Ten-year record of Thalassiosira nordenskioeldii population dynamics: comparison of aquaculture and non-aquaculture sites in the Quoddy Region

A long-term plankton monitoring programme resulted in a tell-year record (1 988-1997) Tor bloom timing and magnitude at four sites in the Quoddy Region . Two sites are located in areas removed from salmonid aquaculture operatio ns, whereas two sites were located close to an aquaculture site. However, a t one of these. cages were removed in 1994. Thalassiosira nordenskioeldii w as detected each spring during the months of April through June. Highest co ncentrations were observed during 1991 or 1992. with cell concentrations at ail four stations two to three times greater than during most other years. This diatom was more variable in its relative contribution to the phytopla nkton community at the non-aquaculture locations and rarely exceeded 60% at any of the stations. Trend analysis of relative abundance data revealed si gnificant positive trends only for the two aquaculture sites (Spearman/Lett enmaier test. slopes 1.40% and slope 0.79% yr(-1)). Concurrently, the aquac ulture site located in a sheltered bay also showed a significant negative t rend in salinity. The analysis suggests that T. nordenskioeldii has increas ed its contribution to the phytoplankton community at the sites located nea r aquaculture activity, one of which also became less saline. These changes may be related to aquaculture activities, although the causative factor(s) is yet unidentified. The trends suggest that this diatom is sensitive to c hanges in the local environment. (C) 2001 International Council for the Exp loration of the Sea.