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11 pages, 6 figures, 1 table, The effects of harmful algae on bivalve physiology are complex and involve both physiological and behavioural responses. Studying those responses is essential to better describe and predict their impact on shellfish aquaculture and health risk for humans. In this study we recorded for two months the physiological response of the blue mussel Mytilus edulis from Eastern Canada to a one-week exposure to a paralytic shellfish poisoning producing dinoflagellate strain of Alexandrium catenella, isolated from the St Lawrence estuary, Canada. Mussels in a ‘control’ treatment were fed continuously with a non-toxic diet, while mussels in a ‘starvation’ treatment were fed the same non-toxic diet the first week and subsequently starved for seven weeks. Mussels in a ‘toxic’ treatment received A. catenella for one week before being starved until the end of the experiment. Over a two-month experiment we monitored shell and tissue growth, filtration capacity, respiration rate, byssal attachment strength, valve opening behaviour, and toxin content in tissues. Mussels fed normally on the toxic dinoflagellate and accumulated an average of 51.6 µg STXeq 100 g–1 after one week of exposure. After seven weeks of depuration, about half of the specimen showed levels around 18 µg STXeq 100 g–1. The condition index of exposed mussels (‘toxic’ treatment) decreased rapidly from the start as compared to mussels that received a one-week non-toxic diet (‘starvation’ treatment). Oxygen consumption rates increased in the ‘toxic’ treatment before leveling out with that of mussels from the ‘starvation’ treatment. Valve opening amplitude was lower in the ‘toxic’ treatment during and following the exposure. Average valve closure duration was higher right after the exposure, during the peak of mussel tissue intoxication. No significant change in byssal thread strength was observed through time in each treatment but less force was required to detach mussels from the ‘toxic’ and ‘starvation’ treatments. The number of byssus threads produced by mussels exposed to the toxic dinoflagellate was also lower than in the control group. These results represent advancements in our understanding of the impacts of harmful algae on bivalves and contribute to the development of mitigation measures necessary to both the safety of consumers and the sustainability of aquaculture operations, This project was undertaken with the financial support of the Government of Canada, specifically by Environment and Climate Change Canada (FDE-CA-2017i002) and Fisheries and Oceans Canada

The effects of ocean acidification and warming on the concentrations of dimethylsulfoniopropionate (DMSP) and dimethylsulfide (DMS) were investigated during a mesocosm experiment in the Lower St. Lawrence Estuary (LSLE) in the fall of 2014. Twelve mesocosms covering a range of pHT (pH on the total hydrogen ion concentration scale) from 8.0 to 7.2, corresponding to a range of CO2 partial pressures (pCO2) from 440 to 2900 μatm, at two temperatures (in situ and C5 °C; 10 and 15 °C) were monitored during 13 days. All mesocosms were characterized by the rapid development of a diatom bloom dominated by Skeletonema costatum, followed by its decline upon the exhaustion of nitrate and silicic acid. Neither the acidification nor the warming resulted in a significant impact on the abundance of bacteria over the experiment. However, warming the water by 5 °C resulted in a significant increase in the average bacterial production (BP) in all 15 °C mesocosms as compared to 10 °C, with no detectable effect of pCO2 on BP. Variations in total DMSP (DMSPt DparticulateCdissolved DMSP) concentrations tracked the development of the bloom, although the rise in DMSPt persisted for a few days after the peaks in chlorophyll a. Average concentrations of DMSPt were not affected by acidification or warming. Initially low concentrations of DMS (< 1 nmol L-1) increased to reach peak values ranging from 30 to 130 nmol L-1 towards the end of the experiment. Increasing the pCO2 reduced the averaged DMS concentrations by 66%and 69%at 10 and 15 °C, respectively, over the duration of the experiment. On the other hand, a 5 °C warming increased DMS concentrations by an average of 240% as compared to in situ temperature, resulting in a positive offset of the adverse pCO2 impact. Significant positive correlations found between bacterial production and concentrations of DMS throughout our experiment point towards temperatureassociated enhancement of bacterial DMSP metabolism as a likely driver of the mitigating effect of warming on the negative impact of acidification on the net production of DMS in the LSLE and potentially the global ocean. Fil: Benard, Robin. Laval University; Canadá Fil: Ferreyra, Gustavo Adolfo. Laval University; Canadá Fil: Michael, Scarratt. Maurice Lamontagne Institute, Fisheries And Oceans; Canadá Fil: Sonia, Michaud. Maurice Lamontagne Institute, Fisheries And Oceans ; Canadá Fil: Michel, Starr. Maurice Lamontagne Institute, Fisheries And Oceans; Canadá Fil: Alfonso, Mucci. Université Mcgill; Canadá Fil: Ferreyra, Gustavo Adolfo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Austral de Investigaciones Científicas; Argentina Fil: Gosselin, Michel. Institut Des Sciences de la Mer de Rimouski; Canadá Fil: Tremblay, Jean-Éric. Laval University; Canadá Fil: Lizotte, Martine. Laval University; Canadá Fil: Yang, Gui Peng. Ocean University; China

The influence of the seasonal development of microplankton communities on the cycling of dimethylsulfide (DMS) and its precursor dimethylsulfoniopropionate (DMSP) was investigated along a South-North gradient (36-59 °N) in the Northwest (NW) Atlantic Ocean. Three surveys allowed the sampling of surface mixed layer (SML) waters at stations extending from the subtropical gyre to the Greenland Current during May, July and October 2003. Pools and transformation rates of DMSP and DMS were quantified and related to prevailing physical and biochemical conditions, phytoplankton abundance and taxonomic composition, as well as bacterioplankton abundance and leucine uptake. The South-North progression of the diatom bloom, a prominent feature in the NW Atlantic, did not influence the production of DMS whereas conditions in the N Atlantic Drift lead to a persistent bloom of DMSP-rich flagellate-dominated phytoplankton community and high net DMS production rates. Macroscale patterns of the observed variables were further explored using principal component analysis (PCA). The first axis of the PCA showed a strong association between the spatio-temporal distribution of DMSP and the abundance of several phytoplankton groups including dinoflagellates and prymnesiophytes, as well as with microbial-mediated DMSP d consumption and yields and rates of the conversion of DMSP into DMS. The second axis revealed a strong association between concentrations of DMS and SML depth and photosynthetically active radiation, a result supporting the prominent role of solar radiation as a driver of DMS dynamics. © 2012 Springer Science+Business Media B.V.