Remineralization Of Organic Matter Research Articles

Water mass gradients of the mid-depth Southwest Atlantic during the past 25,000 years

Antarctic Intermediate Water (AAIW) plays a central role in the Atlantic Meridional Overturning Circulation (AMOC) as the return flow of Northern Sourced Water (NSW) and is therefore of significant importance for the global climate. Past variations of the boundary between AAIW and NSW have been extensively investigated, yet available results documenting the prevailing depth of this boundary and the southern extent of NSW during the last ice age remain ambiguous. Here, we present five new timeseries focusing on the authigenic neodymium isotope signal in sediment cores retrieved from the Southwest Atlantic covering the past 25,000 years. The sites are situated along the southern Brazil Margin and form a bathymetric transect ranging between 1000 and 3000 m water depth, encompassing the modern water mass boundaries of AAIW and NSW and therefore allow their reconstruction since the Last Glacial Maximum (LGM). The new Nd isotope records show little change between the LGM and early deglaciation as well as relatively homogeneous values over the full depth range of the cores during these times. These results strongly contrast with epibenthic foraminiferal stable carbon isotope records (δ13C) from the same sites which exhibit highest glacial values at mid-depths, presumably related to NSW mixing into southern sourced water. We propose that the discrepancy between these two independent water mass proxies is partly related to changes in Nd end member properties of glacial AAIW. The combination of elevated glacial dust fluxes and, as a result, sustained export productivity caused high sinking particle flux in the western South Atlantic, where AAIW is forming. Higher particle flux would have increased the removal (scavenging) of Nd from shallow waters thus reducing the Nd concentration and overprinting the isotopic signature of the glacial AAIW end member. Only under consideration of changes in Nd end member properties along with non-conservative processes such as remineralization of organic matter influencing past seawater δ13C can we reconcile the water mass reconstructions from both proxies.

Carbon and phosphorus processing in a carbonate karst aquifer and delivery to the coastal ocean

In siliciclastic systems, submarine groundwater discharge (SGD) provides a fraction of freshwater and nutrients delivered to coastal waters, but in many carbonate karst terrains SGD represents the predominant source of terrestrial water and solutes. Water compositions may be modified by reactions in subterranean estuaries, altering chemical fluxes via SGD. In carbonate settings, feedbacks between organic carbon remineralization and calcium carbonate mineral (CaCO3) dissolution and precipitation may alter carbon dioxide (CO2) and phosphorus (P) concentrations and fluxes associated with SGD. To assess these effects, we sampled water from multiple submarine springs along the east coast of the Yucatan peninsula, as well as inland cenotes, and a coastal groundwater well. We measured ammonium (NH4), phosphate (PO4), major element, dissolved inorganic and organic carbon concentrations (DIC and DOC), fluorescent characteristics of colored dissolved organic matter (CDOM) and modeled calcite saturation indices (SIcal) and the partial pressure of dissolved CO2 (PCO2). These data indicate that reactions along a hypothetical flow path to the coast control the composition of terrestrial fresh water entering the subterranean estuary. Non-conservative mixing between brackish groundwater and lagoon water reflect changes to groundwater compositions within the subterranean estuary from CO2 produced during organic matter remineralization and CaCO3 dissolution. Although both organic matter remineralization and carbonate dissolution should liberate P, molar N:P ratios in spring discharge are higher than the Redfield Ratio of 16:1, suggesting sequestration of remineralized P through sorption to carbonate minerals within the STE. This result indicates that SGD, the primary source of water and nutrient to this coastal zone, results in P limitation in coastal water and is a source of CO2 despite buffering by CaCO3 dissolution. This result also emphasizes the importance of biogeochemical reactions within subterranean estuaries for estimates of SGD solute delivery to the oceans and impacts to the coastal carbon cycle.

Controls on redox-sensitive trace metals in the Mauritanian oxygen minimum zone

Abstract. The availability of the micronutrient iron (Fe) in surface waters determines primary production, N2 fixation, and microbial community structure in large parts of the world's ocean, and thus it plays an important role in ocean carbon and nitrogen cycles. Eastern boundary upwelling systems and the connected oxygen minimum zones (OMZs) are typically associated with elevated concentrations of redox-sensitive trace metals (e.g., Fe, manganese (Mn), and cobalt (Co)), with shelf sediments typically forming a key source. Over the last 5 decades, an expansion and intensification of OMZs has been observed and this trend is likely to proceed. However, it is unclear how trace-metal (TM) distributions and transport are influenced by decreasing oxygen (O2) concentrations. Here we present dissolved (d; <0.2 µm) and leachable particulate (Lp; >0.2 µm) TM data collected at seven stations along a 50 km transect in the Mauritanian shelf region. We observed enhanced concentrations of Fe, Co, and Mn corresponding with low O2 concentrations (<50 µmol kg−1), which were decoupled from major nutrients and nutrient-like and scavenged TMs (cadmium (Cd), lead (Pb), nickel (Ni), and copper (Cu)). Additionally, data from repeated station occupations indicated a direct link between dissolved and leachable particulate Fe, Co, Mn, and O2. An observed dFe (dissolved iron) decrease from 10 to 5 nmol L−1 coincided with an O2 increase from 30 to 50 µmol kg−1 and with a concomitant decrease in turbidity. The changes in Fe (Co and Mn) were likely driven by variations in their release from sediment pore water, facilitated by lower O2 concentrations and longer residence time of the water mass on the shelf. Variations in organic matter remineralization and lithogenic inputs (atmospheric deposition or sediment resuspension; assessed using Al as indicator for lithogenic inputs) only played a minor role in redox-sensitive TM variability. Vertical dFe fluxes from O2-depleted subsurface-to-surface waters (0.08–13.5 µmol m−2 d−1) driven by turbulent mixing and vertical advection were an order of magnitude larger than atmospheric deposition fluxes (0.63–1.43 µmol m−2 d−1; estimated using dAl inventories in the surface mixed layer) in the continental slope and shelf region. Benthic fluxes are therefore the dominant dFe supply to surface waters on the continental margins of the Mauritanian upwelling region. Overall, our results indicated that the projected future decrease in O2 concentrations in OMZs may result in increases in Fe, Mn, and Co concentrations.

Systematic Variation in Marine Dissolved Organic Matter Stoichiometry and Remineralization Ratios as a Function of Lability

Remineralization of organic matter by heterotrophic organisms regulates the biological sequestration of carbon, thereby mediating atmospheric CO2. While surface nutrient supply impacts the elemental ratios of primary production, stoichiometric control by remineralization remains unclear. Here we develop a mechanistic description of remineralization and its stoichiometry in a marine microbial ecosystem model. The model simulates the observed elemental plasticity of phytoplankton and the relatively constant, lower C:N of heterotrophic biomass. In addition, the model captures the observed decreases in the C:N of more labile dissolved organic matter (DOM) and the C:N of its remineralization with depth, which are driven by a switch in the dominant source of DOM from phytoplankton to heterotrophic biomass. Only a model version with targeted remineralization of N‐rich components is able to simulate the observed profiles of preferential remineralization of N relative to C and the elevated C:N of bulk DOM. The model suggests that more labile substrates are associated with C‐limited heterotrophic growth and not with preferential remineralization, while less labile substrates are associated with growth limited by processing rates and with preferential remineralization. The resulting patterns of variable remineralization stoichiometry mediate the extent to which a proportional increase in carbon production resulting from changes in phytoplankton stoichiometry can increase the efficiency of the biological pump. Results emphasize the importance of understanding the physiology of both phytoplankton and heterotrophs for anticipating changes in biologically driven ocean carbon storage.

Benthic-pelagic coupling of carbon and nitrogen along a tropical estuarine gradient (Gulf of Nicoya, Costa Rica)

Tropical estuaries are very little studied compared to temperate ones, despite their generally high primary production rate and hence potential contribution to global carbon and nitrogen budgets. The Gulf of Nicoya (northwest Costa Rica Pacific coast) is one of the most productive estuaries worldwide. However, how changes in benthopelagic coupling affect biogeochemical cycling of carbon and nitrogen over seasons and along the estuarine gradient is not known for this and, in general, other tropical estuaries. For this purpose, we performed two sampling campaigns during the rainy and dry seasons to investigate the contribution of the pelagic and benthic compartments to the carbon and nutrient pools and dynamics of organic matter remineralization in the estuary. Areal concentrations of chlorophyll a (Chla), organic carbon (C) and total nitrogen (N) integrated along the water column photic layer and in the surface sediment generally reached maximum concentrations in the middle of the estuary. Although, integrated Chla in the water column and sediment were in the same range, integrated C and N in the surface sediment (5 cm) were 2–10 times higher than in the water column. The C:Chla and N:Chla ratios in the suspended particulate matter were much higher than those typical of phytoplankton indicating a strong contribution of allochthonous organic matter. However, the C:N ratio in the water column was frequently lower than the Redfield ratio suggesting an important contribution of particulate inorganic N, particularly in the rainy season. Diffusive O2 sediment uptake rate and the O2 penetration depth within the sediment correlated with the organic carbon content in the sediment. In contrast, total oxygen utilization and total CO2 fluxes showed higher variability along the estuary but no significant statistical correlations with any other sediment biogeochemical variables. Organic carbon flux (JC) to the sediment was highly variable along the estuary and between seasons. Despite differences in absolute terms, the fraction of JC accumulated within the sediment for the whole inner part of the Gulf of Nicoya was about 60% in both seasons. Total C burial in the inner part of the gulf was 390 and 813 Tn C d−1 in the rainy and dry seasons, respectively, representing 1.8 and 3.8% of the daily Costa Rica CO2 emissions.

Carbonate characteristics of the Gulf of Anadyr waters

The first field data describing the dynamics of the carbonate system, aragonite saturation state, and CO2 fluxes between the ocean and the atmosphere in the Gulf of Anadyr in the late autumn season are presented. It was established that during this period the gulf waters absorbed carbon dioxide from the atmosphere at a rate of -22,5 mmol m‑2 day‑1, which determined the “classical” mechanism of seawater acidification due to uptake of excess atmospheric CO2. In general, surface waters of the gulf were supersaturated with respect to aragonite. The exception was the highly dynamic region of Anadyr Strait, where the vertical distribution of the investigated parameters was homogeneous, the surface waters were close to equilibrium with respect to aragonite, and CO2 flux was directed to the atmosphere. Bottom waters of the gulf, in contrast, were characterized by significant seasonal corrosivity with respect to aragonite due primarily to remineralization of organic matter to CO2. It was shown that during the late fall relatively salty and acidic, quasi-equilibrium with respect to aragonite, and oxygen-depleted waters with high concentrations of nutrients and CO2 have been entered the Chirkov Basin and further the Arctic Ocean with the Navarin current.

Nutrient distribution and nitrogen and oxygen isotopic composition of nitrate in water masses of the subtropical southern Indian Ocean

Abstract. The Indian Ocean subtropical gyre (IOSG) is one of five extensive subtropical gyres in the world's ocean. In contrast to those of the Atlantic and Pacific oceans, the IOSG has been sparsely studied. We investigate the water mass distributions based on temperature, salinity and oxygen data, and the concentrations of water column nutrients and the stable isotope composition of nitrate, using water samples collected between ∼30∘ S and the Equator during two expeditions: MSM 59/2 in 2016 and SO 259 in 2017. Our results are the first from this oceanic region and provide new information on nitrogen sources and transformation processes. We identify the thick layer of nutrient-depleted surface waters of the oligotrophic IOSG with nitrate (NO3-) and phosphate (PO43-) concentrations of < 3 and < 0.3 µmol kg−1, respectively (< 300 m; σ < 26.4 kg−1 m−3). Increased nutrient concentrations towards the Equator represent the northern limb of the gyre, which is characterized by typical strong horizontal gradients of the outcropping nutriclines. The influx of the Subantarctic Mode Water (SAMW) from the Southern Ocean injects oxygen-saturated waters with preformed nutrients, indicated by the increased N and O isotope composition of nitrate (δ15N > 7 ‰; δ18O > 4 ‰) at 400–500 m (26.6–26.7 kg−1 m−3), into the subtropical thermocline. These values reflect partial N assimilation in the Southern Ocean. Moreover, in the northern study area, a residue of nitrate affected by denitrification in the Arabian Sea is imported into intermediate and deep water masses (> 27.0 kg−1 m−3) of the gyre, indicated by an N deficit (N* ∼-1 to −4 µmol kg−1) and by elevated isotopic ratios of nitrate (δ15N > 7 ‰; δ18O > 3 ‰). Remineralization of partially assimilated organic matter, produced in the subantarctic, leads to a decoupling of N and O isotopes in nitrate and results in a relatively low Δ(15–18) value of < 3 ‰ within the SAMW. In contrast, remineralization of 15N-enriched organic matter from the Arabian Sea indicates higher Δ(15–18) values of > 4 ‰ within the Red Sea–Persian Gulf Intermediate Water (RSPGIW). Thus, the subtropical southern Indian Ocean is supplied by preformed nitrate from the lateral influx of water masses from regions exhibiting distinctly different N-cycle processes documented in the dual isotope composition of nitrate. Additionally, a significant contribution of N2 fixation between 20.36 and 23.91∘ S is inferred from reduced δ15N–NO3- values towards surface waters (upward decrease of δ15N ∼2.4 ‰), N* values of > 2 µmol kg−1 and a relatively low Δ(15–18) value of < 3 ‰. A mass and isotope budget implies that at least 32 %–34 % of the nitrate in the upper ocean between 20.36 and 23.91∘ S is provided from newly fixed nitrogen, whereas N2 fixation appears to be limited by iron or temperature south of 26∘ S.

Gulf Stream Ring Water Intrusion on the Mid-Atlantic Bight Continental Shelf Break Affects Microbially Driven Carbon Cycling

Warm core, anticyclonic rings that spin off from the Gulf Stream circulate through the region directly offshore of the Mid-Atlantic Bight. If a warm core ring reaches the continental shelf break, its warm, highly saline water may subduct under cooler, fresher continental shelf surface water, resulting in subsurface waters at the shelf break and over the upper continental slope with high temperatures and salinities and distinct physical and chemical properties characteristic of Gulf Stream water. Such intruding water may also have microbial communities with distinct functional capacities, which may in turn affect the rate and nature of carbon cycling in this coastal/shelf environment. However, the functional capabilities of microbial communities within ring intrusion waters relative to surrounding continental shelf waters are largely unexplored. We investigated microbial community capacity to initiate organic matter remineralization by measuring hydrolysis of a suite of polysaccharide, peptide, and glucose substrates along a transect oriented across the Mid-Atlantic Bight shelf, shelf break, and upper slope. At the outermost sampling site, warm and salty water derived from a Gulf Stream warm core ring was present in the lower portion of the water column. This water exhibited hydrolytic capacities distinct from other sampling sites, and exhibited lower heterotrophic bacterial productivity overall. Warm core rings adjacent to the Mid-Atlantic Bight shelf have increased in frequency and duration in recent years. As the influence of warm core rings on the continental shelf and slope increases in the future, the rate and nature of organic matter remineralization on the continental shelf may also shift.

Carbonate Characteristics of the Gulf of Anadyr Waters

The first field data describing the dynamics of the carbonate system, the aragonite saturation state, and CO2 fluxes between the ocean and the atmosphere in the Gulf of Anadyr in the late autumn season are presented. It was established that during this period the gulf waters absorbed carbon dioxide from the atmosphere at a rate of –22.5 mmol m–2 day–1, which determined the “classical” mechanism of seawater acidification due to uptake of excess atmospheric CO2. In general, surface waters of the gulf were supersaturated with respect to aragonite. The exception was the highly dynamic region of Anadyr Strait, where the vertical distribution of the investigated parameters was homogeneous, the surface waters were close to equilibrium with respect to aragonite, and the CO2 flux was directed to the atmosphere. The bottom waters of the gulf, in contrast, were characterized by significant seasonal corrosivity due to remineralization of organic matter. It was shown that, during the late fall, relatively salty and acidic, quasi-equilibrium with respect to aragonite, and oxygen-depleted waters with high concentrations of nutrients and CO2 enter into the Chirikov Basin and further to the Arctic Ocean with Navarin Current.

The Critical Role of Bioturbation for Particle Dynamics, Priming Potential, and Organic C Remineralization in Marine Sediments: Local and Basin Scales

Bioturbation promotes priming and total remineralization of sedimentary organic matter (Corg) in multiple ways. A primary local mode is the injection of reactive Corg from the water column, surface sediment, and mucus secretions into deposits. During feeding, burrowing, and construction activities by benthic fauna, labile substrates are brought into close association with more refractory material over a wide range of time scales, geometries, and depths, enhancing decomposition of the less reactive components (priming). One measure of these local interactions is the particle mixing coefficient, DB, which can be estimated from the averaged penetration of particle-reactive radionuclides into deposits. Patterns of DB in Long Island Sound, an estuarine system with well-defined sources of naturally-occurring radionuclides, show consistent positive correlations between DB and total inventories of 234Th (t1/2 = 24 d) and 210Pb (t1/2 = 22 yr) at local and basin scales. These correlations, maintained seasonally in the case of 234Th, demonstrate not only the penetration of plankton-derived, reactive Corg into deeper regions of deposits during bioturbation over monthly (~ 5 – 10 cm) to decadal timescales (~ 20 – 100 cm) but also the enhanced capture of labile substrates from the water column across basin scales into bioturbated patches as the intensity of reworking increases. In Long Island Sound, sedimentary Chl-a distributions and benthic nutrient regeneration (e.g., NH4+ fluxes) reflect these particle exchange processes. Basin and regional scale capture of labile substrates into bioturbated deposits can be generally demonstrated, for example, along the highly productive Cape Hatteras continental margin. Thus, total and net remineralization necessarily increase with the biogenic enhancement of the quantity of labile particulate substrate in deposits. This capture, intermixing, and close association of reactive and refractory substrates (reductant mixing), and thus the optimization of priming potential, represent important, often overlooked, pathways by which bioturbation generates biogeochemical conditions conducive to maximum efficiency of remineralization.

Characteristics of the Carbonate System in a Semiarid Estuary that Experiences Summertime Hypoxia

In oceanic environments, two sources of CO2 have been found to contribute to acidification of stratified water bodies, i.e., CO2 invasion due to anthropogenic atmospheric CO2 increase and respiration-produced CO2 from organic matter remineralization. Acidification caused by these CO2 sources has been observed frequently in numerous environments spanning from open continental shelves to enclosed estuaries. Here, we report observations on carbonate system dynamics in a relatively well-buffered lagoonal estuary, Corpus Christi Bay (CCB), in a semiarid subtropical region that is influenced by summertime hypoxia as well as strong evaporation and seagrass vegetation in the vicinity. While the relationship between dissolved oxygen (DO) and pH in the bottom waters of CCB was positive as in other coastal and estuarine environments prone to hypoxia, the slope was significantly less than in other systems. We attribute the high buffering capacity in CCB to the presence of abundant seagrass meadows adjacent to CCB and strong evaporation-produced density flow that delivers low CO2 waters to the bottom of CCB. Thus, despite the occurrence of hypoxia, neither bottom water carbonate saturation state with respect to aragonite (Ωarg) nor CO2 partial pressure (pCO2) reached critical levels, i.e., undersaturation (i.e., Ωarag 1000 µatm), respectively.

Mercury spikes at the Devonian-Carboniferous boundary in the eastern part of the Rhenohercynian Zone (central Europe) and in the South China Block

Mercury (Hg) chemostratigraphy is an efficient tool for tracing distal volcanism, as suggested in many recent studies. In our research, we focused on Hg and other trace element contents in two Devonian-Carboniferous boundary sections from two different and spatially distant areas. The Lesní lom section (Czech Republic) was located in the south-eastern part of the Laurussia plate and the Duli (Guangxi, China) section in the South China Plate. In both studied sections, the highest Hg, Hg/Fexs or Hg/Al values occurred at the Devonian-Carboniferous boundary. In Lesní lom, organic matter remineralization was connected with origin of pyrite which sequestered a part of the Hg content. The decoupling of Hg and TOC within the Duli section strata and the significant correlation with Al and Fe may suggest that Hg is hosted primarily by clay particles, derived from the landmass into the marine realm. Mercury was also incorporated into pyrites but the poor correlation between Hg and TS (total sulfur) indicates a possible loss of S during oxygenation events. In the Lesní lom section, and partly in the Duli section, Fe and Mn redox cycling may have played an important role in sediment Hg content. Both sections were influenced by widespread hydrothermal and volcanic activity, indicated by positive Eu anomalies, MSI, Fe/Ti, Zr/Al2O3 and Ti/Al2O3 values. It remains to be clarified whether, besides the interplay of local magmatic activity and redox oscillations, there may have been some additional influence of the multi-phase Viluy Large Igneous Province, although its younger phase (364.4 ± 1.7 Ma) slightly precedes the Hg anomalies obtained.

Effects of Sediment Delivery Changes on Carbon Burial Within the Modern and Abandoned Delta Lobes of the Yellow River, China

Globally, deltaic wetlands sequester large volumes of terrestrial and marine-derived organic carbon. Diminishing sediment supply via river diversions, dams, and/or sea level rise threatens this stored carbon by enhancing erosion, thereby potentially releasing CO2 back to the atmosphere during remineralization of organic matter. The Yellow River delta, located in the Bohai Sea, China, has undergone intense anthropogenic manipulation since the 1950s including rerouting of the river mouth to expand the delta for oil exploitation. The goal of this study is to identify the impacts of river course diversions on sources and rates of carbon burial in the modern and abandoned delta lobes of the Yellow River delta. In 2016, we collected four cores total in abandoned and modern deltaic lobes and measured vertical accretion, total carbon, total nitrogen, δ13C, and n-alkanes. The highest average mass accretion rate of 12,470.1 g m−2 year−1 is observed in the abandoned delta, although it no longer receives direct river sediment input. The modern and abandoned deltas are currently outpacing sea level rise, but vertical accretion rates are influenced by sediment trapping practices in the modern delta and redistribution of eroded sediments in the abandoned. Average carbon burial rates across both delta sites vary between 7.2 and 14.9 g OC m−2 year−1. Sediment-associated carbon at both sites is dominantly sourced from the Loess Plateau. To conserve wetlands across the Yellow River delta, sediment management practices that periodically reintroduce sediment-laden river water to former river courses, such as in the Mississippi delta, are suggested.

Identifying sources and cycling of phosphorus in the sediment of a shallow freshwater lake in China using phosphate oxygen isotopes

Biotic and abiotic pathways for the transformation of phosphorus (P) in the sediment of Taihu Lake, a eutrophic shallow freshwater lake in southeastern China, were studied using the oxygen isotope ratios of phosphate (δ18OP) along with sediment chemistry, X-ray diffraction, and 57Fe-Mössbauer spectroscopic methods. The results showed that δ18OP values of sediment P pools significantly deviated from equilibrium and thus allowed distinguishing potential P sources or pathways of transformation. Isotope values of authigenic P being lighter than equilibrium suggests the re-mineralization of organic matter and subsequent precipitation of apatite as the major pathway of formation of authigenic P. The δ18OP values of the Al-bound P pool (18.9–23.5‰) and ferric Fe-bound P (16.79–19.86‰) could indicate potential terrestrial sources, but the latter being closer to equilibrium values implies partial overprinting of potential source signature, most likely due to reductive dissolution and release of P and followed by partial biological cycling before re-sorption/re-precipitation with newly formed ferric Fe minerals. Oxic/anoxic oscillation and dissolution/re-precipitation reactions and expected isotope excursion are corroborated by sediment chemistry and Mössbauer spectroscopic results. These findings provide improved insights for better understanding the origin and biogeochemical cycling of P associated with eutrophication in shallow freshwater lakes.

Diel population and functional synchrony of microbial communities on coral reefs

On coral reefs, microorganisms are essential for recycling nutrients to primary producers through the remineralization of benthic-derived organic matter. Diel investigations of reef processes are required to holistically understand the functional roles of microbial players in these ecosystems. Here we report a metagenomic analysis characterizing microbial communities in the water column overlying 16 remote forereef sites over a diel cycle. Our results show that microbial community composition is more dissimilar between day and night samples collected from the same site than between day or night samples collected across geographically distant reefs. Diel community differentiation is largely driven by the flux of Psychrobacter sp., which is two-orders of magnitude more abundant during the day. Nighttime communities are enriched with species of Roseobacter, Halomonas, and Alteromonas encoding a greater variety of pathways for carbohydrate catabolism, further illustrating temporal patterns of energetic provisioning between different marine microbes. Dynamic diel fluctuations of microbial populations could also support the efficient trophic transfer of energy posited in coral reef food webs.

Relative Contributions of DNRA and Denitrification to Nitrate Reduction in Thalassia testudinum Seagrass Beds in Coastal Florida (USA)

Seagrass beds are vulnerable to eutrophication (nutrient loading) and declining worldwide. To quantify the fate of nitrogen (N) inputs, intact sediment cores were incubated in a continuous-flow system with 15N enrichments to compare N consumption and efflux pathways within and near seagrass (Thalassia testudinum) beds in St. Joseph Bay, Florida. Sediment oxygen demand and total ammonium (NH4+) efflux were greater (p < 0.001) in vegetated versus unvegetated sediments, suggesting that seagrasses enhance organic matter remineralization. Denitrification rates were 2–20× greater than estimates of potential dissimilatory nitrate reduction to ammonium (DNRA). The effect of vegetation on denitrification rates was inconsistent. Direct denitrification of overlying water nitrate and N fixation rates were low, suggesting tight coupling between remineralization, nitrification, and denitrification. DNRA rates were lower than denitrification and consistently greater in vegetated sediments. DNRA rate measurements are conservative if sediment cation exchange decreases the fraction of 15NH4+ reaching overlying water, especially in vegetated sediments, where rates of nitrate-induced ammonium fluxes were observed. Coupled nitrification-denitrification was the major N loss pathway in this system, as evidenced by the lack of 15N-labeled N2 production in isotope-enriched cores. Using measured sediment oxygen demand and NH4+ fluxes as an indicator of organic matter quality and quantity, these results are consistent with previous work showing that labile organic matter helps regulate the balance between N removal and internal recycling pathways in seagrass systems, which has implications for coastal management strategies to address eutrophication.

Chesapeake Bay Inorganic Carbon: Spatial Distribution and Seasonal Variability

Few estuaries have inorganic carbon datasets with sufficient spatial and temporal coverage for identifying acidification baselines, seasonal cycles and trends. The Chesapeake Bay, though one of the most well-studied estuarine systems in the world, is no exception. To date, there have only been observational studies of inorganic carbon distribution and flux in lower bay sub-estuaries. Here, we address this knowledge gap with results from the first complete observational study of inorganic carbon along the main stem. Dissolved inorganic carbon (DIC) and total alkalinity (TA) increased from surface to bottom and north to south over the course of 2016, mainly driven by seasonal changes in river discharge, mixing, and biological carbon dioxide (CO2) removal at the surface and release in the subsurface. Upper, mid- and lower bay DIC and TA ranged from 1000-1300, 1300-1800, and 1700-1900 µmol kg-1. The pH range was large, with maximum values of 8.5 at the surface and minimums as low as 7.1 in bottom water in the upper and mid-bay. Seasonally, the upper bay was the most variable for DIC and TA, but pH was more variable in the mid-bay. Our results reveal that low pH is a continuing concern, despite reductions in nutrient inputs. There was active internal recycling of DIC and TA, with a large inorganic carbon removal in the upper bay and at salinities <5 most months, and a large addition in the mid-salinities. In spring and summer, waters with salinities between 10 and 15 were a large source of DIC, likely due to remineralization of organic matter and dissolution of CaCO3. We estimate that the estuarine export flux of DIC and TA in 2016 was 40.3 + 8.2 × 109 mol yr-1 and 47.1 + 8.6 × 109 mol yr-1. The estuary was likely a large sink of DIC, and possibly a weak source of TA. These results support the argument that the Chesapeake Bay may be an exception to the long-standing assumption that estuaries are heterotrophic. Furthermore, they underline the importance of large estuarine systems for mitigating acidification in coastal ecosystems, since riverine chemistry is substantially modified within the estuary.

Algal scavenging of mercury in preindustrial Arctic lakes

AbstractThe geochemical speciation of total mercury (THg) was examined in pre‐1800 Arctic lake sediments to improve understanding of the factors controlling “natural baseline” THg. Solid‐phase binding forms of THg were determined by sequential extraction of dated cores from three lakes in different ecozones (barren tundra, grassy tundra, and boreal forest). Sediment organic matter (OM) was mostly of algal origin. Mercury was highly concentrated in the sediment OM fraction (OM‐Hg), comprising 60–87% of THg, while OM (as total organic carbon) constituted only 0.6–13% of sediment dry weight (DW). OM‐Hg concentrations were equivalent to 159 ± 13 to 776 ± 215 ng Hg g−1 DW in algal OM and were enriched 2–39 times compared to sediment THg, indicating that even small changes in algal OM inputs could significantly alter THg. OM‐Hg explained 76–96% of the variation in THg concentrations over many centuries. Concentrations of S2 carbon (an algal productivity proxy) and OM‐Hg were significantly correlated in two lakes but not in the boreal forest lake possibly because of OM remineralization in its deep water column. Fluxes of S2 carbon, OM‐Hg, and THg were highly correlated in the barren tundra lake but could not be calculated for the other lakes. The results overall indicate that high algal Hg concentrations due to scavenging of available Hg controlled OM‐Hg flux to sediments, thus driving changes in THg concentrations and fluxes. These findings improve our understanding of the long‐term stability of baseline THg in northern lakes under a changing climate, including in the modern era.

Organic matter quantity and quality across salinity gradients in conduit‐ vs. diffuse flow‐dominated subterranean estuaries

AbstractSubmarine groundwater discharge (SGD) is a source of water and bioreactive solutes to coastal zones but may be modified by organic matter (OM) remineralization dynamics within subterranean estuaries (STEs). We hypothesize that bioreactive solute fluxes should depend on water residence time in STEs, but links between OM transformations and residence time in STEs are poorly characterized. To test this hypothesis, we compare dissolved OM (DOM) quantity and quality in two hydrologically distinct STE systems: a reef lagoon on the east coast of the Yucatan Peninsula, Mexico, where semidiurnal mixing in submarine springs of a carbonate karst aquifer results in short residence times, and a barrier lagoon on the east coast of Florida, where slow flow through siliciclastic sediments results in long residence times. We measured dissolved organic carbon concentrations and characterized colored DOM (CDOM) with ultraviolet spectroscopy and fluorescence combined with Parallel Factor Analysis. Both sites exhibit similar shifts in OM quality with salinity and reflect a marine source of labile OM to the STEs. Nonconservative mixing and CDOM production occurs at all sites but the long water residence times in the siliciclastic STEs cause orders of magnitude greater production than the carbonate STE. Consistent CDOM production across sites with disparate characteristics indicates that STEs are common sources of CDOM to surface water. However, observed variation in the magnitudes of CDOM production indicates that estimating global, and even regional, solute fluxes associated with SGD will be complicated by hydrologic control on extents of OM remineralization.

Metabolic diversity within the globally abundant Marine Group II Euryarchaea offers insight into ecological patterns

Despite their discovery over 25 years ago, the Marine Group II Euryarchaea (MGII) remain a difficult group of organisms to study, lacking cultured isolates and genome references. The MGII have been identified in marine samples from around the world, and evidence supports a photoheterotrophic lifestyle combining phototrophy via proteorhodopsins with the remineralization of high molecular weight organic matter. Divided between two clades, the MGII have distinct ecological patterns that are not understood based on the limited number of available genomes. Here, I present a comparative genomic analysis of 250 MGII genomes, providing a comprehensive investigation of these mesophilic archaea. This analysis identifies 17 distinct subclades including nine subclades that previously lacked reference genomes. The metabolic potential and distribution of the MGII genera reveals distinct roles in the environment, identifying algal-saccharide-degrading coastal subclades, protein-degrading oligotrophic surface ocean subclades, and mesopelagic subclades lacking proteorhodopsins, common in all other subclades.