Spatial and temporal variability of a dinoflagellate-cyanobacterium community under a complex hydrodynamical influence: A case study at the entrance to the Gulf of Finland

Abstract

Variability of nutrients and pelagic biological parameters (primary production and chlorophyll a [chl a] in flagellate and cyanobacterial size fractions, nitrogen fixation, phytoplankton species abundance) was followed for 12 d in July 1996 at an anchor station at the entrance to the Gulf of Finland. Simultaneously, meso-scale physical fields and plankton distribution were mapped over the surrounding 15 x 30 km area. The study period coincided with the intense blooming of a dinoflagellate Heterocapsa triquetra Ehrenberg and cyanobacterium Aphanizomenon flos-aquae (Linne) Ralfs community. A complex background of hydrodynamical processes was observed in the study area, including downwelling formation and development of an anticyclonic eddy and jet currents. Our hypothesis was that the horizontal scale of patches decreases and the variation of biological parameters increases when moving from the overall community level (chl a) to the size class level and further to the species level. The horizontal distribution of chl a was closely related to the different water masses, but the distribution of the 2 dominant species differed and showed high variability even within water masses. The temporal variability of the pelagic biological parameters at the anchor station (estimated by the coefficient of variation) was between 25 and 95% and it may be explained by horizontal patchiness. The results confirmed our hypothesis by showing that the coefficient of variation of summational parameters (total chl a, total primary production) was always lower than that of parameters specific to plankton size (chl a and primary production in <20 and >20 μm size classes), functional group (diazotrophs) or species. Phytoplankton in the size range equal to or greater than 20 pm exhibited particularly pronounced variability, while the smaller size fractions were less affected.