Environmental variability as a factor controlling spatial patterns in distribution and species diversity of zooplankton in the St. Lawrence Estuary

Abstract

Field studies on a variety of organisms have suggested that environmental variability plays a major role in determining spatial patterns in distribution and species diversity of estuanne organisms due to the effect of abiotic fluctuations on the physiology of animals. However, there is no study examining the effect of environmental variability on zooplankton distribution and diversity in estuaries. As vertical m~gration is obligatory behavior for the retention of planktonic animals at intermediate positions in the St. Lawrence Estuary (Canada), vertical stratification of the water column is considered the major source of environmental variability for zooplankton in this system. To evaluate the importance of this source of variability as a factor controlling the distribution and diversity of zooplankton in the estuary, we examined the relative contribution of each of the environmental factors of salinity, temperature, turbidity and vertical stratification in explaining spatial patterns of summer zooplankton distribution and diversity. Multivariate analyses revealed the presence of a longitudinal succession of seasonally stable species assemblages (tidal freshwater, true-estuarine and euryhalinemarine assemblages) whose spatial distribution was mainly a function of salinity and vertical stratification. Turbidity and temperature played a minor role in explaining spatial distribution. For all sampling periods, the limit between the true-estuarine and euryhaline-marine assemblages, the lowest number of population centers and the lowest zooplankton abundance all corresponded to the most vertically stratified waters. In contrast, population centers of all species were concentrated in the most abiotically stable parts of the estuary. We conclude that environmental variability is a major factor determining zooplankton distribution and diversity in the estuary. The spatio-temporal stability of the species assemblages and the seasonal variabihty in the abundance of some species also suggest that trophic interactions may play an important role in the regulation of zooplankton populations in the estuary. KEY WORDS: Zooplankton assemblages. Salinity. Turbidity. Temperature. Spatio-temporal structure 0 Inter-Research 1994 Resale of fuLJ article not permitted

Responses of crustacean zooplankton to seasonal and tidal salinity changes in the coastal Lake Waihola, New Zealand

“…Many estuaries and systems experiencing substantial intrusions of sea water show transitions from freshwater to euryhaline-marine zooplankton assemblages along salinity gradients (Laprise & Dodson 1994;Lopes 1994). Similarly, in Lake Waihola there was a transition from dominance by the euryhaline copepod G. pectinatus during high salinity periods, to dominance by the cladoceran D. carinata during fresher periods.…”

“…Salinity gradients in estuaries from fresh water to near sea water promote a transition from freshwater to euryhaline-marine zooplankton assemblages, as the degree of marine influence, or salinity, increases (Laprise & Dodson 1994;Lopes 1994). Seasonal changes in salinity, possibly associated with changes in freshwater inflow, may lead to similar transitions over time in the abundance and taxonomic composition of zooplankton communities (Kott 1955;Bayly 1965;Nagaraj 1992).…”

La diversité, la répartition et l’écologie du complexe d’espèces cryptiques Euytemora affinis, dans la zone d’alevinage de l’estuaire moyen du Saint-Laurent

“…Dans la ZTE, la communauté zooplanctonique est composée de 3 assemblages distincts, stables au fil des saisons et dont la répartition spatiale dépend essentiellement de la salinité et de la stratification verticale (Bousfield et collab., 1975;Laprise et Dodson, 1994;Winkler et collab., 2003;Winkler et collab., 2005 (Bousfield et collab., 1975;Laprise et Dodson, 1994;Winkler et collab., 2003). Même si Eurytemora affinis domine seulement l'assemblage estuarien, ce complexe d'espèces cryptiques est aussi présent, mais en moins grande densité, tout au long de la zone de transition estuarienne (Winkler et collab., 2008;Favier et Winkler, 2014).…”

“…Ce phénomène a par ailleurs déjà été modélisé dans la ZTE pour les larves de moules zébrées (Dreissena polymorpha : Simons et collab., 2006). Toutefois, outre les capacités natatoires, (Jeffries, 1962;Laprise et Dodson, 1994). Néanmoins, cette définition ne prend pas en considération les différences potentielles entre les clades de ce …”

“…Par ailleurs, la diminution des populations d'E. affinis (tous clades confondus) durant la période estivale, suivie de l'augmentation de celle-ci à l'automne, semble liée à la dynamique des prédateurs (Laprise et Dodson, 1994). Les prédateurs les plus importants du zooplancton dans la ZTE du Saint-Laurent sont les 2 espèces sympatriques de mysidacés, Mysis stenolepis et Neomysis americana.…”