Size-dependent variations in phytoplankton and protozoan community structure across the St. Lawrence River transition region

Abstract

We evaluated 2 contrasting perspectives on the St Lawrence Estuary maxlmum turb~dlty zone (MTZ) the f~r s t being that ~t is a zone of stress and mortal~ty for the plankton community, the second that it is a biologically productive area with a complex structured food w e b Two crulses were undertaken under different hydrolog~c reglmes one at the end of the spnng flood penod and the other durlng mean summer flow conditions The same pattern of change in conlmunlty structure of the phytoplankton and protozoa (2 to 200 pm) was observed across the saltwater transition in both cruises there was a shift in the size d~stnbutlon of tava towards dominance by larger cells and an increase In the number of taxa (including endemic taxa conf~ned to a specific salinity range) at the bottom of the transition zone There was no e v~d e n c e of a decline in evenness nor of a shlft towards a heterotrophbased food web that would result flom the mortality of autotrophs advected Into the MTZ These changes ~m p l y a prolonged residence tlme for the cells in the MTZ and a comb~natlon of imrnigratlon and emigration pathways that favours larger particles and high taxon dlverslty The iesults are conslstent wlth a 'hydrodynamic entrapment model Our observat~ons do not support the earller vlews that the St Lawrence MTZ IS a reglon of unfavou~able biolog~cal conditions Conversely, the large average cell slze combined w~t h a longer cell-retention time in this region may contnbute towards ~t s productlv~ty as a larval fish nursery

Trophic position of zebra mussel veligers and their use of dissolved organic carbon

“…The d 13 C values of the ETZ consumers are intermediate between those of the freshwater protists, .10-mm seston, and estuarine algae, and consequently these consumers may rely on a combination of these sources. The observations are also consistent with a previous study that found that the maximum turbidity zone of the ETZ acted as a retention zone wherein diatoms and larger particles advected from upstream and produced in situ were concentrated, resulting in a rich feeding zone characterized by high diversity and productivity (Frenette et al 1995). Aggregates and their associated organisms have been documented as an important food source for Eurytemora sp.…”

“…Some studies have argued for a dominant role of allochthonous carbon in sustaining food webs in rivers (McCallister et al 2004), whereas other authors have stressed the greater importance of autochthonous sources (Martineau et al 2004 and references therein). The St. Lawrence River ETZ has long been recognized as supporting high biological productivity and an important larval fish nursery, with large standing stocks of phytoplankton and zooplankton (Frenette et al 1995;Winkler et al 2003). In spite of the ecological importance of this ETZ, there has been no analysis of its overall food web structure.…”

“…These veligers are now recognized as the dominant zooplankton component, constituting between 52% and 90% of the total zooplankton counts in summer (Winkler et al 2005). The larvae are likely retained and concentrated in the ETZ by the same physical mechanisms that cause the high turbidity (Frenette et al 1995). There is now an extensive literature on the effects of adult zebra mussels in aquatic ecosystems; however, the ecological role of their abundant veliger stage has rarely been addressed.…”

Hydrological and biological processes modulate carbon, nitrogen and phosphorus flux from the St. Lawrence River to its estuary (Quebec, Canada)

“…This argument is justifiable given the high rate of mineralization taking place within the estuary (Tremblay and Gagné 2009) and the inclusion of organic N and P fractions highly valuable as substrate for microbial heterotrophs whose respiration further consumes O 2 . Primary production within the turbid transition zone of the upper estuary is light-limited (Levasseur et al 1984) and supports a rich heterotrophic microbiota (Frenette et al 1995). Calculation of the % contribution of SLR to total estuarine flux using TP (14%) and TN (47%) markedly Assuming that all TP exported to the estuary is mineralized within a year c Assuming that all TN exported to the estuary is mineralized within a year increased the importance or river enrichment to estuarine microbial production, making them all the more significant.…”

Advection of freshwater phytoplankton in the St. Lawrence River estuarine turbidity maximum as revealed by sulfur-stable isotopes

“…To explain this apparent paradox, Wofsy (1983) suggested the possibility of an exogenous input of algae, among other hypotheses. This is particularly relevant to the St. Lawrence ETM due to the presence of a hydrodynamic entrapment zone that increases the retention time of inorganic and organic particles because they sink into the landward bottom current and are subject to cyclonic recirculation (see Frenette et al 1995 ; Environnement Canada, Service Météorologique), which contribute to the advection of a large phytoplankton biomass (Vincent et al 1996) further downstream (i.e. towards the ETM).…”