Gesche Winkler

Heterogeneity within the native range: population genetic analyses of sympatric invasive and noninvasive clades of the freshwater invading copepod Eurytemora affinis

Invasive species are often composed of highly differentiated populations or sibling species distributed across their native ranges. This study analysed patterns of distribution and the evolutionary and demographic histories of populations within the native range of the copepod species complex Eurytemora affinis. Genetic structure was analysed for samples from 17 locations from both the invaded and native ranges in the St Lawrence River drainage basin, using 652 base pairs of the mitochondrial cytochrome oxidase subunit I gene. This study revealed a high degree of heterogeneity in genetic structure and habitat type in the native range, as well as a bias in the sources of invasive populations. Two genetically distinct clades showed a pattern of niche partitioning within the St Lawrence basin. The noninvasive North Atlantic clade primarily occupied the central portion of the St Lawrence Middle Estuary, whereas the invasive Atlantic clade was more prevalent along the margins, in the upstream reaches of the estuary and downstream salt marshes. Habitat partitioning and genetic subdivision was also present within the Atlantic clade. The freshwater populations were genetically more proximate to the Atlantic clade populations in the estuary than to those in the salt marsh, suggesting the estuary as the source of the invasive populations. The freshwater invading populations showed evidence of a modest population bottleneck. Populations from both clades showed genetic signatures of demographic population expansions that preceded the timing of the last glacial maximum, supporting the St Lawrence as a secondary contact zone between the two clades. Additional analyses on physiological and evolutionary properties of populations in the native range, along with analysis of the selection regime within native habitats, might yield insights into the evolutionary potential to invade.

Modelling stratification and baroclinic flow in the estuarine transition zone of the St. Lawrence estuary

Abstract This paper presents a hydrodynamic study of the St. Lawrence Estuarys estuarine transition zone, a 100 km region where fresh water from the river mixes with salt water from the estuary. The circulation of the estuarine transition zone is driven by strong tides, a large river flow, and well‐defined salinity gradients. For this study, a three‐dimensional hydrodynamic model was applied to the estuarine transition zone of the St. Lawrence Estuary and used to examine stratification and density‐driven baroclinic flow. The model was calibrated to field observations and subsequently predicted water level elevations, along‐channel currents, and salinity with mean errors of less than 9%, 11%, and 17%, respectively. The baroclinic density‐driven currents were distinguished from the tidal barotropic currents by using principal component analysis. Stratification and baroclinic flow were observed to vary throughout the estuarine transition zone on tidal and subtidal spring‐neap time scales. On a semidiurnal tidal time scale, stratification was periodic, and baroclinic flow was represented by pulses of sheared exchange flow, suggesting that neither buoyancy forcing nor turbulent mixing is dominant at this scale. On a subtidal spring‐neap time scale, stratification and baroclinic flow varied inversely with tidal energy, increasing on weak neap tides and decreasing on strong spring tides.

Life cycle, growth and reproduction of Neomysis americana in the St. Lawrence estuarine transition zone

Neomysis americana dominates the macrozooplankton of the St. Lawrence estuarine transition zone. Our aim was to determine how the growth and reproduction of N. americana may be controlled by environmental factors. The population was sampled fortnightly at Saint-Jean-Port-Joli from May to October 2013. A Bhattacharya cohort analysis was applied to the length–frequency data and the growth of each cohort was described by a von Bertalanffy function. Three cohorts were revealed. Slow growing females of the overwintering cohort produced the spring cohort in May. The summer cohort was released at the end of July producing juveniles of the new overwintering cohort in September and October. Two differential life history strategies were found: an overwintering cohort had an estimated longer life span (8–10 months), grew slowly, showed larger size at maturity and had a larger clutch size compared to those of the spring and summer cohorts. The clutch size was mainly influenced by female size and food quantity, unlike in many other studies, in which clutch size was found to be temperature dependant. These results concerning the life cycle, reproduction and growth of N. americana improve our understanding of the temporal dynamics of this important forage species of the St Lawrence estuarine transition zone.