea) and low (Beaufort Sea) productivity regions of the western Arctic Ocean to investigate the composition and bioavailability of dissolved organic matter (DOM). Concentrations and DOC-normalized yields of TDAA in Chukchi surface waters were relatively high, indicating an accumulation of bioavailable DOM. High concentrations and yields of TDAA were also observed in the upper halocline of slope and basin waters, indicating off-shelf transport of bioavailable DOM from the Chukchi Sea. In contrast, concentrations and yields of TDAA in Beaufort surface waters were relatively low, indicting DOM was of limited bioavailability. Concentrations and yields of TDAA in the upper halocline of slope and basin waters were also low, suggesting the Beaufort is not a major source of bioavailable DOM to slope and basin waters. In shelf waters of both systems, elevated concentrations and yields of TDAA were often observed in waters with higher chlorophyll concentrations and productivity. Surface concentrations of DOC were similar (p \u3e 0.05) in the two systems despite the contrasting productivity, but concentrations and yields of TDAA were significantly higher (p \u3c 0.0001) in the Chukchi than in the Beaufort. Unlike bulk DOC, TDAA concentrations and yields reflect ecosystem productivity in the western Arctic. The occurrence of elevated bioavailable DOM concentrations in the Chukchi Sea implies an uncoupling between the biological production and utilization of DOM and has important implications for sustaining heterotrophic microbial growth and diversity in oligotrophic waters of the central Arctic basins

Improved Algorithms for Accurate Retrieval of UV - Visible Diffuse Attenuation Coefficients in Optically Complex, Inshore Waters

Photochemical processes driven by high-energy ultraviolet radiation (UVR) in inshore, estuarine, and coastal waters play an important role in global bio geochemical cycles and biological systems. A key to modeling photochemical processes in these optically complex waters is an accurate description of the vertical distribution of UVR in the water column which can be obtained using the diffuse attenuation coefficients of down welling irradiance (Kd()). The Sea UV Sea UVc algorithms (Fichot et al., 2008) can accurately retrieve Kd ( 320, 340, 380,412, 443 and 490 nm) in oceanic and coastal waters using multispectral remote sensing reflectances (Rrs(), Sea WiFS bands). However, SeaUVSeaUVc algorithms are currently not optimized for use in optically complex, inshore waters, where they tend to severely underestimate Kd(). Here, a new training data set of optical properties collected in optically complex, inshore waters was used to re-parameterize the published SeaUVSeaUVc algorithms, resulting in improved Kd() retrievals for turbid, estuarine waters. Although the updated SeaUVSeaUVc algorithms perform best in optically complex waters, the published SeaUVSeaUVc models still perform well in most coastal and oceanic waters. Therefore, we propose a composite set of SeaUVSeaUVc algorithms, optimized for Kd() retrieval in almost all marine systems, ranging from oceanic to inshore waters. The composite algorithm set can retrieve Kd from ocean color with good accuracy across this wide range of water types (e.g., within 13 mean relative error for Kd(340)). A validation step using three independent, in situ data sets indicates that the composite SeaUVSeaUVc can generate accurate Kd values from 320 490 nm using satellite imagery on a global scale. Taking advantage of the inherent benefits of our statistical methods, we pooled the validation data with the training set, obtaining an optimized composite model for estimating Kd() in UV wavelengths for almost all marine waters. This optimized composite set of SeaUVSeaUVc algorithms will provide the optical community with improved ability to quantify the role of solar UV radiation in photochemical and photobiological processes in the ocean

Pulsed, Cross‐Shelf Export of Terrigenous Dissolved Organic Carbon to the Gulf of Mexico

The mineralization of terrigenous dissolved organic carbon (tDOC) discharged by rivers can impact nutrient and trace metal cycling, biological productivity, net ecosystem metabolism, and air-sea CO2 exchange in ocean margins. However, the extreme heterogeneity of river-influenced ocean margins represents a major challenge for quantitative assessments of tDOC transformations and thereby obscures the role of tDOC in biogeochemical cycles. Here a lignin-based optical proxy for tDOC and a shelf-wide mass balance approach were used to quantitatively assess the fate of tDOC discharged from the Mississippi-Atchafalaya River System (M-ARS) to the Louisiana shelf. The mass balance revealed that ~40% of the tDOC discharged by the M-ARS during March 2009–2010 was mineralized to CO2 on the Louisiana shelf, with two thirds of the mineralization taking place in the mixed layer. A strong seasonality in tDOC mineralization was observed, with mineralization rates severalfold higher during summer than during winter. Independent assessments of specificmineralization processes indicated biomineralization accounted for ~94%of the tDOCmineralization on an annual basis and suggest that photochemical transformations of tDOC enhanced biomineralization by ~50% in themixed layer. Direct photomineralization accounted for a relatively small fraction (~6%) of the tDOC mineralization on an annual basis. This quantitative assessment directly confirms that ocean margins are major sinks of the tDOC discharged by rivers and indicates that tDOC mineralization rates in the shelf mixed layer are sufficiently large to influence whether the Louisiana shelf is a net sink or source of atmospheric CO2

AQY-M Prediction for CDOM Photobleaching

This DOI contains model data files and MATLAB scripts for predicting the Apparent Quantum Yield Matrix (AQY-M) of Chromophoric Dissolved Organic Matter (CDOM) photobleaching. It includes the necessary model parameters, principal-component variables, MATLAB scripts, and an example file containing input variables from 36 samples (to predict 36 corresponding AQY-M). The details of the model and findings regarding the variability of CDOM photobleaching AQY-M in natural waters are detailed in Zhu et. al. 2023 (10.1016/j.scitotenv.2023.168670).</p

Drivers of suspended sediment dynamics along the shorelines of the Yellow River Delta detected from satellite data

Total suspended solids (TSS) can be a useful indicator of environmental change in nearshore coastal environments. Understanding the mechanisms of TSS variations in response to environmental drivers is of broad interest for ecology and geomorphology. The Yellow River Delta (YRD) in China is a fragile coastal region that has been affected by human activities and climate change. Here, we investigated TSS along the YRD shoreline over two decades with time-series satellite data (2002-2020). We observed that TSS concentration decreased significantly in nearshore waters (5-m isobath) surrounding the YRD, especially in the LaiZhou Bay. During the same time period, wave height (WH) along the deltaic shoreline and sediment load from the Yellow River have decreased, while sea surface height (SSH) has displayed a positive trend. Our results indicate that WH and SSH play a major role in sediment resuspension and dispersion, while the YSD mildly affected TSS variability along the coast. Monthly bed shear stress triggered by waves was then computed using WH, wave period (WP), and SSH. Bed shear stress and TSS displayed a positive correlation. We concluded that seasonal oscillations in SSH in conjunction with wind waves are responsible for TSS variability in the shallow waters in front of the YRD