Aquatic ecosystems, ranging from the open ocean to coastal environments, are driven by intricate nitrogen (N) cycling processes that are essential for maintaining the health of these environments and the food webs they sustain. At the base of these food webs, primary producers such as phytoplankton play a pivotal role by assimilating N from various sources through both photoautotrophic and mixotrophic pathways. Autotrophy converts inorganic N to organic N, predominantly in the form of proteins and amino acids (AAs), which serve as the primary carriers of N as it moves through the food webs.
Identifying the sources of N in aquatic ecosystems and their associated food webs is crucial for understanding and modeling these cycling processes. This can be achieved through contemporary measurements of dissolved inorganic N (e.g., nitrate and ammonia) concentrations and stable isotopes, as well as by reconstructing historical N sources and usage patterns using organic N and their isotopes in biogenic particles and sediments. These investigations provide valuable insights into both the past and present dynamics of N cycling in open ocean and coastal environments.
Moreover, N cycling processes have significant implications for climate, as processes such as denitrification and nitrification can lead to the production of nitrous oxide (N₂O), a potent greenhouse gas with long-term impacts on global warming. Understanding the factors that regulate N₂O production and release is therefore essential for predicting the broader environmental consequences of N cycling.
This session aims to bring together researchers utilizing a diverse array of methodologies, including isotope geochemistry, biomolecular tools, and numerical modeling, to explore N cycling in aquatic ecosystems and their associated food webs across both open ocean and coastal areas. By sharing insights and findings, this session seeks to deepen our understanding of N cycling processes across different aquatic environments, ultimately contributing to a more comprehensive understanding of how N cycling influences ecosystem structure and function across various spatial and temporal scales.
Lead Organizer: Lin Zhang, Texas A and M University Corpus Christi (lin.zhang@tamucc.edu)
Co-organizers:
Mark Altabet, University of Massachusetts Dartmouth (maltabet@umassd.edu)
Annie Bourbonnais, University of South Carolina (abourbonnais@seoe.sc.edu)
Pat Glibert, University of Maryland Center for Environmental Science (glibert@umces.edu)
Wingman (Charlotte) Lee, Texas A&M University-Corpus Christi (wlee4@islander.tamucc.edu)
Presentations
06:00 PM
Impact of a Trichodesmium Bloom on Zooplankton Distribution and Available Nitrogen in Tampa Bay (9473)
Primary Presenter: Rebecca Waggett, The University of Tampa (rwaggett@ut.edu)
Zooplankton serve as an important link within marine food webs – between abundant phytoplankton primary producers and a diversity of consumers. Trichodesmium is an important warm-water cyanobacteria capable of fixing nitrogen in oligotrophic environments and a potential food source for zooplankton predators. Along the Southwest Florida coastline, the toxic dinoflagellate Karenia brevis is notable for creating harmful algal blooms (HABs) on a near annual basis. Nitrogen cycling in Tampa Bay, linked to the nitrogen fixing cyanobacteria Trichodesmium, has been identified as an amplifier of K. brevis blooms. To assess the impacts of ambient nitrogen variation on K. brevis appearance and subsequent impacts on zooplankton community dynamics, sampling took place weekly at three locations within Tampa Bay. Nutrient samples were collected leading up to Trichodesmium bloom events and analyzed for inorganic nitrogen (nitrate, nitrite, and ammonia) and dissolved organic nitrogen using absorption-based spectrophotometry. Zooplankton tows were performed and preserved for quantification and identification. The measured uptake of inorganic nitrogen and release of organic amines during the Trichodesmium bloom provide insight into the nutrient dynamics facilitating K. brevis blooms in estuarine environments. Synergistic analysis of zooplankton further indicated impacts on the community composition, particularly the dominant calanoid copepod species as well as changes in total zooplankton abundance suggesting a bottom-up effect of Trichodesmium produced nitrogen within marine ecosystems.
06:00 PM
Two-dimensional distribution patterns of ammonium in marine sediments revealed by a novel ammonia planar fluorescence sensor (9468)
Primary Presenter: Qingzhi Zhu, Stony Brook University (qing.zhu@stonybrook.edu)
High-resolution distributions of ammonium are critical to study the rates and patterns of remineralization, nitrification, and denitrification processes in sediments. A novel irreversible ammonia planar optical sensor was developed using o-phthalaldehyde (OPA) as indicator to resolve 2-dimensional (2-D) distributions of ammonia with a limit of detection of 9 nM. The sensor is prepared by immobilizing OPA in a thin layer of polyurethane hydrogel (~10 µm thickness) on a transparent polyester sheet, and covered with a gas permeable PTFE membrane to eliminate interferences from various solutes. Fluorescence shows maximum emission at 430 nm with excitation at 365 nm, and fluorescence intensity is proportional to ammonia concentration and time of reaction. No interferences from other major solutes or trace metal ions were observed except hydrogen sulfide when it was higher than 100 µM. The irreversible sensor has been paired with pH sensors and successfully used to measure 1 and 2-D ammonia – ammonium distributions in subtidal sediments. Sensing can be extended to 3-D patterns using stacked images. Images are readily obtained using inexpensive LED excitation and commercial grade digital cameras, with typical pixel resolution of < 50 × 50 mm over areas > 100 cm2. A distinct ammonia consumption zone across the water-sediment interface and complex heterogeneous distribution patterns of ammonia associated with biogenic structures and oxygenation are revealed. Distributions can be related directly to the corresponding visible or x-radiographic images of the sediment.
06:00 PM
Ammonium dynamics and internal nitrogen loading from the water column of eutrophic lakes: implications for managing watershed nutrient loads (8840)
Primary Presenter: Gizem Oguz, Estonian University of Life Sciences (gizemoguz9806@gmail.com)
Synthetic and organic fertilizer use has increased nitrogen (N) levels and combined with climate change, drives eutrophication and cyanobacteria blooms in aquatic systems. Internal nutrient loading from microbial activity in sediments and the water column can intensify these blooms, and ammonium (NH4) is key for internal N cycling and algal growth. NH4 regeneration and potential uptake rates were measured in two medium-sized, shallow, eutrophic lakes in Estonia (Kaiavere and Veisjärv). Water samples from two sites in each lake were amended with 15N-labelled NH4 and incubated in light and dark bottles for 24 hours at near-in situ temperature and light. Results from autumn 2023 and spring 2024 (similar water temperatures, 13 – 14 ˚C) showed 2 – 6 times higher potential NH4 uptake rates in autumn in both lakes, with larger differences in dark bottles. Potential uptake rates in Kaiavere were higher than in Veisjärv, but NH4 regeneration could support ~60% of potential uptake in Veisjärv in spring. In autumn in both lakes and Kaiavere in spring, NH4 regeneration could support 22 – 35% of potential uptake. Combined with high NH4 releases from sediments, internal N loading, ultimately driven by external N loading, could support large proportions of community NH4 demand in the water column (dominated by non-N-fixing cyanobacteria), even when ambient NH4 concentrations were low or undetectable. Thus, watershed management and lake restoration efforts focusing solely on reducing external or internal phosphorus loads are insufficient to mitigate eutrophication in these and other lakes.
06:00 PM
SIMULTANEOUS NITROGEN RELEASES AND REMOVAL FROM EUTROPHIC LAKE SEDIMENTS (8902)
Primary Presenter: Esther Amalachukwu Nwume, Estonian University of Life Sciences (esther_amalachukwu.nwume@emu.ee)
Anthropogenic nutrient inputs, especially from agriculture, lead to eutrophication, harmful algal blooms, and biodiversity loss. Nutrient legacies (both nitrogen (N) and phosphorus (P)) accumulate in lake sediments and can later be released into the water column, delaying water quality improvements after external load reductions. Sediments in eutrophic lakes release both N and P, representing internal nutrient loads, which can be used by harmful algae to produce biomass (and toxins). To determine whether sediments in eutrophic lakes Kaiavere and Veisjärv (central Estonia) are a net source or sink for bioavailable N, intact sediment cores were incubated in a continuous-flow system, with and without 15N-ammonium or 15N-nitrate tracers, to measure microbial N transformations, such as denitrification/anammox (N removal), N fixation (N source), and dissimilatory nitrate reduction to ammonium (DNRA) rates and net nutrient fluxes. Low/no anammox or DNRA was detected in either lake, but denitrification was stimulated by 15N-nitrate in both lakes. N fixation consistently exceeded denitrification in unamended Veisjärv sediments, but Kaiavere sediments varied between net N fixation and net denitrification. Bioreactive N (e.g., ammonium and urea) and phosphate were also released from sediments in both lakes, but molar N:P of these sediment nutrient releases was very high, indicating N excess, relative to P. External N loads should thus also be reduced, in addition to P, to minimize internal N loading from legacy N and to effectively manage eutrophication and algal blooms in lakes.
06:00 PM
SINK OR SOURCE: INVESTIGATING N2O DYNAMICS IN NATURAL PONDS ACROSS NEW YORK STATE (9379)
Primary Presenter: Kaci Zarek, Cornell University (kacizarek10@gmail.com)
Nitrous oxide (N2O) is an important greenhouse gas (GHG) with a global warming potential 273 times greater than carbon dioxide in a 100-year time period. Ponds are biogeochemical hotspots that may be key sinks for N2O. However, what is driving the potential N2O sink or source dynamics in ponds and variability across ponds remains poorly understood. Therefore, we are exploring: (i) How variable N2O concentrations are across natural ponds and in surface vs. bottom waters, and (ii) What the main physicochemical drivers are of N2O sources versus sinks in these natural ponds. We sampled 14 natural ponds across the four dominant ecoregions of New York State (northeastern, U.S.) twice in summer 2024. In each sampling round, we measured dissolved N2O gas concentrations in surface and bottom waters, and collected a suite of physicochemical variables (e.g., temperature, oxygen, pH, nitrate, and organic carbon). Our preliminary results show during both rounds of sampling, the bottom waters of our ponds remained more undersaturated in N2O while our surface waters may shift from sources to sinks of N2O. Together, our findings will provide a more holistic understanding of N2O dynamics in natural ponds and potential drivers of N2O production and consumption.
06:00 PM
USING CSIA-AA OF ANCIENT FISH BONE PROTEINS TO ESTIMATE CHANGES IN BASAL NITROGEN IN THE GULF OF MAINE OVER THE LAST 4,400 YEARS (9413)
Primary Presenter: Samantha Turtle, Bates College (turtlesam@outlook.com)
Previous nitrogen isotope studies of bulk proteins extracted from ancient Atlantic cod (Gadus morhua) tissues document a 1-2‰ decrease in d15N values over the last couple of centuries (Harris, 2011; Lueders-Dumont et al., 2018). Due to the nature of the nitrogen isotope signal in bulk proteins, this isotopic shift may be attributed to a decrease in trophic level and/or a change in baseline nitrogen in the Gulf of Maine over this time period. Here, we analyze the d15N composition of individual amino acids from ancient cod bone collagen to tease out the relative importance of shifts in trophic level vs baseline nitrogen sources to cod diets through time. Preliminary data indicate that d15N values of phenylalanine (“source” amino acid) extracted from cod bone collagen became more depleted in d15N over the last 500 years and into the modern record. These shifts in d15N phenylalanine are in agreement with those found in d15N phenylalanine of deep-sea corals (Sherwood et al., 2011) and bivalves (Whitney et al., 2019) from the Gulf of Maine over the last 100+ years. The fact that similar trends are seen in three different species occupying different ecological niches suggests the shift in source nitrogen may reflect broad changes in hydrographic conditions in the Gulf of Maine. More work is needed to corroborate these preliminary findings and is currently underway.
06:00 PM
CULTIVATION EFFORTS AND SIZE FRACTIONATED RATES SUPPORT POTENTIAL SURFACE WATER NITROUS OXIDE CYCLE (9245)
Primary Presenter: Jessica Hexter, Princeton University (jh6959@princeton.edu)
Nitrous oxide (N2O) is a potent greenhouse gas that is predicted to be the single greatest ozone depleting agent of the 21st century. Despite making up only 3% of the surface ocean, oxygen minimum zones (OMZs) are responsible for as much as 15-21% of global N2O emissions, predominately from denitrification. The only known biological sink for N2O is the last step of the denitrification pathway, mediated by an enzyme that is thought to be oxygen intolerant. Therefore, N2O respiration should not be favored in surface-living microbes. Previous work in the Eastern Tropical North Pacific OMZ, however, provided evidence for significant potential rates of surface water N2O consumption, and implied a potential role for particles to serve as anoxic microsites. We tested this hypothesis by enriching for surface water N2O reducers from the Eastern Tropical South Pacific (ETSP) OMZ targeting several size fractions and different organic substrates. Having obtained a few dozen N2O-respiring isolates from surface water, we then characterize them in terms of their phylogeny, their capabilities for nitrous oxide consumption, and other anaerobic respiratory reactions. Additionally, we report simulated in-situ and potential N2O production and consumption rates in surface water from varying size fractions and oxygen concentrations. These rates imply a potential cryptic N2O cycle in surface water that is distributed across the particle size spectrum.
06:00 PM
Mixing it up: Nitrous Oxide Air-Sea Gas Flux in the Gulf of Mexico: accounting for hurricane mixing (9281)
Primary Presenter: William Love, University of South Carolina (lovewd@email.sc.edu)
Nitrous oxide (N2O) is a trace gas with a warming potential approximately 270 times that of carbon dioxide. N2O is also the primary source of atmospheric nitrogen oxides, which deplete the stratospheric ozone layer. Marine N2O production from bacterial processes (nitrification and denitrification) represents, globally, up to 30% of N2O emitted to the atmosphere. N2O samples were collected at various locations and depths in the Gulf of Mexico hypoxic zone that develops during summer to investigate N2O cycling. Air-sea gas fluxes of N2O were calculated in accordance with Wanninkhof 2014. Although air-sea gas fluxes are generally negligible during summer due to strong stratification, hurricanes are a major source of vertical mixing. Yet, the effects of hurricane mixing on sea-air N2O fluxes is never accounted for, due to sampling limitations. During hurricanes, shallow waters in the Gulf of Mexico are being uniformly mixed, at depths down to 150m . A simple vertical mixing model of N2O was created to estimate hurricane-like conditions and then air-sea gas fluxes were recalculated considering hurricane speed winds.
06:00 PM
Impacts of Mesoscale Eddies on Nitrous Oxide Production in the Eastern Tropical North Pacific Oxygen Deficient Zone (9706)
Primary Presenter: Annie Bourbonnais, University of South Carolina (abourbonnais@seoe.sc.edu)
Nitrous oxide (N2O) is a potent greenhouse gas and ozone depleting substance with important oceanic sources to the atmosphere. Yet the roles of mesoscale or sub mesoscale features (e.g., eddies, chlorophyl intrusions), oxygen concentrations, or organic matter fluxes on N2O dynamics are still poorly constrained. We sampled 23 stations along a transect from Costa Rica to San Diego in the Eastern Tropical North Pacific (ETNP) for N2O concentrations, stable isotopes and isotopomers in January 2022. The transect crossed contrasting oxygen and productivity regimes in the ETNP Oxygen Deficient Zone (ODZ), including the highly productive Costa-Rica Dome. In addition, we observed at least two mesoscale eddies along the cruise track. We measured rates of N2O production from nitrification and denitrification using 15N-labeled incubations. We also performed kinetic experiments to evaluate the effect of substrate additions on N2O production rates. We observed highest N2O concentrations and production rates at low-O2 concentrations near the ODZ oxycline in elevated productivity areas, as evaluated by satellite-derived surface chlorophyll a and ship transmissometer data. Consistent with previous studies in ODZs, our isotopomer data suggest that incomplete denitrification is responsible for these high N2O accumulations above the ODZ. We will further expand on the role of mesoscale eddies for N2O production both along vertical and horizontal gradients.
06:00 PM
Quantitative Recovery of Dissolved Organic Nitrogen? Exploring a New Method Integrating Ultrafiltration with C18 and PPL Solid-Phase Extraction Resins (9237)
Primary Presenter: Lingyu Ma, University of California, Santa Cruz (lma104@ucsc.edu)
Marine Dissolved organic nitrogen (DON) is central to the ocean’s nitrogen cycle. However, in particular the refractory lower molecular weight (LMW) DON pool, constituting the large majority of all DON, remains poorly understood, primarily due to isolation and characterization challenges. Although early work targeting LMW DON composition used PPL solid-phase extraction (SPE), modest recoveries limit interpretation. Nevertheless, recent experiments suggest that new approaches may achieve vastly larger DON recoveries. NMR data indicating that heterocyclic nitrogen compounds quantitatively dominate LMW DON are consistent independent work indicating far higher potential DON recovery with non-polar SPE, exceeding 90% recovery in some small-scale isolations. If born out at larger scale, these observations suggest that coupling HMW DON filtrations together with multiple resin types targeting polar and non-polar LMW compounds might enable nearly quantitative DON recovery, while simultaneously separating more labile and refractory fractions. Here we test this idea, reporting a new method coupling C18 and PPL resins with 1 kDa ultrafiltration (UF). We first report LMW DON recovery efficiencies by PPL vs. C-18, evaluating the composition and representativeness of each fraction with a combination of isotopic analysis (δ15N-DON), biomarkers (DL, δ15N, δ13C -amino acids), and ¹⁵N CP/MAS NMR. Finally, we evaluate a unified three part method coupling UF, PPL and C-18 for quantitative DON recovery relative composition at each stage.
06:00 PM
Compound-Specific and Intramolecular δ¹⁵N of Histidine in Marine Particles and Organisms: A New Tool for Investigating Nitrogen (N) Metabolism and Cycling (8983)
Primary Presenter: Charlotte Wing Man Lee, Texas A&M University - Corpus Christi (wlee4@islander.tamucc.edu)
Histidine (HIS) exhibits minimal ¹⁵N fractionation during trophic transfer, potentially preserving the δ¹⁵N baseline of the food web. With its three N atoms incorporated into the HIS molecule and catabolized via distinct enzymatic pathways, site-specific δ¹⁵N values of HIS can provide insights into N metabolism and associated isotopic fluxes. While molecular average δ¹⁵NHIS (δ¹⁵NHIS-Total) is rarely reported and no intramolecular δ¹⁵NHIS data for natural samples exist due to technical challenges with conventional analytical methods. We developed a new approach using ion-exchange chromatography (IC) to analyze δ¹⁵N of underivatized HIS. δ¹⁵NHIS-Total is determined by converting all N to nitrate through UV-persulfate oxidation. Site-specific δ¹⁵N values of the α-N and sidechain N (δ¹⁵NHIS-α and δ¹⁵NHIS-s, respectively) are calculated by isotope mass balance. δ¹⁵NHIS-Total matched corresponding EA/IRMS δ¹⁵N values within ±1‰ for in-house isotopic standards, and uncertainty ≤ ±0.6‰. In both commercial HIS standards and biological samples, including cyanobacteria, zooplankton, and fish, the α-N was found to be significantly enriched in ¹⁵N relative to the side-N (Δδ¹⁵Nα-s = +4 – 26‰), suggesting greater catabolic processing of α-N compared to side-N. This study establishes a new framework for combining chromatographic purification, chemical oxidation, and isotope analysis to obtain both molecular average and intramolecular δ¹⁵N values for poly-N amino acids, offering a valuable tool for investigating N metabolic pathways.
SS18P - Nitrogen Cycling Processes in Aquatic Ecosystems and Associated Food Webs
Description
Time: 6:00 PM
Date: 29/3/2025
Room: Exhibit Hall A