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

ABSTRACT. Variability of nutrients and pelagic biological parameters (pnmary 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 bloorning 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 downweiiing, formation and development of an anticyclonic eddy and Iet currents. Our hypothesis was that the horizontal scale of patches decreases and the variation of biological parameters increases when moving from the overall comrnunity level (chl a) to the size class level and further to the species level. The horizontal distnbution of chl a was closely related to the different water masses, but the distribution of the 2 dominant species differed and showed high vanability 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 surnrnational parameters (total chl a, total primary production) was always lower than that of parameters specific to plankton size (chl a and pnmary production in <20 and >20 pm 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.

No sign of denitrification in a Baltic Sea cyanobacterial bloom

“…Temporal comparisons of rates were not possible due to overall low or absent denitrification. The N 2 fixation rates measured were within the range of previously published rates from the Baltic Sea (Lindahl et al 1980, Lindahl & Wallström 1985, Niemistö et al 1989, Kononen et al 1999. Maximum N 2 fixation rates of approximately 5-fold (Niemistö et al 1989) as well as half the ones measured here (Kononen et al 1999) have been reported from the Baltic Sea.…”

“…The N 2 fixation rates measured were within the range of previously published rates from the Baltic Sea (Lindahl et al 1980, Lindahl & Wallström 1985, Niemistö et al 1989, Kononen et al 1999. Maximum N 2 fixation rates of approximately 5-fold (Niemistö et al 1989) as well as half the ones measured here (Kononen et al 1999) have been reported from the Baltic Sea. When comparing these results to previous studies, one has to bear in mind that the conversion factor from ethylene to N 2 (4:1 mol used in this study) can vary in natural conditions (Moisander et al 1996, Kononen et al 1999.…”

Bio-optical modelling of primary production in the SW Finnish coastal zone, Baltic Sea: fast repetition rate fluorometry in Case 2 waters

“…In summary, our study site is part of a non-steadystate oceanic system from an algal ecological point of view, with a pronounced spatial, vertical and temporal variability in phytoplankton species composition and biomass (Kononen et al 1999), thus making any attempt to resolve seasonal trends in biological activity extremely challenging.…”

Seasonal niche strategy of the bloom-forming dinoflagellate Heterocapsa triquetra

“…While Heterocapsa triquetra is not a toxic algal species, many of the seasonal and environmental factors that allow it to form dense (>10 6 l -1 ), nearly monospecific blooms in estuaries and coastal regions throughout the world (Kim 1997, Kononen et al 1999, Lindholm & Nummelin 1999) are important to most harmful species as well. H. triquetra blooms are nutrient-driven and are identified with seasonal coastal dynamics similar to those proposed to promote the formation of Alexandrium tamaranse blooms in NE US waters (Keafer & Anderson 1993).…”