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The Bay of Quinte, a long, shallow, high-phosphorus system in Lake Ontario, spans a trophic range from eutrophic (upper Bay) to meso-oligotrophic (mouth). Phosphorus control (PC) and the invasion of dreissenid mussels and Cercopagis pengoi have influenced its biology and environment. We elucidated the principal drivers behind zooplankton production in the Bay from pre-PC times to the present (1975-2008). Production of abundant species was calculated using the egg-ratio method. Production estimates were improved by determining system-specific production/biomass (P/B) relationships for rarer species. Planktivory governed zooplankton production initially, likely preventing a response to PC. With changes in the fish community, zooplankton production increased. Thereafter, the principal drivers were the structure and size of the fish community, shifts in the predatory arena associated with changing light penetration, temperature, and addition of C. pengoi--all within a general context of nutrient availability. Dreissenid impacts were indirect. Interactive variables combining measures of predation, nutrient level, light, and temperature were the best at accounting for variability in zooplankton production. La baie de Quinte, un systeme long, peu profond et riche en phosphore du lac Ontario, couvre une gamme de milieux trophiques allant d'eutrophe (haute baie) a mesooligotrophe (embouchure). Le controle du phosphore (PC) et l'invasion de moules dreissenidees et de Cercopagis pengoi ont influence sa biologie et son milieu. Nous avons caracterise les principaux facteurs agissant sur la production de zooplancton depuis la periode precedant l'application de mesures de PC jusqu'au present (1975-2008). La production des especes abondantes a ete calculee par la methode des proportions d'oeufs. Pour les especes plus rares, la determination des relations entre la production et la biomasse (P/B) pour le systeme a permis de meilleures estimations de la production. La planctonophagie regissait initialement la production de zooplancton, reprimant vraisemblablement toute reponse aux mesures de PC. Des changements a la communaute de poissons se sont traduits par une augmentation de la production de zooplancton apres quoi les principaux facteurs influant sur la production etaient la structure et la taille de la communaute de poissons, des modifications de la predation associees a des variations de la penetration de lumiere et de la temperature et l'introduction de C. pengoi, tous ces facteurs s'exercant dans un contexte global de disponibilite de nutriments. Les impacts des dreissenides etaient indirects. Des variables interactives combinant des mesures de la predation, des teneurs en nutriments, de la lumiere et de la temperature expliquaient le mieux la variabilite de la production zooplanctonique. [Traduit par la Redaction], Introduction Production of upper trophic levels in aquatic food webs hinges on the productivity of zooplankton in addition to benthos. Zooplankton support the growth of young-of-the-year fish as well as [...]

We investigated population responses of Mysis to ecosystem changes induced by invasion of dreissenids and predatory cladocerans, Cercopagis and Bythotrephes. Lake productivity declined as dreissenids invaded the offshore region. Whole-lake mysid biomass was compared before (early 1990s) and after (2002-2007) the invasion period; it declined 40%45%. Abundance of young mysids and presence of a summer cohort increased with summer, epilmnetic, nighttime zooplankton biomass (i.e., food biomass index). Cercopagis + Bythotrephes biomass was negatively correlated with this index, implicating them in the mysid decline. Eggs per gravid female increased with autumn, total-water-column zooplankton biomass, reflecting the greater use of hypolimnetic waters by adults. Reproductive success was below replacement during the period 2002-2005. First-year mysid growth rate was maintained while population abundance declined, suggesting selection for individuals that feed effectively at low food concentrations. Mortality rates in the first and second years were dependent on cohort density, indicating that competition for food limited abundance in the first 2 years. Fish predation indices (smelt and alewife combined) were correlated positively with mortality rates and negatively with abundance in the third year. Thus, mysids cannot support as many fish in invaded compared with non-invaded lakes. They may also not be a stable food resource; unusual cohort losses occurred in some years. Nous etudions les reactions demographiques des Mysis aux changements dans l'ecosysteme causes par l'invasion des dreissenides et des cladoceres predateurs, Cercopagis et Bythotrephes. La productivite lacustre diminue a mesure que les dreissenides envahissent le large. Nous comparons la biomasse des mysides dans le lac entier avant (debut des annees 1990) et apres (2002-2007) l'invasion; elle a decline de 40-45 %. L'abondance de jeunes mysides et la presence d'une cohorte d'ete sont en relation positive avec la biomasse nocturne et epilimnetique du zooplancton en ete (indice de biomasse alimentaire). La biomasse de Cercopagis + Bythotrephes est en correlation negative avec cet indice, ce qui les implique dans le declin des mysides. Les oeufs par femelle gravide augmentent avec la biomasse du zooplancton dans toute la colonne d'eau en automne, ce qui montre la plus grande utilisation des eaux de l'hypolimnion par les adultes. Le succes de la reproduction n'a pas atteint la valeur de remplacement en 2002-2005. Le taux de croissance des mysides durant leur premiere annee se maintient malgre le declin de l'abondance de la population, ce qui laisse croire qu'il existe une selection des individus qui s'alimentent de facon efficace aux faibles concentrations de nourriture. Les taux de mortalite (MR) durant la premiere et la seconde annee dependent de la densite de la cohorte, ce qui indique que la competition pour la nourriture limite l'abondance durant les deux premieres annees. Les indices de predation par les poissons (eperlans et gaspareaux reunis) sont en correlation positive avec MR et negative avec l'abondance durant la troisieme annee. Les mysides ne peuvent ainsi assurer le maintien d'autant de poissons dans les lacs envahis que dans les lacs non envahis. Ils peuvent aussi ne pas etre une source stable de nourriture; il se produit en effet des pertes inhabituelles de cohortes certaines annees. [Traduit par la Redaction], Introduction During the later part of the twentieth century, North American freshwater systems experienced a series of invasions by aquatic invasive species (AIS) capable of substantially altering the structure and [...]

We investigated population responses of to ecosystem changes induced by invasion of dreissenids and predatory cladocerans, and . Lake productivity declined as dreissenids invaded the offshore region. Whole-lake mysid biomass was compared before (early 1990s) and after (2002-2007) the invasion period; it declined 40%-45%. Abundance of young mysids and presence of a summer cohort increased with summer, epilmnetic, nighttime zooplankton biomass (i.e., food biomass index). Cercopagis + Bythotrephes biomass was negatively correlated with this index, implicating them in the mysid decline. Eggs per gravid female increased with autumn, total-water-column zooplankton biomass, reflecting the greater use of hypolimnetic waters by adults. Reproductive success was below replacement during the period 2002-2005. First-year mysid growth rate was maintained while population abundance declined, suggesting selection for individuals that feed effectively at low food concentrations. Mortality rates in the first and second years were dependent on cohort density, indicating that competition for food limited abundance in the first 2 years. Fish predation indices (smelt and alewife combined) were correlated positively with mortality rates and negatively with abundance in the third year. Thus, mysids cannot support as many fish in invaded compared with non-invaded lakes. They may also not be a stable food resource; unusual cohort losses occurred in some years. [ABSTRACT FROM AUTHOR]