David Didier

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 its removal 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 outside the breakwater system). Landward sites exhibited finer sediment with higher organic carbon content compared to other sites and supported distinct macrobenthic communities dominated by opportunistic Oligochaetes, while reference sites were dominated by Molluscs and Crustaceans. The intermediate seaward site consistently resembled the unvegetated reference site across all variables measured at both sites, including sediment characteristics, oxygen dynamics and benthic fluxes, thereby showing reference-like conditions. Surprisingly, despite these substantial physical and biological changes, dark total benthic oxygen uptake, nitrate and ammonium benthic fluxes showed no consistent separation among impacted, intermediate, and reference sites, while benthic denitrification rates were comparable among the impacted and reference sites where they were measured. This functional similarity reflects an apparent functional convergence arising from distinct combinations of abiotic and biotic factors, including root-mediated rhizosphere oxygenation, macrofauna-mediated sediment irrigation, and contrasting reliance on internally produced versus water-column nitrate. However, this convergence might not apply to all ecosystem services offered by natural salt marshes and may depend on specific biological communities, emphasizing the importance of careful consideration in coastal management decisions.

Arctic fjords are experiencing rapid environmental change driven by amplified high‐latitude warming, with implications for submarine slope stability. This study investigates the distribution, timing and triggering mechanisms of submarine landslides in Grise Fiord (High Arctic) to evaluate the role of past climate variability in sediment preconditioning and slope failure. High‐resolution multibeam bathymetry and sub‐bottom profiler data were combined with a multiproxy analysis of sediment cores, including physical properties, dinoflagellate cyst assemblages and geochemistry. Bathymetric mapping reveals more than 100 submarine landslides throughout the fjord, with morphologies indicating triggering by deltaic progradation, iceberg groundings and the downslope extension of subaerial debris flows. Core stratigraphy and acoustic backscatter data suggest that these slope failures are not modern, having occurred more than 1000 years ago. Available chronologies on three of the submarine landslides suggest that they were initiated during transitions from colder to warmer climatic conditions, pointing to a climatic control on slope instability, potentially linked to reduced sea‐ice cover and increased meltwater and sediment delivery. These findings contrast with lower latitude fjords on southern Baffin Island, where modern submarine landslides are already occurring, highlighting a spatial lag in geomorphic response to Arctic warming. Grise Fiord may therefore represent a fjord system in a preinstability state, with modern slope failures likely to emerge once climatic thresholds similar to those in southern Baffin Island are reached. Continued warming underscores the need for monitoring, particularly given implications for future coastal and port infrastructure in Ausuittuq.

This study is the first of a two-part paper that summarizes the development of a prototype coastal hazard prediction system providing short-term (+48 h) forecasts of the total water level (TWL) at 50 m resolution for the province of Quebec, Eastern Canada. In this first part, the implementation of the offshore wave model component of the system, which is a regional 1 km-resolution WAVEWATCH III™ (WW3) configuration for the Estuary and Gulf of St. Lawrence (EGSL), is presented and discussed. The configuration is forced by high resolution atmosphere, ocean and sea ice forecasts provided by Environment and Climate Change Canada (ECCC) and includes a state-of-the-art parameterization of wave propagation and attenuation in sea ice that has been tuned with observations from the EGSL. Performances are assessed against wave data collected over a 2-year period during which the forecasting system was running operationally, and against historical storm data using a model hindcast. Results demonstrate reasonable forecast skills both for normal and extreme wave conditions during ice-free periods with errors ranging from 15 % to 31 % of the mean wave height. However, when sea ice is present, performances are drastically reduced, primarily due to inaccuracies in the predicted ice fields at spatial scales over which wave energy typically dissipates in sea ice.

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.

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.

This study is the first of a two-part paper that summarizes the development of a prototype coastal hazard prediction system providing short-term (+48 h) forecasts of the total water level (TWL) at 50 m resolution for the province of Quebec, Eastern Canada. In this first part, the implementation of the offshore wave model component of the system, which is a regional 1 km-resolution WAVEWATCH III™(WW3) configuration for the Estuary and Gulf of St. Lawrence (EGSL), is presented and discussed. The configuration is forced by high resolution atmosphere, ocean and sea ice forecasts provided by Environment and Climate Change Canada (ECCC) and includes a state-of-the-art parameterization of wave propagation and attenuation in sea ice that has been tuned with observations from the EGSL. Performances are assessed against wave data collected over a two-year period during which the forecasting system was running operationally, and against historical storm data using a model hindcast. Results demonstrate reasonable forecast skills both for normal and extreme wave conditions during ice-free periods with errors ranging from 15 % to 31 % of the mean wave height. However, when sea ice is present, performances are drastically reduced, primarily due to inaccuracies in the predicted ice fields at spatial scales over which wave energy typically dissipates in sea ice.

Coastal hazards affect many Arctic communities by threatening the integrity of low-lying infrastructures, yet coastal flooding remains poorly constrained at high latitudes because local observations of waves and water levels are scarce. Here, we map coastal flood-prone areas in the community of Ausuittuq (Grise Fiord, Nunavut), Canada’s northernmost community located in Jones Sound, a semi-enclosed basin. The study uses a community-based approach that combines coastal topographic surveys, hydrodynamic observations, wave runup measurements, and historical hindcasts. A site-specific empirical wave runup equation is developed from beach slope and offshore wave conditions, and total water levels (TWL) are reconstructed from wave and water-level hindcasts (1979-2017). Extreme TWL events are identified from annual maxima, and flood scenarios are mapped for five return periods under present-day conditions and two 2100 relative sea-level scenarios adapted to the region. Results show that extreme TWLs in Ausuittuq are controlled primarily by tides and wave runup rather than storm surge, and that numerous infrastructures are exposed to coastal flooding under all scenarios. Under present-day conditions, the 10-year event floods 40,159m ² of land, exposing multiple homes to flooding. Under the median RCP~8.5 scenario, projected relative sea-level fall due to postglacial isostatic uplift reduces flood extent, but does not eliminate hazard, whereas the enhanced RCP~8.5 scenario increases exposure, reaching 45,400m ² of flooded area, 1282~m of affected road, and 15 exposed homes for the 10-year event. These results show that even in a fetch-limited High Arctic environment, wave runup can generate significant coastal flood exposure and should be integrated into community-scale hazard assessments.

This study is the first of a two-part paper that summarizes the development of a prototype coastal hazard prediction system providing short-term (+48 h) forecasts of the total water level (TWL) at 50 m resolution for the province of Quebec, Eastern Canada. In this first part, the implementation of the offshore wave model component of the system, which is a regional 1 km-resolution WAVEWATCH III™(WW3) configuration for the Estuary and Gulf of St. Lawrence (EGSL), is presented and discussed. The configuration is forced by high resolution atmosphere, ocean and sea ice forecasts provided by Environment and Climate Change Canada (ECCC) and includes a state-of-the-art parameterization of wave propagation and attenuation in sea ice that has been tuned with observations from the EGSL. Performances are assessed against wave data collected over a two-year period during which the forecasting system was running operationally, and against historical storm data using a model hindcast. Results demonstrate reasonable forecast skills both for normal and extreme wave conditions during ice-free periods with errors ranging from 15 % to 31 % of the mean wave height. However, when sea ice is present, performances are drastically reduced, primarily due to inaccuracies in the predicted ice fields at spatial scales over which wave energy typically dissipates in sea ice.

The Canadian Arctic Archipelago (CAA) is warming at an unprecedented rate, leading to sea ice loss and glacial retreat. Marine-terminating (tidewater) glaciers can fuel summertime marine productivity by delivering nutrient-rich deep waters via upwelling to the surface ocean. While the impact of glacier-induced upwelling has been well-studied in the context of phytoplankton and primary productivity, its effects on broader marine microbial communities remain poorly understood. We investigated how glacier-driven upwelling shapes marine microbial (bacterial and archaeal) communities across a series of sites in the CAA. At upwelling sites, the upper 50 m of the water column exhibited elevated nutrient concentrations and physical characteristics that resembled deeper waters, which were associated with differences in microbial community composition relative to non-upwelling sites. Our results indicate that upwelling influences microbial communities in surface waters in two ways. It directly introduces typically deeper-water-associated taxa into surface waters and reshapes ecological niches by enhancing nutrient supply and stimulating primary production, indirectly driving changes in microbial communities. The enrichment of Candidatus Nitrosopumilus , a deep water nitrifier, likely affects nitrogen cycling and raises the possibility of active nitrification in surface waters. Likewise, the increased abundance of taxa known to be associated with phytoplankton-derived organic matter in upwelling regions suggests an enhanced capacity to process organic matter generated from elevated primary productivity. Ultimately, as tidewater glaciers continue to retreat, the resulting changes in the glacially-driven upwelling regime will likely shift marine microbial communities towards assemblages adapted to less productive ecosystems, with implications for nutrient cycling in these systems. Importance: Climate change has a disproportionate impact on the Arctic, with rising temperatures causing increased marine-terminating glacier retreat and changes in the marine water column structure. The consequent loss of the ability of these glaciers to upwell deep water to the surface ocean results in a reduction of nutrient delivery and mixing in these ecosystems. Previous work has highlighted the importance of marine-terminating glaciers in sustaining phytoplankton productivity during the summer season through this delivery of deep-water nutrients to the surface ocean. The impact of glacially-induced upwelling on marine bacterial and archaeal communities, however, remains underexplored. We found that in regions with glacially-driven upwelling, the surface ocean showed enrichment of phytoplankton-associated taxa and nitrifiers commonly associated with deep waters. This work underscores the role of glacially-driven upwelling in structuring both microbial communities and nutrient cycling, suggesting that glacier loss could reshape community composition and biogeochemical processes in a rapidly changing Arctic.