MEPS 400:143-154 (2010)  -  DOI: https://doi.org/10.3354/meps08413

ABSTRACT: Nine stations were sampled across the St. Lawrence estuarine transition zone (ETZ) in summer 2006 in order to investigate the variability in Eurytemora affinis growth condition, as revealed by RNA:DNA ratios, and its relationship to the nutritional quality of seston, expressed in terms of fatty acid (FA) composition. The population of E. affinis in the St. Lawrence ETZ was under the influence of brackish waters, as well as fresh and saline waters landwards and seawards of the frontal mixing zone, respectively. The RNA:DNA ratio for E. affinis throughout the ETZ ranged from 2.14 to 23.01, with an average value of 10.30 ± 4.35. When compared with the previously reported RNA:DNA ratio for copepods under optimal growth conditions, our values suggest that the summertime population of E. affinis in the St. Lawrence ETZ was in good growth condition. The hydrodynamic imprint within the St. Lawrence ETZ translated into the distribution pattern of seston having various FA compositions. Freshwater seston was of better nutritional quality (i.e. richer in essential fatty acids) than that from brackish and more saline waters. However, this did not translate into a significantly different E. affinis RNA:DNA ratio between the 3 types of waters. Yet, the growth condition of E. affinis under the influence of fairly stable fresh waters was associated with the relative abundance of 20:5n3-rich seston advected from upstream despite the low contribution of chl a to total suspended particulate matter. Moreover, a comparison between seston and zooplankton FA composition revealed the potential trophic transfer of diatom-related FA from producers to consumers within fresh waters. No such correlations were evidenced within the most dynamic parts of the ETZ (brackish and more saline waters) where the relationship between E. affinis growth condition and seston quality was likely constrained by a spatial-temporal decoupling between seston and zooplankton.

KEY WORDS: Eurytemora affinis · RNA:DNA ratio · Fatty acids · Estuarine transition zone · St. Lawrence River