6 research outputs found
RĂŽle nutritionnel du picophytoplancton pour les bivalves d'Ă©levage
RĂSUMĂ: Lâaquaculture des bivalves illustre une forte croissance de la biomasse produite au Canada. Cette augmentation est notamment due Ă lâinnovation constante des techniques dâĂ©levage et une meilleure comprĂ©hension des interactions de cette industrie avec lâenvironnement. Ceci Ă©tant dit, la compĂ©tition inter- et intra spĂ©cifique sur la matiĂšre organique disponible comme source de nourriture demeure un enjeu pouvant avoir des rĂ©percussions sur la croissance des bivalves dâĂ©levage, telles que la moule bleue (Mytilus edulis) et lâhuĂźtre amĂ©ricaine (Crassostrea virginica). De fortes densitĂ©s de bivalves, possĂ©dant une capacitĂ© de filtration importante, peuvent efficacement induire un certain contrĂŽle sur les communautĂ©s phytoplanctoniques dâun estuaire tout en rĂ©duisant considĂ©rablement la concentration de matiĂšre organique en suspension (seston). La disponibilitĂ© de la nourriture nâest toutefois pas uniquement limitĂ©e Ă la biomasse prĂ©sente dans le systĂšme, mais aussi Ă la taille des particules. LâefficacitĂ© de rĂ©tention de celles-ci par les bivalves augmente gĂ©nĂ©ralement en fonction de la taille des particules. Inversement, la rĂ©tention des petites cellules phytoplanctoniques, tel que le picophytoplancton (0,2â2,0 ÎŒm), est souvent perçue comme Ă©tant moindre et sans intĂ©rĂȘt nutritionnel. Ce picophytoplancton est pourtant trĂšs abondant en milieux riches en nutriments dissouts et domine parfois la biomasse phytoplanctonique des estuaires. Les baies aquacoles du Canada Atlantique, comme Ă lâĂle-du-Prince-Ădouard (Ă.-P.-Ă), ne dĂ©rogent pas Ă cette situation, mais la production de coquillages Ă lâintĂ©rieur de ces estuaires ne semble pas ĂȘtre affectĂ©e pour autant. Est-ce que M. edulis et C. virginica ont la capacitĂ© dâutiliser cette ressource abondante comme source nutritionnelle?
Ainsi, cette thĂšse est constituĂ©e de trois axes de recherche dans lesquels nous explorons le potentiel nutritionnel des cellules picophytoplanctoniques en relation avec les bivalves en Ă©levage intensif. Le tout est regroupĂ© en trois chapitres distincts cherchant Ă : i) DĂ©terminer si le picophytoplancton est une source majeure de lâalimentation des moules dâĂ©levages (M. edulis); ii) Analyser la capacitĂ© des huĂźtres dâĂ©levage (C. virginica) Ă filtrer, ingĂ©rer et assimiler le picophytoplancton comme source de nourriture; iii) VĂ©rifier le potentiel de compĂ©tition entre M. edulis et le tunicier envahisseur (Styela clava) en lâingestion et lâassimilation du picophytoplancton. Dans un premier temps, nous avons testĂ© la contribution potentielle du picophytoplancton Ă la croissance des moules dâĂ©levage dans la baie de St. Peters (Ă.-P.-Ă.) en complĂ©tant un suivi sur le terrain du phytoplancton (biomasse fractionnĂ©e), de la croissance de deux cohortes de moules (1 an et 2 ans), ainsi quâun volet expĂ©rimental oĂč nous avons analysĂ© la capacitĂ© de rĂ©tention du picophytoplancton naturel par M. edulis qui dĂ©montra une efficacitĂ© de rĂ©tention de 20±2 % de ce petit phytoplancton. Nous avons par la suite intĂ©grĂ© ces donnĂ©es de biomasses phytoplanctoniques, de croissances des moules et dâefficacitĂ© de rĂ©tention de picophytoplancton Ă lâintĂ©rieur dâun modĂšle numĂ©rique de type « Dynamic Energy Budget (DEB) ». Les simulations DEB excluant le picophytoplancton xii (rĂ©tention de 0 % par les moules) prĂ©dirent une diminution de la croissance des moules de 14 Ă 29 %. Cette contribution du picophytoplancton (< 2,0 ÎŒm) au budget Ă©nergĂ©tique de lâaquaculture de la moule est donc non nĂ©gligeable et devrait ĂȘtre considĂ©rĂ©e lors des Ă©tudes dâinteractions entre lâaquaculture des bivalves et la dynamique du phytoplancton.
En second lieu, grĂące Ă des mĂ©thodes analytiques utilisant des traceurs diĂ©tĂ©tiques (profilage des isotopes stables et des acides gras), ainsi que lâutilisation de 13C comme marqueur de traçabilitĂ© du picophytoplancton, nous avons explorĂ© les sources nutritionnelles distinctes des huĂźtres cultivĂ©es (C. virginica) dans la baie de Foxley (Ă.-P.-Ă.) en plus dâexaminer leur capacitĂ© dâassimilation du picophytoplancton. Les huĂźtres cultivĂ©es en suspension dĂ©montrĂšrent une concentration en lipides significativement supĂ©rieure Ă celles provenant de la culture sur le fond. Les microalgues demeurent la source principale de nutrition des huĂźtres. De plus, malgrĂ© leur proximitĂ© au benthos, les huĂźtres de fond ne prĂ©sentĂšrent aucun lien significatif envers les sources dĂ©tritiques ou bactĂ©riennes. Les rĂ©sultats dâenrichissement (13C) dĂ©montrĂšrent concrĂštement une assimilation du picophytoplancton par C. virginica, le tout en intĂ©grant ce carbone isotopiquement marquĂ© directement dans leurs tissus, et ce, mĂȘme lorsque nourris dâune diĂšte ne comprenant que 20% de picophytoplancton. De surcroĂźt, les huĂźtres montrĂšrent mĂȘme un enrichissement en 13C de lâacide gras spĂ©cifique 22:2 (ou NMI), un acide gras uniquement bio synthĂ©tisĂ© par les bivalves. En terminant, des manipulations expĂ©rimentales Ă lâintĂ©rieur de notre laboratoire mobile (in situ) ont permis de dĂ©terminer le potentiel dâassimilation du picophytoplancton par les moules (M. edulis) et, par le fait mĂȘme, de dĂ©terminer si celles-ci sont compĂ©titrices face Ă lâune des espĂšces envahissantes retrouvĂ©es sur lâĂ.-P.-Ă., soit le tunicier solitaire Styela clava. Utilisant le marquage isotopique (13C) du picophytoplancton, les rĂ©sultats obtenus nous ont permis de dĂ©montrer clairement une assimilation importante, et ce pour les deux espĂšces de filtreurs.
La moule sembla toutefois prĂ©senter un avantage allomĂ©trique associĂ© au taux dâassimilation du picophytoplancton comparativement au tunicier. Une compĂ©tition semble effectivement prĂ©sente entre lâespĂšce cultivĂ©e et son envahisseur face Ă cette source distincte de nourriture. La proximitĂ© de ces deux espĂšces dans le contexte aquacole nous a incitĂ©s Ă mener Ă terme une derniĂšre comparaison expĂ©rimentale afin de vĂ©rifier si un transfert secondaire dâĂ©nergie (13C) est possible. Ce processus fut dĂ©montrĂ© via lâingestion des fĂšces provenant dâindividus avoisinants. Pour les deux espĂšces, les individus ingĂ©rant les particules fĂ©cales produites par S. clava sâenrichissent de niveaux supĂ©rieurs en 13C. GrĂące Ă cette Ă©tude, ces rĂ©sultats apportent de nouvelles pistes de rĂ©flexion quant Ă lâimportance des petites cellules phytoplanctoniques, telle que le picophytoplancton, en tant que source de nutrition non nĂ©gligeable pour les bivalves dâĂ©levage. Ce projet de doctorat prĂ©sente des conclusions trĂšs pertinentes nous permettant dâamĂ©liorer nos connaissances sur les interactions entre les bivalves dâĂ©levages et les communautĂ©s phytoplanctoniques. -- Mot(s) clĂ©(s) en anglais : picophytoplankton, Mytilus edulis, Crassostrea virginica, Styela clava, aquaculture, assimilation, biomarkers. -- ABSTRACT: Bivalve aquaculture is an industry in constant growth in Canada. This growth can be attributed to thanks to the innovation of husbandry techniques, which translates to a slight increase in biomass production annually. This said, inter- and intra-specific competition for food sources is a concern which can have important repercussions on cultured bivalve species in the area such as the blue mussel (Mytilus edulis) and the eastern oyster (Crassostrea virginica). It is known that high densities of filter feeders, with their important filtration pressure, can control phytoplankton communities and deplete significantly suspended organic matter (seston) from the water column. Although important, food availability is not the only limiting factor to be considered in research on bivalve particle selection. Particle size is also an important parameter often dictating the retention efficiency of variable sources of organic matter during the filter-feeding process. Since, the retention efficiency is generally positively correlated with particle size, thus the retention of small phytoplankton cells such as picophytoplankton (0.2â2.0 ÎŒm) is notably low. Picophytoplankton is often abundant in nutrient rich estuaries and occasionally dominates the overall phytoplankton biomass of those productive ecosystems. Shellfish aquaculture areas in Atlantic Canada are not the exception, even though the bivalve production in those estuaries does not display any negative trends in relation to the presence of picophytoplankton. Are M. edulis and C. virginica benefiting from the strong availability of picophytoplankton as an energy source to thrive in these estuaries?
In this context, the present thesis is founded on three research questions relating to the nutritional contribution of picophytoplankton in intensive bivalvesâculture settings. The information provided is divided in three distinct chapters focusing on the following specific themes: i) Determine the potential contribution of picophytoplankton is an important food source for cultured mussels (M. edulis). ii) Analyze the capacity of oysters (C. virginica) to filter, ingest and assimilate picohytoplankton as an energy source. iii) Explore the potential competition between M. edulis and its invader, the tunicate (Styela clava), for the ingestion and assimilation of picophytoplankton. To begin with, we verified the contribution of picophytoplankton towards the growth of cultured mussels from St. Peters Bay (P.E.I.) using field monitoring data, specifically phytoplankton size-fractionated biomass and mussel growth from two cohorts (1- and 2-year old crop). Experimental data based on natural seston was exploited to assess the efficiency of mussels to retain picophytoplankton cells; M. edulis demonstrated a retention efficiency of 20±2%. We then integrated phytoplankton biomass, mussel growth and picophytoplankon retention data in a Dynamic Energy Budget (DEB) numerical model. Simulations excluding picophytoplankton (retention efficiency adjusted to 0% by mussels) predicted a reduction in mussel growth of 14â29%. Consequently, the contribution of xiv picophytoplankton (< 2.0ÎŒm) to the energy budget of mussel aquaculture is not negligible and should be considered within studies on shellfish aquaculture interactions with phytoplankton dynamics as well as the environment.
Subsequently, using analytical methods such as dietary tracers (stable isotopes and fatty acids profiling) on the field, as well as 13C for picophytoplankton traceability in situ laboratory experiments, we investigated the different nutritional sources for cultivated oysters (C. virginica) in the Foxley River system (P.E.I.). Oysters cultivated in suspension contained significantly higher lipids in their tissues than their bottom cultured counterparts. Microalgae remained the principal food source for oysters independent of culture method, and bottom cultured oysters did not demonstrate significant relations with bacterial and detrital sources. The enrichment experiment with isotopically labelled (13C) picophytoplankton showed important assimilation of picophytoplankton carbon in oystersâ tissues, even when fed a diet containing as low as 20% of picophytoplankton cells. Moreover, the fatty acid 22:2 (or NMI) biosynthesized by oysters showed 13C enrichment. The latter fatty acid is only biosynthesized by bivalves, thus demonstrate enhance complexity of usage of integrated carbon from picophytoplankton. To conclude, one last series of experimental trials were conducted using natural sea water to help us determine the potential of picophytoplankton assimilation by cultured mussels (M. edulis) and invasive tunicates (S. clava). This enabled us to determine if there is inter-specific competition for those small phytoplankton cells between the later cultured and invasive species.
Our results using 13C as dietary tracer clearly demonstrated an assimilation of picophytoplankton by both filter-feeder species and consequently indicated competition for that specific food source. This said, cultured mussels demonstrated a superior allometric relation in regards to the assimilation rates of picophytoplankton in comparison to their invader. In the context of aquaculture, the close proximity of both species on cultivation structures initiated our last series of trials looking at potential secondary transfer of energy through feces ingestion. We concluded that indeed, there is a transfer originating from picophytoplankton assimilation through feces ingestion of individuals in close proximity. For both species, individuals ingesting S. clava feces acquired important levels of enrichment (13C). Results in the present thesis bring new reflexions on the contribution of small phytoplankton cells, such as picophytoplankton, as an important food source for cultured bivalves. This doctoral project presented significant conclusions and novel information which will help better understand the complex relations between shellfish aquaculture and phytoplankton assemblages. -- Mot(s) clé(s) en anglais : picophytoplankton, Mytilus edulis, Crassostrea virginica, Styela clava, aquaculture, assimilation, biomarkers
Revisiting ecological carrying capacity indices for bivalve culture
Ecological carrying capacity (ECC) indices for bivalve culture rely on key ecosystem turnover rates: 1. clearance time (CT), the time needed for the cultured bivalves to filter the entire bay volume; 2. renewal time (RT), the time required to replace the entire bay volume with external water; and 3. production time (PT), the time needed for phytoplankton biomass renewal via local primary production. These turnover rates are conceptually straightforward but lack measurement standardizations in the context of ECC assessments. This study compares simple turnover rate methods with more complex approaches designed to address key assumptions and improve accuracy. Method comparisons were performed across multiple embayments (systems) in Prince Edward Island, Canada. When crop aggregation and system-scale refiltration effects were considered, CT increased by a factor of 14 to 22 depending on the system and species under cultivation. Seasonal temperature considerations further impacted CT by a factor of 38 to 142. Regarding RT, validated hydrodynamic models and tidal prism models produced remarkably different outcomes; the tidal prism approach underestimated RT by 77â94% across the studied systems. Conversely, PT was unaffected by contrasting phytoplankton parameterization; pre-aquaculture (1969â1970) and contemporary (2011â2012) datasets led to similar PT outcomes. However, other metrics revealed a contemporary shift towards low phytoplankton biomass and smaller phytoplankton cells (picophytoplankton); these observations suggest that PT provides insufficient granularity regarding microalgae biomass replacement. Overall, the study rejects a common assumption that the bay-scale turnover rates serving the conventional CT/RT and CT/PT indices can be easily and accurately parameterized; these indices should be used cautiously in assessing the sustainability of farming activities.publishedVersio
RĂŽle nutritionnel du picophytoplancton pour les bivalves d'Ă©levage
RĂSUMĂ: Lâaquaculture des bivalves illustre une forte croissance de la biomasse produite au Canada. Cette augmentation est notamment due Ă lâinnovation constante des techniques dâĂ©levage et une meilleure comprĂ©hension des interactions de cette industrie avec lâenvironnement. Ceci Ă©tant dit, la compĂ©tition inter- et intra spĂ©cifique sur la matiĂšre organique disponible comme source de nourriture demeure un enjeu pouvant avoir des rĂ©percussions sur la croissance des bivalves dâĂ©levage, telles que la moule bleue (Mytilus edulis) et lâhuĂźtre amĂ©ricaine (Crassostrea virginica). De fortes densitĂ©s de bivalves, possĂ©dant une capacitĂ© de filtration importante, peuvent efficacement induire un certain contrĂŽle sur les communautĂ©s phytoplanctoniques dâun estuaire tout en rĂ©duisant considĂ©rablement la concentration de matiĂšre organique en suspension (seston). La disponibilitĂ© de la nourriture nâest toutefois pas uniquement limitĂ©e Ă la biomasse prĂ©sente dans le systĂšme, mais aussi Ă la taille des particules. LâefficacitĂ© de rĂ©tention de celles-ci par les bivalves augmente gĂ©nĂ©ralement en fonction de la taille des particules. Inversement, la rĂ©tention des petites cellules phytoplanctoniques, tel que le picophytoplancton (0,2â2,0 ÎŒm), est souvent perçue comme Ă©tant moindre et sans intĂ©rĂȘt nutritionnel. Ce picophytoplancton est pourtant trĂšs abondant en milieux riches en nutriments dissouts et domine parfois la biomasse phytoplanctonique des estuaires. Les baies aquacoles du Canada Atlantique, comme Ă lâĂle-du-Prince-Ădouard (Ă.-P.-Ă), ne dĂ©rogent pas Ă cette situation, mais la production de coquillages Ă lâintĂ©rieur de ces estuaires ne semble pas ĂȘtre affectĂ©e pour autant. Est-ce que M. edulis et C. virginica ont la capacitĂ© dâutiliser cette ressource abondante comme source nutritionnelle?
Ainsi, cette thĂšse est constituĂ©e de trois axes de recherche dans lesquels nous explorons le potentiel nutritionnel des cellules picophytoplanctoniques en relation avec les bivalves en Ă©levage intensif. Le tout est regroupĂ© en trois chapitres distincts cherchant Ă : i) DĂ©terminer si le picophytoplancton est une source majeure de lâalimentation des moules dâĂ©levages (M. edulis); ii) Analyser la capacitĂ© des huĂźtres dâĂ©levage (C. virginica) Ă filtrer, ingĂ©rer et assimiler le picophytoplancton comme source de nourriture; iii) VĂ©rifier le potentiel de compĂ©tition entre M. edulis et le tunicier envahisseur (Styela clava) en lâingestion et lâassimilation du picophytoplancton. Dans un premier temps, nous avons testĂ© la contribution potentielle du picophytoplancton Ă la croissance des moules dâĂ©levage dans la baie de St. Peters (Ă.-P.-Ă.) en complĂ©tant un suivi sur le terrain du phytoplancton (biomasse fractionnĂ©e), de la croissance de deux cohortes de moules (1 an et 2 ans), ainsi quâun volet expĂ©rimental oĂč nous avons analysĂ© la capacitĂ© de rĂ©tention du picophytoplancton naturel par M. edulis qui dĂ©montra une efficacitĂ© de rĂ©tention de 20±2 % de ce petit phytoplancton. Nous avons par la suite intĂ©grĂ© ces donnĂ©es de biomasses phytoplanctoniques, de croissances des moules et dâefficacitĂ© de rĂ©tention de picophytoplancton Ă lâintĂ©rieur dâun modĂšle numĂ©rique de type « Dynamic Energy Budget (DEB) ». Les simulations DEB excluant le picophytoplancton xii (rĂ©tention de 0 % par les moules) prĂ©dirent une diminution de la croissance des moules de 14 Ă 29 %. Cette contribution du picophytoplancton (< 2,0 ÎŒm) au budget Ă©nergĂ©tique de lâaquaculture de la moule est donc non nĂ©gligeable et devrait ĂȘtre considĂ©rĂ©e lors des Ă©tudes dâinteractions entre lâaquaculture des bivalves et la dynamique du phytoplancton.
En second lieu, grĂące Ă des mĂ©thodes analytiques utilisant des traceurs diĂ©tĂ©tiques (profilage des isotopes stables et des acides gras), ainsi que lâutilisation de 13C comme marqueur de traçabilitĂ© du picophytoplancton, nous avons explorĂ© les sources nutritionnelles distinctes des huĂźtres cultivĂ©es (C. virginica) dans la baie de Foxley (Ă.-P.-Ă.) en plus dâexaminer leur capacitĂ© dâassimilation du picophytoplancton. Les huĂźtres cultivĂ©es en suspension dĂ©montrĂšrent une concentration en lipides significativement supĂ©rieure Ă celles provenant de la culture sur le fond. Les microalgues demeurent la source principale de nutrition des huĂźtres. De plus, malgrĂ© leur proximitĂ© au benthos, les huĂźtres de fond ne prĂ©sentĂšrent aucun lien significatif envers les sources dĂ©tritiques ou bactĂ©riennes. Les rĂ©sultats dâenrichissement (13C) dĂ©montrĂšrent concrĂštement une assimilation du picophytoplancton par C. virginica, le tout en intĂ©grant ce carbone isotopiquement marquĂ© directement dans leurs tissus, et ce, mĂȘme lorsque nourris dâune diĂšte ne comprenant que 20% de picophytoplancton. De surcroĂźt, les huĂźtres montrĂšrent mĂȘme un enrichissement en 13C de lâacide gras spĂ©cifique 22:2 (ou NMI), un acide gras uniquement bio synthĂ©tisĂ© par les bivalves. En terminant, des manipulations expĂ©rimentales Ă lâintĂ©rieur de notre laboratoire mobile (in situ) ont permis de dĂ©terminer le potentiel dâassimilation du picophytoplancton par les moules (M. edulis) et, par le fait mĂȘme, de dĂ©terminer si celles-ci sont compĂ©titrices face Ă lâune des espĂšces envahissantes retrouvĂ©es sur lâĂ.-P.-Ă., soit le tunicier solitaire Styela clava. Utilisant le marquage isotopique (13C) du picophytoplancton, les rĂ©sultats obtenus nous ont permis de dĂ©montrer clairement une assimilation importante, et ce pour les deux espĂšces de filtreurs.
La moule sembla toutefois prĂ©senter un avantage allomĂ©trique associĂ© au taux dâassimilation du picophytoplancton comparativement au tunicier. Une compĂ©tition semble effectivement prĂ©sente entre lâespĂšce cultivĂ©e et son envahisseur face Ă cette source distincte de nourriture. La proximitĂ© de ces deux espĂšces dans le contexte aquacole nous a incitĂ©s Ă mener Ă terme une derniĂšre comparaison expĂ©rimentale afin de vĂ©rifier si un transfert secondaire dâĂ©nergie (13C) est possible. Ce processus fut dĂ©montrĂ© via lâingestion des fĂšces provenant dâindividus avoisinants. Pour les deux espĂšces, les individus ingĂ©rant les particules fĂ©cales produites par S. clava sâenrichissent de niveaux supĂ©rieurs en 13C. GrĂące Ă cette Ă©tude, ces rĂ©sultats apportent de nouvelles pistes de rĂ©flexion quant Ă lâimportance des petites cellules phytoplanctoniques, telle que le picophytoplancton, en tant que source de nutrition non nĂ©gligeable pour les bivalves dâĂ©levage. Ce projet de doctorat prĂ©sente des conclusions trĂšs pertinentes nous permettant dâamĂ©liorer nos connaissances sur les interactions entre les bivalves dâĂ©levages et les communautĂ©s phytoplanctoniques. -- Mot(s) clĂ©(s) en anglais : picophytoplankton, Mytilus edulis, Crassostrea virginica, Styela clava, aquaculture, assimilation, biomarkers. -- ABSTRACT: Bivalve aquaculture is an industry in constant growth in Canada. This growth can be attributed to thanks to the innovation of husbandry techniques, which translates to a slight increase in biomass production annually. This said, inter- and intra-specific competition for food sources is a concern which can have important repercussions on cultured bivalve species in the area such as the blue mussel (Mytilus edulis) and the eastern oyster (Crassostrea virginica). It is known that high densities of filter feeders, with their important filtration pressure, can control phytoplankton communities and deplete significantly suspended organic matter (seston) from the water column. Although important, food availability is not the only limiting factor to be considered in research on bivalve particle selection. Particle size is also an important parameter often dictating the retention efficiency of variable sources of organic matter during the filter-feeding process. Since, the retention efficiency is generally positively correlated with particle size, thus the retention of small phytoplankton cells such as picophytoplankton (0.2â2.0 ÎŒm) is notably low. Picophytoplankton is often abundant in nutrient rich estuaries and occasionally dominates the overall phytoplankton biomass of those productive ecosystems. Shellfish aquaculture areas in Atlantic Canada are not the exception, even though the bivalve production in those estuaries does not display any negative trends in relation to the presence of picophytoplankton. Are M. edulis and C. virginica benefiting from the strong availability of picophytoplankton as an energy source to thrive in these estuaries?
In this context, the present thesis is founded on three research questions relating to the nutritional contribution of picophytoplankton in intensive bivalvesâculture settings. The information provided is divided in three distinct chapters focusing on the following specific themes: i) Determine the potential contribution of picophytoplankton is an important food source for cultured mussels (M. edulis). ii) Analyze the capacity of oysters (C. virginica) to filter, ingest and assimilate picohytoplankton as an energy source. iii) Explore the potential competition between M. edulis and its invader, the tunicate (Styela clava), for the ingestion and assimilation of picophytoplankton. To begin with, we verified the contribution of picophytoplankton towards the growth of cultured mussels from St. Peters Bay (P.E.I.) using field monitoring data, specifically phytoplankton size-fractionated biomass and mussel growth from two cohorts (1- and 2-year old crop). Experimental data based on natural seston was exploited to assess the efficiency of mussels to retain picophytoplankton cells; M. edulis demonstrated a retention efficiency of 20±2%. We then integrated phytoplankton biomass, mussel growth and picophytoplankon retention data in a Dynamic Energy Budget (DEB) numerical model. Simulations excluding picophytoplankton (retention efficiency adjusted to 0% by mussels) predicted a reduction in mussel growth of 14â29%. Consequently, the contribution of xiv picophytoplankton (< 2.0ÎŒm) to the energy budget of mussel aquaculture is not negligible and should be considered within studies on shellfish aquaculture interactions with phytoplankton dynamics as well as the environment.
Subsequently, using analytical methods such as dietary tracers (stable isotopes and fatty acids profiling) on the field, as well as 13C for picophytoplankton traceability in situ laboratory experiments, we investigated the different nutritional sources for cultivated oysters (C. virginica) in the Foxley River system (P.E.I.). Oysters cultivated in suspension contained significantly higher lipids in their tissues than their bottom cultured counterparts. Microalgae remained the principal food source for oysters independent of culture method, and bottom cultured oysters did not demonstrate significant relations with bacterial and detrital sources. The enrichment experiment with isotopically labelled (13C) picophytoplankton showed important assimilation of picophytoplankton carbon in oystersâ tissues, even when fed a diet containing as low as 20% of picophytoplankton cells. Moreover, the fatty acid 22:2 (or NMI) biosynthesized by oysters showed 13C enrichment. The latter fatty acid is only biosynthesized by bivalves, thus demonstrate enhance complexity of usage of integrated carbon from picophytoplankton. To conclude, one last series of experimental trials were conducted using natural sea water to help us determine the potential of picophytoplankton assimilation by cultured mussels (M. edulis) and invasive tunicates (S. clava). This enabled us to determine if there is inter-specific competition for those small phytoplankton cells between the later cultured and invasive species.
Our results using 13C as dietary tracer clearly demonstrated an assimilation of picophytoplankton by both filter-feeder species and consequently indicated competition for that specific food source. This said, cultured mussels demonstrated a superior allometric relation in regards to the assimilation rates of picophytoplankton in comparison to their invader. In the context of aquaculture, the close proximity of both species on cultivation structures initiated our last series of trials looking at potential secondary transfer of energy through feces ingestion. We concluded that indeed, there is a transfer originating from picophytoplankton assimilation through feces ingestion of individuals in close proximity. For both species, individuals ingesting S. clava feces acquired important levels of enrichment (13C). Results in the present thesis bring new reflexions on the contribution of small phytoplankton cells, such as picophytoplankton, as an important food source for cultured bivalves. This doctoral project presented significant conclusions and novel information which will help better understand the complex relations between shellfish aquaculture and phytoplankton assemblages. -- Mot(s) clé(s) en anglais : picophytoplankton, Mytilus edulis, Crassostrea virginica, Styela clava, aquaculture, assimilation, biomarkers
Cultured eastern oysters (
In this study, we investigated the food sources of eastern oysters Crassostrea virginica cultivated in Atlantic Canada. Stable isotopes (13C and 15N) and fatty acid biomarkers were used to identify these sources under in situ conditions for suspended (âŒ0.5âm below surface) and bottom (âŒ2âm) culture stocks. It was found that particulate organic matter represented the main food source, with major contributions from live phytoplankton. Higher lipid contents were detected in the digestive glands of suspended oysters compared to bottom oysters (pâ2âÎŒm, 1.05â±â0.15âÎŒgâlâ1, meanâ±âSEM). To determine whether the small size PPP was captured and assimilated by C. virginica, feeding trials were conducted in the laboratory using three PPP/NPP diets (20%, 50%, and 80% PPP), consisting of isotopically-labelled (ÎŽ13C) PPP cells (Nannochloropsis oculata) and non-labelled NPP cells (Tisochrysis lutea). An isotopically-labelled fatty acids analysis indicated PPP assimilation in various tissues (digestive gland, gills, mantle, and abductor muscle), including from oysters fed the reduced (20%) PPP diet. Isotopic enrichment (13C) in the FA 22:2 (non-methylene-interrupted or NMI) showed that precursors of NMIs utilized PPP carbon in its biosynthesis process. In conclusion, C. virginica assimilated primarily particulate organic matter (POM), including PPP, which dominated the phytoplankton community in near surface waters. C. virginica can exploit PPP carbon during fatty acid production and further biosynthesis
Validation of trophic and anthropic underwater noise as settlement trigger in blue mussels
International audienceLike the majority of benthic invertebrates, the blue mussel Mytilus edulis has a bentho-pelagic cycle with its larval settlement being a complex phenomenon involving numerous factors. Among these factors, underwater noise and pelagic trophic conditions have been weakly studied in previous researches. Under laboratory conditions, we tested the hypothesis that picoplankton assimilation by the pediveliger blue mussel larvae acts as a food cue that interacts with anthropic underwater sound to stimulate settlement. We used 13C-labeling microalgae to validate the assimilation of different picoplankton species in the tissues of pediveliger larvae. Our results clearly confirm our hypothesis with a significant synergic effect of these two factors. However, only the picoeukaryotes strains assimilated by larvae stimulated the settlement, whereas the non-ingested picocyanobacteria did not. Similar positive responses were observed with underwater sound characterized by low frequency vessel noises. The combination of both factors (trophic and vessel noise) drastically increased the mussel settlement by an order of 4 compared to the control (without picoplankton and noise). Settlement levels ranged from 16.5 to 67% in 67âh