Small-scale variations in mussel (Mytilus spp.) dynamics and local production

A mussel bed was sampled monthly at four intertidal levels (mid: 2.15; mid-low: 1.65; low: 1.2 and sublittoral fringe: 0.7 m from chart datum) from July 1979 to July 1980 at Pointe-Mitis in the St. Lawrence estuary. A strong spring reduction of abundance (both in density and biomass) suggested that the mussel bed was being degraded. Community perturbation was attributed to ice scour. Partial reestablishment of the mussel bed (all age classes) was observed during late spring and early summer and occurred mainly at the mid-low intertidal level. Changes in the size structure of the mussel bed with level suggest that the annual windstorm regime may be an important factor in the dynamics of the bed. Mean body mass decreased at the three lower shore levels but increased at the highest shore level. Overall, net secondary production (assessed by the increment-summation method) was negative due to the decrease in mean body mass. Annual production rates (kJ m−2 y−1) from the mid intertidal level to the sublittoral fringe were 1130, − 4072, − 4013 and − 3258, respectively, while P/B ratios (y−1) were 0.17, − 0.69, − 0.50 and − 0.45. The calculated production and the productivity (potential production) are compared and used to provide insight into the condition of the mussel bed

Modeling the depuration potential of blue mussels (Mytilus spp.) in response to thermal shock

When contaminated, most molluscs destined for human consumption require a depuration regime lasting 48 h at a minimum temperature of 5 °C to ensure elimination of coliforms. However, this regime is unsatisfactory in northern temperate regions, where temperatures are frequently below 5 °C. A series of tests were undertaken to determine the filtration activity of mussels (Mytilus spp.) under cold temperatures. We used physiological measures rather than the more traditional method of bacterial analysis to evaluate mussel acclimation to cold temperatures. Mussels were acclimated for 4 weeks at three different temperatures (8, 4, and − 1 °C) and their scope for growth was evaluated each week to determine the level of acclimation. Mussels were then exposed to a thermal shock and clearance rates were measured after 2 h and 72 h. We observed a clearance rate of 2.45 l h− 1 g− 1 (g DW) for the 8 °C control group. Thus, within a 48-h depuration period, Mytilus spp. could filter a standard volume of 117.47 l. We used a von Bertalanffy exponential model to estimate the time required for an individual from each thermal shock treatment to filter that standard volume. We found that thermal shock had an important effect on the volume filtered by a mussel in 48 h. For example, mussels acclimated at 8 °C were able to filter the standard volume of 117.47 l in an average of 75 h at 4 °C, whereas those acclimated at 4 °C and transferred to 8 °C required only 23 h on average

Spatial organisation of fish communities in the St. Lawrence River: a test for longitudinal gradients and spatial heterogeneities in a large river system

Typified by heterogeneous habitats, large rivers host diversified communities throughout their course. As the spatial organisation of fish communities within these ecosystems remains little studied, longitudinal gradients and spatial heterogeneities of fish diversity were analysed in the large temperate St. Lawrence River, Canada. We used two distinct datasets obtained from either seine nets or gillnets from governmental standardised fish surveys (1995–2012) consisting of a total of 299,662 individuals from 76 fish species captured in 1,051 sites. Results from diversity indices and multivariate analysis revealed a gradual downstream increase in taxonomic diversity, and a gradual change of the community structure along the river. In addition, we observed different fish communities within fluvial lakes and corridors and found significant differences in fish community structure between opposite shores. The fish communities described along the river using seine nets are spatially more heterogeneous than when described using gillnets. This discrepancy is likely resulting both from the more mobile species targeted by gillnets and sampling sites located farther from the shallower shoreline habitat targeted by seine nets. The organisation of fish communities stresses the need to implement science-based policies and actions to preserve biodiversity and restore communities distributed over large heterogeneous ecosystems

The relation between productivity and species diversity in temperate-arctic marine ecosystems

Energy variables, such as evapotranspiration, temperature, and productivity explain significant variation in the diversity of many groups of terrestrial plants and animals at local to global scales. Although the ocean represents the largest continuous habitat on earth with a vast spectrum of primary productivity and species richness, little is known about how productivity influences species diversity in marine systems. To search for general relationships between productivity and species richness in the ocean, we analyzed data from three different benthic marine ecosystems (epifaunal communities on subtidal rock walls, on navigation buoys in the Gulf of St. Lawrence, and Canadian Arctic macrobenthos) across local to continental spatial scales (1000 km) using a standardized proxy for productivity, satellite-derived chlorophyll a. Theoretically, the form of the function between productivity and species richness is either monotonically increasing or decreasing, or curvilinear (hump- or U-shaped). We found three negative linear and three hump-shaped relationships between chlorophyll a and species richness out of 10 independent comparisons. Scale dependence was suggested by more prevalent diversity-productivity relationships at smaller (local, landscape) than larger (regional, continental) spatial scales. Differences in the form of the functions were more closely allied with community type than with scale, as negative linear functions were restricted to sessile epifauna while hump-shaped functions occurred in Arctic macrobenthos (mixed epifauna, infauna). In two of the data sets, (St. Lawrence epifauna and Arctic macrobenthos) significant effects of chlorophyll a co-varied with the effects of salinity, suggesting that environmental stress as well as productivity influences diversity in these marine systems. The co-varying effect of salinity may commonly arise in broad-scale studies of productivity and diversity in marine ecosystems when attempting to sample the largest range of productivity, often encompassing a coastal-oceanic gradient

Dissolved organic matter concentration, optical parameters and attenuation of solar radiation in high-latitude lakes across three vegetation zones

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. Les lacs en hautes latitudes sont généralement transparents due à de faibles productivité et concentration en matière organique dissoute (MOD). Toutefois, ceux-ci présentent une grande variabilité en propriétés optiques. Nous avons étudié le régime lumineux avec la MOD dans 18 lacs oligotrophes des forêts (bouleau pubescent) et toundras (arbustive et herbacée) du nord-ouest de la Laponie finlandaise. Dans 12 lacs, >1% du rayonnement photosynthétique actif (RPA) a atteint le fond, tandis que le UVB 1% atteint entre 0.1 et >12m. Les lacs de la toundra herbacée avaient les transparences les plus élevées mais des concentrations en carbone organique dissout (COD) et en MOD chromophore (a440) les plus faibles (moyennes: RPA Kd 0,3m–1, COD 2,1mg.l –1, a440 0,4m–1), tandis que ceux de la toundra arbustive et des forêts étaient moins transparents (COD 4,7mg.l–1, a440 1,4m–1). L’atténuation de la radiation solaire et la transparence étaient liées à a440. Notre étude montre l’importance du type de bassin versant pour les caractéristiques de la MOD et l’optique des lacs. Nous prédisons que de faibles changements de la MOD pourront causer des changements dans l’exposition aux UV, mais des changements du RPA pourraient avoir de faibles effets biologiques dans ces lacs déjà illuminés jusqu’au fond

Toxicité des hydrocarbures et impacts des déversements sur les organismes marins et leur environnement

Les risques liés aux déversements opérationnels et accidentels d’hydrocarbures seront intensifiés par l’augmentation des besoins mondiaux en pétrole. Les accidents, bien que rares, se multiplient et causent d’innombrables effets sur l’environnement et sur les organismes qui s’y trouvent. Ce chapitre fait un survol de l’impact potentiel des hydrocarbures sur les organismes et sur les habitats ainsi que des conséquences sur le fonctionnement et les services écologiques que procurent les écosystèmes marins. La nature et le comportement des pétroles déversés ainsi que les risques associés aux méthodes d’intervention, par exemple l’utilisation de dispersants ou le nettoyage physique, y sont abordés. Les voies d’exposition des divers organismes (microorganismes, phytoplancton, zooplancton, invertébrés, oiseaux et mammifères marins) et les effets sur leur population sont illustrés en prenant exemple parmi les événements malheureux de l’échouage de l’Exxon Valdez en 1989 ou de l’explosion de la plateforme Deepwater Horizon en 2010. L’influence des conditions de déversement sur la résilience des écosystèmes touchés sera abordée

Photoacclimation and light thresholds for cold temperate seagrasses

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

Remote sensing of coastal vegetation phenology in a cold temperate intertidal system: Implications for classification of coastal habitats

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

Hidden stores of organic matter in northern lake ice : Selective retention of terrestrial particles, phytoplankton and labile carbon

Around 50% of the world's lakes freeze seasonally, but the duration of ice-cover is shortening each year and this is likely to have broad limnological consequences. We sampled freshwater ice and the underlying water in 19 boreal and polar lakes to evaluate whether lake ice contains an inoculum of algae, nutrients, and carbon that may contribute to lake ecosystem productivity. Boreal and Arctic lakes differed in ice duration (6 vs. >10 months), thickness (70 vs. 190 cm), and quality (predominantly snow ice vs. black ice), but in all lakes, there were consistent differences in biological and biogeochemical composition between ice and water. Particulate fractions were often more retained while most dissolved compounds were excluded from the ice; for example, the ice had more terrestrial particulate carbon, measured as fatty acid biomarkers (averages of 1.1 vs. 0.3 µg L−1) but lower dissolved organic carbon (2.2 vs. 5.7 mg C L−1) and inorganic phosphorus concentrations (4.0 vs. 7.5 µg C L−1) than the underlying water. The boreal ice further had three times higher chlorophyll-a, than the water (0.9 vs. 0.3 µg L−1). Of the dissolved fractions, the contribution of protein-like compounds was higher in the ice, and this in all lakes. These labile compounds would become available to planktonic microbes when the ice melts. Our results show that freshwater ice has an underestimated role in storage and transformation in the biogeochemical carbon cycle of ice-covered lake ecosystems

Impacts potentiels cumulés des facteurs de stress liés aux activités humaines sur l’écosystème marin du Saint-Laurent

Les activités humaines modifient l’environnement naturel, perturbant par le fait même les organismes qui y habitent. Dans l’écosystème marin du golfe du Saint-Laurent, les diverses perturbations affectent les écosystèmes à différents degrés. Pour certains d’entre eux, les effets sont mal connus ou simplement inconnus. De plus, plusieurs perturbations peuvent affecter simultanément une composante de l’écosystème ou un système en entier. Les effets cumulés sont encore moins connus. Dans ce chapitre, nous synthétisons les connaissances actuelles sur les facteurs de stress liés aux activités humaines, puis essayons de déterminer leurs interactions et leurs effets cumulés sur l’écosystème du Saint-Laurent