Due to the dynamic physical environment of the Lower St. Lawrence Estuary, the spring phytoplankton bloom in the Laurentian Channel occurs late in the season, typically in mid-June, but the high phytoplankton biomass is sustained throughout the summer months. In this study, relationships between the phytoplankton production cycle, water temperature, and the reproductive cycle of Calanus finmarchicus Gunnerus, a predominant planktonic copepod in the Lower Estuary, were investigated during spring-summer 1991. Field observations showed that the final stages of oocyte maturation in C. finmarchicus females did not begin until the onset of the spring phytoplankton bloom in midJune. High egg production rates, as estimated by the number of eggs released by females incubated immediately after capture, commenced 1 wk later and persisted until late August. Egg production rates were significantly correlated with an index of gonadal maturity in females and were consistent with a rectilinear or curvilinear relationship with chlorophyll a standing stock. Laboratory experiments showed that: (1) in presence of food (the diatom Thalassiosira weissfloggii), maturation of oocytes would proceed and females could spawn eggs at least 2 mo before the spring bloom; (2) without food, the oocytes did not develop past immature stages, except in a small minority of the population; and (3) colder temperatures in early spring would prolong the lag between the onset of the spring bloom and the start of egg production by less than 4 d. Combined with concurrent microscopic measurements of oil sac volume, the results do not rule out the possibhty that lipid reserves were used to support the early stages of oogenesis, but do show that the majority of females did not use lipid reserves for vitellogenesis prior to the spring phytoplankton bloom. It is suggested that the Lower St. Lawrence Estuary is an important region of C. finmarchicus production in summer which, because of the residual surface circulation, may act as a Calanus 'pump' to influence levels of zooplankton biomass in the Gulf of St. Lawrence and on the shelf off Nova Scotia.
Sinking of pigmented particulate matter during and immediately after an ice algal bloom was studied in southeastern Hudson Bay at a 45 m deep station, using consecutive short-term deployments of sediment traps 30 m below the undersurface of the ice. Cell sinking and grazing by herbivores were assessed by measuring the vertical fluxes of chlorophyll a and phaeopigments. There were 2 peaks of cell sedimentation, a minor one during the first half of the bloom (mid-April) and a major one at the end of the bloom (mid-May); both were related to atmospheric warming events. The fluxes of both phaeopigments and fecal pellets (mainly from large herbivores) increased at the end of the bloom as ice algae were released from the ice environment. The total export of ice algae to the benthos (sinking cells + fecal pellets) was estimated to be ca 20 % of the ice algal production during the measurement period. A large proportion of ice algal production (ca 30 %) was still suspended in the upper water column at the end of the sampling season. The fate of the remaining 50 '10 is not known, but it is hypothesized that a significant fraction was retained in the pelagic environment.
Due to the dynamic physical environment of the Lower St. Lawrence Estuary, the spring phytoplankton bloom in the Laurentian Channel occurs late in the season, typically in mid-June, but the high phytoplankton biomass is sustained throughout the summer months. In this study, relationships between the phytoplankton production cycle, water temperature, and the reproductive cycle of Calanus finmarchicus Gunnerus, a predominant planktonic copepod in the Lower Estuary, were investigated during spring-summer 1991. Field observations showed that the final stages of oocyte maturation in C. finmarchicus females did not begin until the onset of the spring phytoplankton bloom in mid-June. High egg production rates, as estimated by the number of eggs released by females incubated immediately after capture, commenced 1 wk later and persisted until late August. Egg production rates were significantly correlated with an index of gonadal maturity in females and were consistent with a rectilinear or curvilinear relationship with chlorophyll a standing stock. Laboratory experiments showed that: (1) in presence of food (the diatom Thalassiosira weissfloggii), maturation of oocytes would proceed and females could spawn eggs at least 2 mo before the spring bloom; (2) without food, the oocytes did not develop past immature stages, except in a small minority of the population; and (3) colder temperatures in early spring would prolong the lag between the onset of the spring bloom and the start of egg production by less than 4 d. Combined with concurrent microscopic measurements of oil sac volume, the results do not rule out the possibhty that lipid reserves were used to support the early stages of oogenesis, but do show that the majority of females did not use lipid reserves for vitellogenesis prior to the spring phytoplankton bloom. It is suggested that the Lower St. Lawrence Estuary is an important region of C. finmarchicus production in summer which, because of the residual surface circulation, may act as a Calanus 'pump' to influence levels of zooplankton biomass in the Gulf of St. Lawrence and on the shelf off Nova Scotia. KEY WORDS: Calanus. Egg production. Primary production cycle. Zooplankton
We use inverse optimization techniques and data from Landry (1978) to predict natural mortality rates in a population of the marine, planktonic copepod Acartia clausii. Predicted mortality rates are those that make the observed seasonal pattern of life-history characteristics evolutionarily stable. Our predictions closely approximate the rates observed by Landry. The results imply (1) that the inverse relationship between adult body size and temperature in A. clausii (also widely observed in other copepods and poikilotherms) is consistent with the hypothesis that it is an evolutionary adaptation to a seasonal environment, and (2) that demographic parameters, such as natural mortality rates, can be usefully estimated from life-history theory.
The planktonic copepod Calanus finmarchicus is a fundamental prey resource for the critically endangered North Atlantic right whale Eubalaena glacialis. Incorporation of prey information into E. glacialis decision support tools could improve management. Zooplankton time series are usually analyzed with respect to abundance, but predators such as E. glacialis forage based on whether prey aggregations exceed energetic thresholds. In order to better understand the distribution and dynamics of the high-abundance end of C. finmarchicus on the northeastern US continental shelf, where E. glacialis feed, we modeled the environmental conditions associated with C. finmarchicus densities that exceed nominal feeding thresholds. Threshold values were chosen based on a review of E. glacialis feeding behavior throughout the domain. Following model selection procedures, we used a random forest model with bathymetry, bottom temperature, bottom salinity, day of year, sea surface temperature, sea surface temperature gradient, bathymetric slope, time-integrated chlorophyll, current velocity gradient, and wind covariates. Model performance was highest with thresholds that matched reported E. glacialis feeding thresholds equivalent to 10000 copepods m-2. The high-density aggregations of C. finmarchicus had some different covariate responses compared to previous statistical abundance models, such as a warmer temperature range at both the surface and at depth, as well as a much higher degree of spatial variability. The output data layers of the model are designed to link with E. glacialis models used in US governmental decision support tools. Including this type of foraging information in decision support tools is a step forward in managing this critically endangered species.
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