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In the subarctic region, climate warming and permafrost thaw are leading to emergence of ponds and to an increase in mobility of catchment carbon. As carbon of terrestrial origin is increasing in subarctic freshwaters the resource pool supporting their microbial communities and metabolism is changing, with consequences to overall aquatic productivity. By sampling different subarctic water bodies for a one complete year we show how terrestrial and algal carbon compounds vary in a range of freshwaters and how differential organic carbon quality is linked to bacterial metabolism and community composition. We show that terrestrial drainage and associated nutrients supported higher bacterial growth in ponds and river mouths that were influenced by fresh terrestrial carbon than in large lakes with carbon from algal production. Bacterial diversity, however, was lower at sites influenced by terrestrial carbon inputs. Bacterial community composition was highly variable among different water bodies and especially influenced by concentrations of dissolved organic carbon (DOC), fulvic acids, proteins and nutrients. Furthermore, a distinct preference was found for terrestrial vs. algal carbon among certain bacterial tribes. The results highlight the contribution of the numerous ponds to cycling of terrestrial carbon in the changing subarctic and arctic regions.

In a context of reduced global biodiversity, the potential impacts from the loss of habitat-forming species (HFS) on ecosystem structure and functioning must be established. These species are often the main community primary producers and have a major role in the establishment of organisms through facilitation processes. This study focuses on macroalgae and mussels as HFS within an intertidal zone along the St. Lawrence estuary (Quebec, Canada). Over a 16-week period, we manipulated the in situ diversity profile (richness, evenness, identity, and abundance) of the dominant HFS (Fucus distichus edentatus, F. vesiculosus, and Mytilus spp.) in order to define their role in both the establishment of associated species and community primary production. Contrary to expectation, no general change in HFS richness, evenness, abundance, or identity on associated species community establishment was observed. However, over the study period, the HFS diversity profile modified the structure within the trophic guilds, which may potentially affect further community functions. Also, our results showed that the low abundance of HFS had a negative impact on the primary productivity of the community. Our results suggest that HFS diversity profiles have a limited short-term role in our study habitat and may indicate that biological forcing in these intertidal communities is less important than environmental conditions. As such, there was an opportunistic establishment of species that ensured rapid colonization regardless of the absence, or the diversity profile, of facilitators such as HFS.

Intertidal vegetation provides important ecological functions, such as food and shelter for wildlife and ecological services with increased coastline protection from erosion. In cold temperate and subarctic environments, the short growing season has a significant impact on the phenological response of the different vegetation types, which must be considered for their mapping using satellite remote sensing technologies. This study focuses on the effect of the phenology of vegetation in the intertidal ecosystems on remote sensing outputs. The studied sites were dominated by eelgrass (Zostera marina L.), saltmarsh cordgrass (Spartina alterniflora), creeping saltbush (Atriplex prostrata), macroalgae (Ascophyllum nodosum, and Fucus vesiculosus) attached to scattered boulders. In situ data were collected on ten occasions from May through October 2019 and included biophysical properties (e.g., leaf area index) and hyperspectral reflectance spectra (Rrs(λ)). The results indicate that even when substantial vegetation growth is observed, the variation in Rrs(λ) is not significant at the beginning of the growing season, limiting the spectral separability using multispectral imagery. The spectral separability between vegetation types was maximum at the beginning of the season (early June) when the vegetation had not reached its maximum growth. Seasonal time series of the normalized difference vegetation index (NDVI) values were derived from multispectral sensors (Sentinel-2 multispectral instrument (MSI) and PlanetScope) and were validated using in situ-derived NDVI. The results indicate that the phenology of intertidal vegetation can be monitored by satellite if the number of observations obtained at a low tide is sufficient, which helps to discriminate plant species and, therefore, the mapping of vegetation. The optimal period for vegetation mapping was September for the study area.

Water quality deterioration is expected to worsen the light conditions in shallow coastal waters with increasing human activities. Temperate seagrasses are known to tolerate a highly fluctuating light environment. However, depending on their ability to adjust to some decline in light conditions, decreases in daily light quantity and quality could affect seagrass physiology, productivity, and, eventually, survival if the Minimum Quantum Requirements (MQR) are not reached. To better understand if, how, and to what extent photosynthetic adjustments contribute to light acclimation, eelgrass ( Zostera marina L.) shoots from the cold temperate St. Lawrence marine estuary (Rimouski, QC, Canada) were exposed to seven light intensity treatments (6, 36, 74, 133, 355, 503, and 860 μmol photons m –2 s –1 , 14:10 light:dark photoperiod). Photosynthetic capacity and efficiency were quantified after five and 25 days of light exposure by Pulse Amplitude Modulated (PAM) fluorometry to assess the rapid response of the photosynthetic apparatus and its acclimation potential. Photoacclimation was also studied through physiological responses of leaves and shoots (gross and net primary production, pigment content, and light absorption). Shoots showed proof of photosynthetic adjustments at irradiances below 200 μmol photons m –2 s –1 , which was identified as the threshold between limiting and saturating irradiances. Rapid Light Curves (RLC) and net primary production (NPP) rates revealed sustained maximal photosynthetic rates from the highest light treatments down to 74 μmol photons m –2 s –1 , while a compensation point (NPP = 0) of 13.7 μmol photons m –2 s –1 was identified. In addition, an important package effect was observed, since an almost three-fold increase in chlorophyll content in the lowest compared to the highest light treatment did not change the leaves’ light absorption. These results shed new light on photosynthetic and physiological processes, triggering light acclimation in cold temperate eelgrass. Our study documents an MQR value for eelgrass in the St. Lawrence estuary, which is highly pertinent in the context of conservation and restoration of eelgrass meadows.

Form–function relationships in plants underlie their ecosystem roles in supporting higher trophic levels through primary production, detrital pathways, and habitat provision. For widespread, phenotypically-variable plants, productivity may differ not only across abiotic conditions, but also from distinct morphological or demographic traits. A single foundation species, eelgrass Zostera marina, typically dominates north temperate seagrass meadows, which we studied across 14 sites spanning 32–61°N latitude and two ocean basins. Body size varied by nearly two orders of magnitude through this range, and was largest at mid-latitudes and in the Pacific Ocean. At the global scale, neither latitude, site-level environmental conditions, nor body size helped predict productivity (relative growth rate 1–2% day–1 at most sites), suggesting a remarkable capacity of Z. marina to achieve similar productivity in summer. Furthermore, among a suite of stressors applied within sites, only ambient leaf damage reduced productivity; grazer reduction and nutrient addition had no effect on eelgrass size or growth. Scale-dependence was evident in different allometric relationships within and across sites for productivity and for modules (leaf count) relative to size. Zostera marina provides a range of ecosystem functions related to both body size (habitat provision, water flow) and growth rates (food, carbon dynamics). Our observed decoupling of body size and maximum production suggests that geographic variation in these ecosystem functions may be independent, with a future need to resolve how local adaptation or plasticity of body size might actually enable more consistent peak productivity across disparate environmental conditions.

Benthic fauna refers to all fauna that live in or on the seafloor, which researchers typically divide into size classes meiobenthos (32/64 µm–0.5/1 mm), macrobenthos (250 µm–1 cm), and megabenthos (>1 cm). Benthic fauna play important roles in bioturbation activity, mineralization of organic matter, and in marine food webs. Evaluating their role in these ecosystem functions requires knowledge of their global distribution and biomass. We therefore established the BenBioDen database, the largest open-access database for marine benthic biomass and density data compiled so far. In total, it includes 11,792 georeferenced benthic biomass and 51,559 benthic density records from 384 and 600 studies, respectively. We selected all references following the procedure for systematic reviews and meta-analyses, and report biomass records as grams of wet mass, dry mass, or ash-free dry mass, or carbon per m 2 and as abundance records as individuals per m 2 . This database provides a point of reference for future studies on the distribution and biomass of benthic fauna. Measurement(s) marine benthic feature • organic material • mass density Technology Type(s) digital curation Factor Type(s) geographic location • water depth • organism size class Sample Characteristic - Organism meiobenthos • macrobenthos • megabenthos Sample Characteristic - Environment marine benthic biome • ocean Sample Characteristic - Location Atlantic Ocean • Pacific Ocean • Arctic Ocean • Southern Ocean • Indian Ocean Machine-accessible metadata file describing the reported data: https://doi.org/10.6084/m9.figshare.12481985

Aim We examined the relationships between bathymetry, latitude and energy and the diversity of marine benthic invertebrates across wide environmental ranges of Canada's three oceans. Location Canadian Pacific, Arctic and Atlantic Oceans from the intertidal zone to upper bathyal depths, encompassing 13 marine ecoregions. Methods We compiled 35 benthic datasets that encompass 3,337 taxa (70% identified to species and 21% to genus) from 13,172 samples spanning 6,117 sites. Partitioning the analyses by different gear types, ecoregions or sites, we used Hill numbers to examine spatial patterns in α‐diversity. We used resampling and extrapolation to standardized sampling effort and examined the effects of depth, latitude, chemical energy (export particulate organic carbon [POC] flux), thermal energy (bottom temperature) and seasonality of primary production on the benthic biodiversity. Results The Canadian Arctic harboured the highest benthic diversity (e.g. epifauna and common and dominant infauna species), whereas the lowest diversity was found in the Atlantic. The Puget Trough (Pacific), Beaufort Sea, Arctic Archipelago, Hudson Bay, Northern Labrador and Southern Grand Bank (Atlantic) were the “hotspots\" of diversity among the ecoregions. The infauna and epifauna both exhibited hump‐shaped diversity–depth relationships, with peak diversity near shelf breaks; latitude (positively) predicted infaunal diversity, albeit weakly. Food supply, as inferred from primary production and depth, was more important than thermal energy in controlling diversity patterns. Limitations with respect to calculating POC flux in coastal (e.g. terrestrial runoff) and ice‐covered regions or biological interactions may explain the negative POC flux–infaunal diversity relationship. Main Conclusions We show previously unreported diversity hotspots in the Canadian Arctic and in other ecoregions. Our analyses reveal potential controlling mechanisms of large‐scale benthic biodiversity patterns in Canada's three oceans, which are inconsistent with the prevailing view of seafloor energy–diversity relationships. These results provide insightful information for conservation that can help to implement further MPA networks.

Multi-trophic conservation and management strategies may be necessary if reciprocal linkages between primary producers and their consumers are strong. While herbivory on aquatic plants is well-studied, direct top-down control of seagrass populations has received comparatively little attention, particularly in temperate regions. Herein, we used qualitative and meta-analytic approaches to assess the scope and consequences of avian (primarily waterfowl) herbivory on temperate seagrasses of the genus . Meta-analyses revealed widespread evidence of spatio-temporal correlations between and waterfowl abundances as well as strong top-down effects of grazing on . We also documented the identity and diversity of avian species reported to consume and qualitatively assessed their potential to exert top-down control. Our results demonstrate that and their avian herbivores are ecologically linked and we suggest that bird herbivory may influence the spatial structure, composition, and functioning of the seagrass ecosystem. Therefore, the consequences of avian herbivory should be considered in the management of seagrass populations. Of particular concern are instances of seagrass overgrazing by waterfowl which result in long-term reductions in seagrass biomass or coverage, with subsequent impacts on local populations of waterfowl and other seagrass-affiliated species. While our results showed that bird density and type may affect the magnitude of the top-down effects of avian herbivory, empirical research on the strength, context-dependency, and indirect effects of waterfowl- interactions remains limited. For example, increased efforts that explicitly measure the effects of different functional groups of birds on seagrass abundance and/or document how climate change-driven shifts in waterfowl migratory patterns impact seagrass phenology and population structure will advance research programs for both ecologists and managers concerned with the joint conservation of both seagrasses and their avian herbivores.

Ecosystems are under pressure from multiple human disturbances whose impact may vary depending on environmental context. We experimentally evaluated variation in the separate and combined effects of the loss of a key functional group (canopy algae) and physical disturbance on rocky shore ecosystems at nine locations across Europe. Multivariate community structure was initially affected (during the first three to six months) at six locations but after 18 months, effects were apparent at only three. Loss of canopy caused increases in cover of non-canopy algae in the three locations in southern Europe and decreases in some northern locations. Measures of ecosystem functioning (community respiration, gross primary productivity, net primary productivity) were affected by loss of canopy at five of the six locations for which data were available. Short-term effects on community respiration were widespread, but effects were rare after 18 months. Functional changes corresponded with changes in community structure and/or species richness at most locations and times sampled, but no single aspect of biodiversity was an effective predictor of longer-term functional changes. Most ecosystems studied were able to compensate in functional terms for impacts caused by indiscriminate physical disturbance. The only consistent effect of disturbance was to increase cover of non-canopy species. Loss of canopy algae temporarily reduced community resistance to disturbance at only two locations and at two locations actually increased resistance. Resistance to disturbance-induced changes in gross primary productivity was reduced by loss of canopy algae at four locations. Location-specific variation in the effects of the same stressors argues for flexible frameworks for the management of marine environments. These results also highlight the need to analyse how species loss and other stressors combine and interact in different environmental contexts.

High-latitude lakes are usually transparent, due to their low productivity and low concentration of dissolved organic matter (DOM), but large variations in lake optical properties can be found within and between regions. We investigated the light regimes in relation to DOM in 18 oligotrophic, high-latitude lakes across mountain birch woodland, shrub tundra and barren tundra in north-west Finnish Lapland. In 12 lakes >1% of photosynthetically active radiation (PAR) reached the lake bottom, while 1% UV-B depth ranged from 0.1 to >12 m. Lakes located in barren tundra had highest transparency, lowest dissolved organic carbon (DOC) concentration and lowest DOM absorption (a440) (mean values: Kd PAR 0.3m-1, DOC 2.1mg l-1, a440 0.4m-1), while lakes in shrub tundra and mountain birch forest were less transparent (DOC 4.7 mg l-1, a440 1.4 m-1). Solar attenuation and lake transparency was best explained by a440. Our survey emphasizes the importance of catchment type on DOM characteristics and lake optics. We predict that even small changes in DOM quality may largely change the UV radiation exposure of lakes while changes in PAR may have smaller biological effects in these shallow lakes that are already illuminated to the bottom.