Marine non-cyanobacterial diazotrophs (NCDs) are recognized as globally distributed, however, few representatives have been isolated in pure cultures. As a result, understanding the physiology, growth rate, substrate preference and dinitrogen (N2) fixation capabilities proves difficult. Thalassolituus haligoni . sp. nov., BB40 was isolated from a fjord-like inlet within Kjipuktuk (Halifax), Nova Scotia. The fully sequenced genome displayed all necessary genes required for N2 fixation, and various carbon uptake pathways. The gram-negative flagellated rod shape bacterium displayed significantly higher growth rates in medium amended with nitrate (NO3-) or ammonia (NH3), compared to dissolved N2, as the sole nitrogen source. Biological N2 fixation rates were detectable across all conditions, measuring a range from 9.34 × 10-6 to 1.4 × 10-1 fmol N cell-1 day-1. Growth of the isolate was successful between 4 °C up to 35 °C, with a Topt of 20 °C for N2, and between 27 - 30 °C for fixed nitrogen (NO3- and NH3). The closest relatives to T. haligoni , were found to be the uncultured Arc-gamma-03 (99% average nucleotide identity (ANI)) and Oceanobacter antarcticus (81% ANI). T. haligoni also displays versatile capabilities for growth on various carbon, and nitrogen sources, and antibiotics. Collectively this study provides an in-depth physiological assessment of an Oceanospirillales diazotrophic species which we presently have limited knowledge of. ### Competing Interest Statement The authors have declared no competing interest.

This paper presents the Gulf of St. Lawrence and Estuary Dataset (GOSLED), a quality-controlled compilation of biogeochemical observations collected during 21 research cruises in the St. Lawrence Estuary, Gulf of St. Lawrence, and Saguenay Fjord between 2003 and 2023. This dataset integrates hydrographic measurements and a broad suite of discrete biogeochemical variables into a single, standardized compilation suitable for reuse, synthesis, and long-term analysis. GOSLED includes discrete measurements of dissolved oxygen, carbonate-system parameters, macronutrients, dissolved organic carbon, selected biogeochemical gases, stable isotope ratios of carbon and water, and transient and deliberate tracers. Data were compiled from multiple independent research cruises and laboratory archives (2003-2020), including contributions from the Marine Environmental Observation, Prediction and Response Network (MEOPAR) - R & eacute;seau Qu & eacute;bec maritime (RQM) Gulf of St. Lawrence Tracer Release Experiment (TReX; 2021-2023), RQM Odyss & eacute;e Saint Laurent program (2018-2023), and the Fisheries and Oceans Canada (DFO) Atlantic Zone Monitoring Program (AZMP; fall 2022). Sampling was conducted predominantly during the ice-free season, resulting in limited winter coverage across much of the system. All data were harmonized and processed following primary quality-control procedures adapted from GLODAP and CODAP-NA standards. Secondary crossover analysis was not possible due to a lack of deep-water (>1500 m) sampling. This paper documents the data provenance, quality-control procedures, known limitations, and recommended considerations for dataset usage. GOSLED is archived at the Canadian Integrated Ocean Observing System - St. Lawrence Global Observatory (CIOOS-SLGO) and is publicly accessible at 10.26071/d6f3fdfc-788d-48ff (Nesbitt et al., 2026).

Abstract. Human activities are increasingly affecting coastal ecosystems, with shoreline stabilization structures becoming a prevalent response to sea-level rise and extreme climatic events. While these structures aim to protect coastal communities and infrastructures, their effects on ecosystem functioning, and particularly nitrogen cycling and fixed-nitrogen loss processes such as denitrification, remain poorly understood. To assess the ecological impacts of breakwater construction, we employed a space-for-time substitution approach to examine changes in sediment biogeochemistry, macrobenthic community structure, and nitrogen cycling in an intertidal salt marsh in the St. Lawrence Estuary, Canada. We measured benthic fluxes, denitrification rates, and macrofaunal assemblages at five locations: two landward of the breakwater (impacted sites), one situated immediately seaward of the breakwater (intermediate site) and two reference sites (one vegetated and one unvegetated). Landward sites exhibited finer sediment with higher organic carbon content and supported distinct macrobenthic communities dominated by opportunistic Oligochaetes while reference sites were dominated by Molluscs and Crustaceans. The intermediate seaward site closely resembled the unvegetated reference site in both sediment characteristics and oxygen dynamics. Surprisingly, despite these substantial physical and biological changes, dark total benthic oxygen uptake, NO3−, NH4+ benthic fluxes and benthic denitrification rates showed no significant differences between impacted and reference sites. This functional similarity suggests a degree of ecosystem plasticity, where different combinations of abiotic and biotic factors can maintain similar ecosystem function. However, such functional plasticity might not apply to all ecosystem services offered by natural salt marshes, emphasizing the importance of careful consideration in coastal management decisions.

Bioavailable nitrogen governs ocean productivity and carbon fixation by regulating phytoplankton growth and community composition. Nitrogen input primarily results from fixation, while denitrification and anammox remove bioavailable nitrogen in oxygen‐depleted conditions. Traditionally considered limited to highly suboxic (i.e., <5 μM) waters, recent studies suggest that fixed‐nitrogen removal processes may extend beyond, elevating global nitrogen loss estimates. This study directly quantifies fixed‐nitrogen loss across oxygen gradients (from 140 to 32 μM) along the Estuary and Gulf of St. Lawrence using N cycle tracers (, , and ). Notably, we observe significant production when ambient concentrations fall below a threshold value of 58.9 ± 1.1 μM, including potential water column fixed‐nitrogen removal processes above suboxia. We hypothesis that ambient deoxygenation eases the formation of suboxic microareas in suspended organic matter. Benthic production remains unaffected under intensifying water column deoxygenation from 50 down to 32 μM, but the contribution of produced through nitrification in the sediment to denitrification diminishes as deoxygenation intensifies. Combined, water column and benthic fixed‐nitrogen removal processes drive anomalies and strong deficiency in bottom waters. Additionally, the observed threshold also triggers production. Overall, our study highlights the profound impact of coastal ocean deoxygenation on nitrogen cycling, suggesting unexpected shifts even at ambient oxygen concentrations traditionally considered well above suboxic conditions.

Dissolved oxygen is a major environmental driver in aquatic environments, and its decline in the global ocean over recent decades threatens marine fauna, particularly benthic invertebrates. These organisms, often sessile or sedentary, cannot escape persistent environmental hypoxia and must rely on the adjustment of physiological mechanisms, such as energy metabolism and cell functioning pathways, underpinning their ability to cope with these challenging conditions. However, the molecular bases of such mechanisms, particularly under in situ conditions, are yet poorly understood. Here, we characterised the proteomic profile of the annelid Neoleanira tetragona, a species widespread in the North Atlantic Ocean, across the permanent deoxygenation gradient of the Estuary and Gulf of St. Lawrence (EGSL). Specifically, whole specimens were collected from four regions of the EGSL deoxygenation gradient and were analysed using high-resolution LC-MS/MS with a shotgun proteomics approach. Region pairwise comparisons through linear models (LIMMA) showed no differentially abundant proteins, but generalised linear latent variables models identified 59 proteins with differential abundance linked to environmental oxygen and/or food availability. An overrepresentation of tricarboxylic acid cycle via citrate synthase activity was supported in response to low oxygen and high food availability for the annelid. Our results suggest that N. tetragona possesses a compensatory mechanism to cope with the in situ persistent deoxygenation, which involves the accumulation of key proteins that are responsible for maintaining steady energy metabolism under in situ persistent deoxygenation. Our findings contribute to shed light on physiological strategies that benthic marine invertebrates can employ to cope with ongoing and future environmental challenges.

The Bedford Basin is a a 70 m deep, seasonally stratified and hypoxic semi-enclosed fjord on the West Atlantic coast (Nova Scotia, Canada). The basin is connected to the Atlantic Ocean (Scotian Shelf) via a narrow 20 m deep sill that restricts exchange and mixing of surface and bottom waters. Bedford Basin is located in an urban setting (Halifax) and receives considerable wastewater input. The Basin has benefitted from a weekly multidisiciplinary time-series of physical, chemical and microbiological data over several decades. Over the past decade, the intensity of hypoxia has increased due to warmer winters and reduced convective renewal of the deeper water. The presentation will highlight understanding of physical-microbiological-chemical interactions, and sediment-water exchanges, that affect the concentration and speciation of nitrogen and iodine species in relation to variable levels of oxygen. A focus will be on interannual and short-term variability in production of nitrous oxide and iodide in relation to variations in oxygen, microbial diversity and sediment-water exchange. The potential of coastal basins to act as living laboratories for studying complex, redox-dependent processes through comprehensive, multidisciplinary, time-series study will be demonstrated. The closely related potential of urban fjords to act as testbeds for evaluation of emerging approaches to the mitigation of coastal hypoxia will be emphasized.

Bioavailable nitrogen governs ocean productivity and carbon fixation by regulating phytoplankton growth and community composition. Nitrogen input primarily results from fixation, while denitrification and ANAMMOX removes bioavailable nitrogen in oxygen-depleted conditions. Traditionally considered limited to highly suboxic (i.e., < 5 µM) waters, recent studies suggest fixed-nitrogen removal processes may extend beyond, elevating global nitrogen loss estimates. This study directly quantifies fixed-nitrogen loss across oxygen gradients (from 140 to 35 µM) along the Estuary and Gulf of St. Lawrence using N cycle tracers (, and ). Notably, we observe significant production when concentrations fall below 57-52 µM, including unexpected water column fixed-nitrogen removal processes above suboxia. Benthic production remains unaffected under intensifying deoxygenation from 50 down to 34 µM, but sedimentary nitrification contribution to denitrification diminishes with intensifying deoxygenation. Combined, water column and benthic fixed-nitrogen removal processes drive anomalies and strong deficiency in bottom waters. Additionally, observed concentration threshold triggers production, unveiling the profound impact of ocean deoxygenation on nitrogen cycling, challenging conventional expectations even at hypoxic concentrations.

The O2 content of the global ocean has been declining progressively over the past decades, mainly because of human activities and global warming. Nevertheless, how long-term deoxygenation affects macrobenthic communities, sediment biogeochemistry and their mutual feedback remains poorly understood. Here, we evaluate the response of the benthic assemblages and biogeochemical functioning to decreasing O2 concentrations along the persistent bottom-water dissolved O2 gradient of the Estuary and Gulf of St. Lawrence (QC, Canada). We report several of non-linear biodiversity and functional responses to decreasing O2 concentrations, and identify an O2 threshold that occurs at approximately at 63 μM. Below this threshold, macrobenthic community assemblages change, and bioturbation rates drastically decrease to near zero. Consequently, the sequence of electron acceptors used to metabolize the sedimentary organic matter is squeezed towards the sediment surface while reduced compounds accumulate closer (as much as 0.5-2.5 cm depending on the compound) to the sediment-water interface. Our results illustrate the capacity of bioturbating species to compensate for the biogeochemical consequences of hypoxia and can help to predict future changes in benthic ecosystems.

Environmental stressors have profound implications for species, communities, and ecosystems by altering fundamental processes. With increasing human impacts on aquatic ecosystems, two main scenarios have been reported: (1) the spatiotemporal superposition of multiple stressors, leading to interactions among them and (2) intensifying environmental gradients, leading to threshold responses. However, studies designed to assess the effects of multiple stressors (e.g., pulse stressors) along environmental gradients (e.g., press stressors) are uncommon, and interactions between pulse and press stressors may cause abrupt changes in biological responses. We conducted a laboratory experiment to investigate the effects of osmotic stress along a nutrient enrichment gradient on a freshwater community composed of periphyton, microorganisms, and zebra mussels (Dreissena polymorpha). Our objectives were to (1) quantify the individual and combined effects of stressors, (2) delineate thresholds along the nutrient gradient (press) in the absence and presence of osmotic stress (pulse), and (3) test for interactions between the two stressors. We evaluated effects on metabolic rates in D. polymorpha and on microbial activity, as well as phototrophic periphyton biomass and physiological status. We observed interactions between the two stressors for metabolic rates in D. polymorpha and periphytic phaeopigments. In contrast, we found an individual effect of osmotic stress on microbial activity and chlorophyll a content. Thresholds were only identified in the presence of osmotic stress for metabolic rates in D. polymorpha. Our work highlights the importance of combining multiple stressors with environmental gradients and the need to consider multiple biological compartments when evaluating the impacts of stressors on ecosystems.

Anthropogenic impacts and global changes have profound implications for natural ecosystems and may lead to their modification, degradation or collapse. Increases in the intensity of single stressors may create abrupt shifts in biotic responses (i.e. thresholds). The effects of multiple interacting stressors may create non-additive responses, known as synergistic or antagonistic interactions. Here we combine both concepts—ecological thresholds and interactions between multiple stressors—to understand the effects of multiple interacting stressors along environmental gradients, and how this can affect the occurrence of thresholds. Using an experimental approach to investigate the effect of nutrient enrichment and saltwater intrusion on mortality in the zebra mussel, Dreissena polymorpha, we show that multiple stressors can create thresholds at lower levels of an environmental gradient. Our results reveal a major shortcoming in how we currently investigate these two ecological concepts, as considering them separately may be causing underestimation of thresholds and stressor-interaction impacts.

As land use intensifies, many coastal waters are becoming enriched with otherwise limiting nutrients, leading to eutrophication. While the extreme effects of eutrophication on benthic communities are well documented, there is still a lack of knowledge about how nutrient enrichment alters biogeochemical interactions occurring at the sediment-water interface. Using ex-situ experiments, this study explores the consequences of nutrient enrichment on sediment characteristics, macrofauna community and benthic fluxes. The quantity of sedimentary organic matter and porewater concentration of NH4+, NOx and PO(4)(3-)increased in enriched treatments. These changes did not affect the macrobenthic community structure. However, macroinfauna buried less deep and increased their ventilation activity. As consequences, nutrient efflux increased, thereby favouring eutrophication processes. These effects were reduced in presence of seagrass, thus illustrating the buffering capacity of seagrass in the context of environmental changes, and particularly, of eutrophication. Overall, this study highlights that the functional consequences of nutrient enrichment involve interconnected processes that are variable in space and time.

Natural ecosystems are experiencing unprecedented rates of change due to anthropogenic activities and global change, leading to either gradual changes in a given response or tipping points. While the tipping point concept has been tested in an array of habitats since the 1960s, the spatiotemporal superposition of multiple drivers in different ecosystems needs to be considered when investigating the response of species, communities, populations, and ecosystems along environmental gradients. Here, we (1) develop a historical and current perspective of tipping point studies in terrestrial, freshwater, and marine ecological systems; (2) portray the research effort in different freshwater and marine habitats; and (3) explore the results of experimental studies focusing on tipping points measured at the individual, communities, ecosystem level, as well as ecosystem functions and services in a context of single and multiple stressors. The number of studies mentioning the concept of tipping points increases every year, but very few studies have specific objective to identify them. Even fewer studies consider how the addition of another stressor into an ecosystem may alter a tipping point. In addition, many studies investigated multiple responses, but only one-fourth (7 out of 28) of them concentrate their effort on multiple biological or ecological levels of complexity. This review allowed us to identify shortcomings in this research field and propose ways to make this ecological concept anew.

Chronic hypoxia and anoxia have strong impacts on coastal ecosystems worldwide. In shallow coastal ecosystems, such situations are essentially driven by high benthic oxygen (O2) demand resulting from organic matter mineralization in surface sediment and amplified by a low mixing of the water column. However, the benthic O2 demand may greatly vary according to the O2 availability, sediment biogeochemical properties, and bioturbation by macrobenthic fauna. Here we examined how the sediment O2 demand varies in response to seasonal and long-lasting (pluri-decadal) hypoxia in the Berre lagoon, a coastal ecosystem impacted by chronic hypoxia events since 60 years. Oxygen penetration depth, diffusive and total O2 fluxes were measured in situ using a microelectrode autonomous profiler and benthic chamber deployments at three sites impacted by quite-permanent (PA), seasonal (PI) and occasional (PO) hypoxia in August 2016. They were seasonally repeated at site PI between August 2015 and August 2016. Additional physical and chemical characteristics were also measured in surface sediment. Sediment profile images and characteristics of benthic macrofauna communities enabled to estimate the quality of the benthic ecosystem. The highest benthic O2 demand was observed after seasonal anoxia in relation to the important accumulation of reduced chemical species in surface sediment. Interestingly, both pluri-decadal hypoxia and normoxia produced relatively high benthic O2 demand related to a higher accumulation of organic matter and to the presence of reduced chemical species at site dominated by hypoxia, and to the presence of fresher organic matter and active bioturbating macrofaunal communities in normoxic site. The low benthic O2 demand at site seasonally impacted by hypoxia likely resulted from the degraded state of the macrofaunal community and from the lower accumulation of reduced chemical species. The occurrence of hypoxia and anoxia situations in the Berre lagoon was predicted from the competition between kinetics of benthic O2 demand and water column reoxygenation events induced by strong wind. The good agreement between the measured and predicted hypoxia/anoxia occurrence clearly indicates that the chronic deoxygenation events in the Berre lagoon, and the resulting degraded ecological state of the benthic ecosystem are driven both by the benthic O2 demand and by the intensity and duration of the water column stratification.

Although parasites represent a substantial part of marine communities' biomass and diversity, their influence on ecosystem functioning, especially via the modification of host behaviour, remains largely unknown. Here, we explored the effects of the bopyrid ectoparasite Gyge branchialis on the engineering activities of the thalassinid crustacean Upogebia pusilla and the cascading effects on intertidal ecosystem processes (e.g. sediment bioturbation) and functions (e.g. nutrient regeneration). Laboratory experiments revealed that the overall activity level of parasitized mud shrimp is reduced by a factor 3.3 due to a decrease in time allocated to burrowing and ventilating activities (by factors 1.9 and 2.9, respectively). Decrease in activity level led to strong reductions of bioturbation rates and biogeochemical fluxes at the sediment-water interface. Given the world-wide distribution of mud shrimp and their key role in biogeochemical processes, parasite-mediated alteration of their engineering behaviour has undoubtedly broad ecological impacts on marine coastal systems functioning. Our results illustrate further the need to consider host-parasite interactions (including trait-mediated indirect effects) when assessing the contribution of species to ecosystem properties, functions and services.

Sea ice plays an important role in subpolar seagrass meadows. It protects meadows against wave action and extreme temperatures. On the other hand, sea ice destroys seagrass leaves and removes plots of sediments and organics debris, leaving long-lasting ice-made tidal pools of various shapes and sizes within the meadow. The present study aimed at investigating the effect of sea ice on benthic community structure and biogeochemical processes in a subpolar seagrass meadow. Vegetated areas (V), artificially-created (aTP), and natural (nTP) tidal pools were sampled from April to October 2018 in a seagrass meadow located at Manicouagan Peninsula (Québec; 49°5′36″N, 68°12′44″W). aTP and nTP showed similar sediment characteristics with coarser sediment and lower particulate organic carbon and total nitrogen content but also lower NOx and higher NH4+ and PO43− porewater concentrations as compared to V. Benthic macrofauna communities showed a strong seasonality with very reduced total density, biomass and species richness during wintertime (from December to April) relatively to summertime (from June to September). Benthic macrofauna communities were also more diversified and abundant in V than in aTP and nTP. Species assemblages in aTP and nTP represented a subset of species assemblages in V with any species found exclusively in tidal pools. However, total biomass was similar among treatments, suggesting that tidal pools sheltered larger individuals than vegetated areas. These results underline the importance of considering the spatial heterogeneity of seagrass meadows when assessing the functioning of these ecosystems.

Seagrass meadows are among the most productive ecosystems in the world: they store a large amount of carbon and host highly diverse macrobenthic communities. They also play a key role in biogeochemistry at the sediment-water interface. The light requirements of seagrasses limit their development to shallow coastal areas where they are facing various natural and anthropogenic disturbances, which has induced a global loss of these ecosystems over the last decades. Nutrient enrichment of coastal waters, resulting from anthropogenic activities is one of the leading causes of this decline. Subpolar seagrass meadows present a strong seasonal dynamic, with a long winter when seagrasses rely on carbon reserves that they build up during the short growing season (limited to two to three months during summer time). Hence, it has been hypothesized that the effects of nutrient enrichment on seagrass ecosystem functioning depend on seasonal dynamics. In this study, we performed a series of mesocosm experiments over a month period to investigate the effects of the timing, duration and intensity of disturbance on macrofauna bioturbation, oxygen and nutrients porewater concentration profiles and benthic fluxes using three levels (including control) of realistic nutrient enrichments at the beginning (June) and at the end (August) of the growing season. In May, effects of intermediate level of nutrient enrichment were only visible on total oxygen uptake by the sediment at day 30 of disturbance while it affected oxygen and nutrients benthic fluxes at day 15 in August. The highest level of nutrient enrichment affected oxygen and nutrients benthic fluxes in May and August. Overall, our results highlight the importance of considering the time (period and duration) in the assessment of the functional consequences of disturbances.

The iron isotope composition of sedimentary deposits is a key tool for tracking changes in the biogeochemical and redox conditions of modern and geologically ancient aquatic systems. The use of iron isotopes to reconstruct oxic, anoxic and redox conditions is based on iron isotope fractionation associated with the iron(II)-iron(III) redox reaction and the formation of various iron oxy, hydroxy and sulfur species. However, the degree of iron isotope fractionation varies within the sedimentary record, and the processes leading to isotope fractionation within aquatic systems also vary. Here we investigate iron isotope fractionation within the water column of Lake Cadagno, Switzerland. Lake Cadagno is a 21 m deep alpine meromictic lake that is permanently stratified. It has two chemically distinct water layers: an oxic mixolimnion separated by a narrow chemocline from an anoxic monimolimnion The chemocline is located across a narrow band between 10 and 13.5 m and is defined by steep chemical gradients in dissolved oxygen, redox potential, nutrients (nitrite + nitrate, ammonia), dissolved and particulate trace metals (iron and manganese) and sulfur species. Iron isotope determination (Fe-56/Fe-54 ratio; expressed as delta Fe-56) of both dissolved (delta Fe-56(dissolved)) and particulate (delta Fe-56(particulate)) iron reveals a sharp transition within the chemocline with a heavy delta Fe-56(disssolved) value (+0.75 parts per thousand) at 11.5 m and light delta Fe-56(dissolved) value (-0.61 parts per thousand) below at 12 m. The large shift in delta Fe-56(dissolved) occurs where anoxygenic phototrophic bacteria become abundant. Modelling of the dissolved iron isotope fractionation within the chemocline produced fractionation factors of -0.60 parts per thousand (kinetic model) and -1.52 parts per thousand (equilibrium model) assuming a two-step process involving iron oxidation followed by precipitation. Changes in the isotope composition of particulate iron with depth are subtler to that of dissolved iron, with slightly lighter values (-0.18 parts per thousand) above the chemocline and slightly heavier values (+0.13 parts per thousand) below. The production of reduced iron(II) within and below the chemocline is likely coupled to dissimilatory iron(III) reduction and dissolution reactions associated with sinking iron(III) oxy and hydroxy species. These processes lead to a peak in dissolved iron concentrations at 12.5 m. The decline in dissolved iron concentration below this depth and its change in isotope composition is consistent with the formation of iron sulphide species, e.g. mackinawite, under mildly euxinic conditions. Overall our results indicate that variations in delta Fe-56(dissolved) likely involve a combination of biotic and abiotic processes with biological mixing enhancing the rate at which iron(II) is transferred across the chemocline. The observed isotope transformation of dissolved iron across the chemocline of Lake Cadagno may make it a reasonable analogue to past shallow-water systems with mild euxinia. (C) 2019 Elsevier Ltd. All rights reserved.