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
04:30 PM
Nitrogen reduction rapidly reverses eutrophication in shallow phosphorus-rich lakes and reservoirs (8981)
Primary Presenter: Thad Scott, Baylor University (Thad_Scott@Baylor.edu)
Here we report the results of a five-year shallow-lake mesocosm experiment testing the role of nitrogen (N) in inducing and reversing lake eutrophication. We tested two conditions: 1) the effect of combined but imbalanced N and P fertilization on inducing lake eutrophication, and 2) the effect of subsequently eliminating N fertilization while maintaining P fertilization to reverse lake eutrophication. The mesocosms were open to both the atmosphere and the lake sediments which ensured that the experimental conditions mimicked the complex biogeochemical interactions that occur in lakes. We fertilized all mesocosms with 1.3 g P m-2 y-1 for five years. We also fertilized with four different N rates (1.3, 9.4, 32, and 64 g N m-2 y-1) in a randomized block design and utilized the lake outside the mesocosms as a control. After four years, we halted all N fertilization but continued P fertilization for a fifth year. Nutrient concentrations and phytoplankton biomass responded approximately proportion to N and P fertilization rates during the first four years of the experiment. Total N concentrations declined rapidly after the cessation of N fertilization but total P concentrations actually increased despite constant P fertilization. Chlorophyll-a concentrations decreased by 50-70% and was proportional to total N concentration reductions. Combining our results with the 2022 National Lakes Assessment data we show that reducing N inputs has the potential to reverse eutrophication in over 1 million lakes across the conterminous US.
04:45 PM
WATER COLUMN AMMONIUM REGENERATION SUPPORTS PRODUCTIVITY IN TWO LARGE, EUTROPHIC LAKES (8820)
Primary Presenter: Margot Sepp, Estonian University of Life Sciences (margot.sepp@emu.ee)
Harmful cyanobacterial blooms often rely on recycling of ammonium to produce biomass and nitrogen-rich toxins, despite low or unmeasurable concentrations in water. Thus, measuring ammonium turnover rates (uptake and regeneration) is necessary to determine its actual availability. Here, water column ammonium dynamics, in-lake water quality parameters driving these dynamics, and the importance of internal nitrogen loading in supporting community ammonium demand were explored in two large, shallow, eutrophic lakes (Võrtsjärv and Peipsi, Estonia). Stable isotope incubations were conducted almost monthly (including during winter) in Võrtsjärv and several times per year in Peipsi (from March 2019 to March 2022). Ammonium turnover rates in Võrtsjärv and Peipsi were similar to those reported for other large eutrophic lakes, despite being located at much higher latitude. Ammonium dynamics were strongly related to seasonally changing water quality variables, such as temperature, nutrient concentrations, and phytoplankton biomass, which, combined, explained 68–71% of variation in measured rates. Water column ammonium regeneration supported, on average, 65% (Võrtsjärv) and 76% (Peipsi) of community ammonium demand during the warm season (May – October). Internal nitrogen loading from ammonium regeneration in the water column vastly exceeded external loading into Võrtsjärv. These results emphasize the importance of internal nitrogen loading in driving primary productivity in eutrophic lakes and the necessity to reduce external nitrogen loading, in addition to phosphorus, into lakes.
05:00 PM
Cycling of Nitrogen in Marine Dissolved Organic Matter Revealed by Radiocarbon Analysis of Individual Amino Acids (8986)
Primary Presenter: Yuchen Sun, University of California, Santa Cruz (sonyuushin@gmail.com)
Dissolved organic nitrogen (DON) is one of the largest active pools of reduced N in the ocean, playing an important role in marine ecosystems. However, DON-specific ages and cycling rates remain largely unknown. Radiocarbon (14C) content indicates age since synthesis, representing a powerful tool for tracing the formation and cycling of organic molecules. However, the lack of 14C approaches specific to N-containing biomolecules has meant that direct estimates of DON cycling and age are essentially nonexistent. Here we report a new method for the 14C analysis of individual amino acids (AAs), the largest DON component which can be characterized at the molecular level. We show that by coupling multidimensional high-performance liquid chromatography (HPLC) with ultra-sensitive accelerator mass spectrometry (AMS), 14C analyses of individual AAs, including both their L- and D-enantiomers, can be achieved from ocean DON isolates. We use this method to report the first L- and D- AA 14C ages in both semi-labile high molecular weight (HMW) DON and refractory low molecular weight (LMW) DON, from the central CA margin. Specifically, we will use relative 14C ages of D-AA as direct tracers for persistence of bacterially synthesized DON components, comparing these with 14C ages of both L-AA as well as the total DOM fractions from which they derive, toward the ultimate goal of understanding the relative ages of DOC vs. DON pools, and the importance of bacterial sources in influencing DON persistence and cycling.
05:15 PM
NITROGEN CYCLING AND NITROUS OXIDE FLUXES IN A TIDAL WETLAND IN AUSTRALIA (9685)
Primary Presenter: Britte van Haastregt, Southern Cross University (b.van.haastregt.10@student.scu.edu.au)
Tidal wetlands can remove excess nitrogen through processes such as denitrification and anammox. Denitrification thereby generates nitrous oxide (N2O), a potent greenhouse gas. While wetlands are often an N2O source, some pristine systems can be sinks under certain conditions. Habitat complexity can enhance biogeochemical cycling and thereby increase N2O fluxes. However, the underlying mechanisms are not well understood. This study measured denitrification, anammox, DNRA and N2O fluxes across different habitats, including mangrove bare sediment, crab burrows, pneumatophores, and salt marsh, in different seasons at a low-disturbance tidal wetland near Jacobs Well, QLD, Australia. Sediment-air N2O fluxes were measured using static chamber incubations in exposed habitats and sediment-water fluxes were measured with benthic chamber when these habitats were inundated. Denitrification, anammox and DNRA rates were measured in situ using a novel whole-system 15NO3- labelling approach. Results show substantial variation in N2O fluxes across different habitats, tides, and seasons. Generally, sediment-air N2O fluxes are higher than sediment-water N2O fluxes, with the highest fluxes observed from pneumatophores and bare sediment sites. Denitrification rates are also highest in those habitats. Crab burrows and salt marsh sites generally have lower N2O emissions or slight uptakes and the highest DNRA rates. All fluxes and process rates will be upscaled using a hydrodynamic model and habitat map to construct an annual nitrogen budget, which will improve our understanding of nitrogen dynamics and N2O fluxes in tidal wetlands.
05:30 PM
Investigating the Role of Microseira wollei mats in Nitrogen and Phosphorus Cycling at Lake Wateree, South Carolina (9263)
Primary Presenter: Archana Venkatachari, University of South Carolina (avenkatachari@seoe.sc.edu)
Harmful Cyanobacterial Blooms (HCBs) increasingly threaten human health and aquatic ecosystems in freshwater environments. Microseira (Lyngbya) wollei, a non-heterocystous cyanobacterium prevalent in the southeastern United States, forms perennial benthic and seasonal pelagic mats that can release toxins and fix atmospheric nitrogen. This study focuses on Lake Wateree, a hydroelectric reservoir characterized by extensive M. wollei mats, aiming to elucidate the role of this cyanobacterium in benthic nutrient cycling dynamics. From March to October 2023, excluding April and June, we sampled two sites: one with M. wollei mats (HCB) and one without. We established two pairs of sites, sampling them alternately over six months. Denitrification rates, representing the conversion of nitrate to N2 gas, were measured alongside anammox and dissimilative nitrate reduction to ammonium, using 15N-labeled incubations in whole sediment cores. Our results showed that nitrogen removal via denitrification was 3.9 times higher in sediments associated with M. wollei mats (up to (0.96 mmol N/m²/day), establishing a correlation between mat thickness and denitrification rates (R² = 0.98). Anammox rates were generally below detection limits. Notably, we observed net negative N2 fluxes from sediments to the water column in M. wollei cores, indicating biological nitrogen fixation. Additionally, elevated phosphorus and iron concentrations in sediments with M. wollei suggest active regulation of redox conditions, facilitating the release of legacy phosphorus from organometallic complexes. These findings are crucial for management strategies at Lake Wateree and enhance the understanding of nutrient dynamics associated with benthic HCBs, informing future mitigation efforts.
05:45 PM
Understanding Nitrogen Fixation in Phototrophic Diazotrophs: Insights from a Stoichiometric model (9282)
Primary Presenter: Nicole Wagner, Oakland University (nicolegouldingwagner@gmail.com)
Nitrogen (N) plays a pivotal role in regulating productivity and imposes constraints on the adaptability of ecosystems to climate change and biogeochemical cycles. Most N exists in the inert form of di-nitrogen gas (N2), inaccessible to many organisms except diazotrophs, which convert N2 into bioavailable ammonia (NH3) using the nitrogenase enzyme. While the stoichiometry of N2-fixation reaction is well understood, the organismal stoichiometric controls on N2-fixation remain elusive. Here, we developed a mechanistic model, representing a chemostat culture, to explore population and nutrient stoichiometry dynamics of phototrophic diazotrophs. It tracks biomass in terms of carbon and energetic ATP, and the organismal (cellular) content of the essential nutrients N, phosphorus (P), iron (Fe), and molybdenum (Mo). Our model tracks multiple N components including ammonium and nitrate, nitrogenase, nitrate reductase, and other protein forms, allowing us to explore stoichiometric constraints on C fixation, biosynthesis, nutrient uptake, and N2 fixation rates. Our model examines how these factors interact and predicts when any given potentially limiting factor takes prominence over others across a gradient of N:P:Fe input concentration ratios. Through model analyses, we can predict how these limiting elements affect diazotrophs with different physiological traits. Our model is a novel approach that betters our understanding of phototrophic diazotrophs, N2 fixation, and biogeochemical cycling.
SS18C - Nitrogen Cycling Processes in Aquatic Ecosystems and Associated Food Webs
Description
Time: 4:30 PM
Date: 29/3/2025
Room: W207CD