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.
Johnson, C. L., Leising, A. W., Runge, J. A., Head, E. J. H., Pepin, P., Plourde, S., and Durbin, E. G. 2008. Characteristics of Calanus finmarchicus dormancy patterns in the Northwest Atlantic. – ICES Journal of Marine Science, 65: 339–350. Demographic time-series from four fixed stations in the Northwest Atlantic Ocean demonstrate variable timing of entry into and emergence from dormancy in subpopulations of the planktonic copepod Calanus finmarchicus. A proxy for timing of entry was established as the date each year when the proportion of the fifth copepodid stage (CV) in the subpopulation rose to half its overall climatological maximum CV proportion at that station. The proxy for timing of emergence at each station was set as the first date when adults were more than 10% of the total abundance of copepodid stages. An alternate emergence proxy date was determined by back-calculating the spawning dates of the first early copepodid stages appearing in spring, using a stage-structured, individual-based model. No single environmental cue (photoperiod, surface temperature, or average surface-layer chlorophyll a concentration) consistently explained entry or emergence dates across all stations. Among hypotheses put forward to explain dormancy in Calanus species, we cannot eliminate the lipid accumulation window hypothesis for onset of dormancy or a lipid-modulated endogenous timer controlling dormancy duration. The fundamental premise of these hypotheses is that individuals can only enter dormancy if their food and temperature history allows them to accumulate sufficient lipid to endure overwintering, moult, and undergo early stages of gonad maturation.
Calanus glacialis and Calanus finmarchicus dominate the zooplankton community in the Arctic and North Atlantic, respectively. The vast zone of sympatry between these species, the potential for overlap between reproductive seasons, and the evidence for intermediate values of discriminant traits suggest that these species hybridize. We genotyped 684 individuals from 14 Arctic to Atlantic stations using one mitochondrial (16S) and 10 nuclear loci (microsatellites). Strong genetic differentiation between parental species confidently identified hybrids in areas of sympatry. Hybrid frequency was highly variable among stations and did not covary with mean annual sea surface temperature. In the St. Lawrence Estuary, parental and hybrid genotypes were nonrandomly distributed between depth layers (300-100 m and 100-0 m) and across sampling dates, and hybrids seemed more frequent in July than in May and September. Overall, the bimodal frequency distribution of parental and hybrid genotypes suggests that reproductive barriers limit gene flow between these species. The opportunity for interbreeding is more likely restricted by differences in species reproductive phenology than by dispersal. Hybridization also affects prosome length, a morphological trait widely used to discriminate Calanus species, and thus possibly contributes to species misidentification. Highly introgressed individuals indicate that hybrids are fertile and reproduce, suggesting that hybrid fitness could affect estimates and models of these species population dynamics. This is the first evidence for interspecific hybridization between marine zooplankton species, but similar cases could be uncovered using nuclear genetic markers in groups of closely related and morphologically similar marine zooplankton species.
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