Kaisa Kononen

Toxicity and isolation of the cyanobacterium Nodularia spumigena from the southern Baltic Sea in 1986

Three water bloom samples were collected in August 1986 from the southern Baltic Sea. Acute toxicity of the samples was determined by mouse bioassay and the toxins were further studied by HPLC. The bloom samples contained equal amounts of cyanobacteria Nodularia spumigena and Aphanizomenon flos-aquae and were hepatotoxic. Two hepatotoxic Nodularia spumigena strains were isolated from the samples. The isolates produce a toxic peak indistinguishable from the bloom material in the HPLC analysis. The toxicity of the fractions was verified by mouse bioassay. Thus the toxicity of the bloom samples was in all likelihood caused by Nodularia spumigena.

Copepod grazing in a summer cyanobacteria bloom in the Gulf of Finland

Blooms of the cyanobacteria Nodularia spumigena and Aphanizomenon flos-aquae dominated the phytoplankton assemblages of the western Gulf of Finland and the eastern side of the northern Baltic Sea in late July–August, 1992. The bloom overlapped the peak seasonal contributions of the dominant mesozooplankton herbivores in the region, the copepods Acartia bifilosa and Eurytemora affinis and the cladoceran Bosmina longispina maritima. Using radio-labelling techniques; the copepods were offered one of the cyanobacteria, Nodularia, as well as the 10–54 µm fraction of the natural phytoplankton assemblage. In general, incorporation rates of the labelled phytoplankton into the copepods declined with increasing contributions of the cyanobacteria. For both copepods, incorporation was inversely related to total phytoplankton biomass, whether measured as chlorophyll, total cells or cyanobacteria biomass. The very low rates for Acartia (< 0.8 µl [copepod h]−1) indicated that this copepod was likely starving in the cyanobacteria bloom, consistent with the generally poor condition of the animal observed in the laboratory. The other major mesozooplanktor, B. longispina maritima, ingested substantially more cyanobacterial biomass than the two copepods, based on HPLC-identified cyanobacteria-specific pigment echinenone in the gut. Bloom carbon provided < 1% and < 4% of the daily rations for Acartia and Eurytemora, respectively. Total copepod demand in the cyanobacteria blooms was trivial, < 1% of bloom biomass consumed daily. These results suggest that copepod herbivory is relatively unimportant in dissipating summer cyanobacteria blooms in the Gulf of Finland.

Physiological and community responses of summer plankton to nutrient manipulation in the Gulf of Finland (Baltic Sea) with special reference to phosphorus

Abstract Summer plankton communities from two locations in the central and western Gulf of Finland (Stns LL6 and JML, respectively), characterized by different nutrient dynamics, were manipulated with nitrogen and phosphorus additions in the laboratory. The aim was to study whether these populations would show indices of different phosphorus physiology or community structure. We were especially interested to determine whether the growth of diazotrophic filamentous cyanobacteria could be stimulated by phosphorus additions. No growth of these species was observed with daily addition of nutrients. The most evident population response was that of the small-size fraction organisms ( 10 µm the species favored by the nutrient additions were Dictyosphaerium subsolitarium van Goor in the central part and Snowella sp. in the western part of the Gulf. Phosphorus turnover rates measured by 32P-uptake were faster in the control and N-supplemented ...

Quasi-permanent density front at the entrance to the Gulf of Finland: response to wind forcing

Abstract A quasi-permanent density front separates the saltier water of the northern Baltic Proper spreading along the Estonian coast from the less saline waters originating from the Gulf of Finland. Daily mapping of the area with a high-horizontal-resolution towed CTD was performed over a 12-day period onboard R/VAranda in July 1993. Easterly winds (parallel to the front) cause offshore movement of denser water, as well as a stronger inclination of the front to the sea surface. With westerly winds, the less dense water moves onshore and overrides the denser water, thus forming a shallower upper mixed layer. Spatio-temporal changes in the frontal zone are tested by a simplified semi-empirical model, with consideration only to wind forcing. Deviations from the assumed relationship are explained by the along-front buoyancy advection.

Community development and modes of phosphorus utilization in a late summer ecosystem in the central Gulf of Finland, the Baltic Sea

The development of a filamentous, nitrogen-fixing cyanobacterial bloom was followed during July–August 1990 in a stratified basin in the central Gulf of Finland, Baltic Sea. Hydrography, dissolved inorganic, particulate and total nutrients, chlorophyll a, alkaline phosphatase activity, 32PO4-uptake and phytoplankton species were measured. The study period was characterized by wind-induced mixing events, followed by marked nutrient pulses and plankton community responses. Phosphate uptake was highest throughout the study period in the size fraction dominated by bacteria and picocyanobacteria (< 2 µm) and the proportion of uptake in the size fraction 2–10 µm remained low (2–6%). Higher phosphate turnover times were observed in a community showing signs of enhanced heterotrophic activity. The bloom of filamentous, nitrogen-fixing cyanobacteria Aphanizomenon flos-aquae was promoted by a nutrient pulse with an inorganic nutrient ratio (DIN:DIP) of 15. The results show that the quality, frequency and magnitude of the physically forced nutrient pulses have an important role in determining the relative share of the different modes of phosphorus utilization and hence in determining the cyanobacterial bloom intensity and species composition in the Baltic Sea.

Spatio-Temporal Dynamics of the Cyanobacterial Blooms in the Gulf of Finland, Baltic Sea

High-resolution on-line measurements of chlorophyll a, particles, salinity and temperature supplemented with stationary vertical casts of CTD, nutrients and phytoplankton, from August 1990 were used to investigate the spatio-temporal patterns of summer cyanobacterial blooms along the axis of the Gulf of Finland, the Baltic Sea. These measurements were supplemented with information about phytoplankton species composition and the vertical distributions of chlorophyll and nutrients while the ship was stationary. The results showed that the Gulf of Finland is divided into several sub-basins which are separated from each other by quasi-stationary fronts related to the bottom topography and circulation patterns of the Gulf. The basins and fronts had their own characteristic bloom dynamics — wind-induced external energy inputs controlled bloom formation within the sub-basins, whereas the vertical, front-related transport processes had an additional impact on the bloom in frontal areas. Nutrient reserves and inorganic N:P ratios below the seasonal thermocline differed between sub-areas.

Phytoplankton community in relation to salinity fronts at the entrance to the Gulf of Finland, Baltic Sea

Abstract Horizontal and temporal distributions of phytoplankton populations, including the most abundant mixotrophic and heterotrophic flagellates, were studied during a cyanobacterial bloom at the entrance to the Gulf of Finland. The study area was characterized by quasi-persistent salinity fronts. We examined the responses of different species to horizontal discontinuities in surface layer salinity, and compared their distributions. Several species showed significant (p<0.05) differences (analysis of variance and t-test) in abundance in different water masses. In the early phase of the bloom, the front had a marked effect on the spatial distribution of the filamentous cyanobacteria Nodularia spumigena Mertens and Aphanizomenon flos-aquae Ralfs ex Bernet et Flahault. Biomass decreased steeply at the low salinity side of the front. During later bloom stages, spatial separations of cyanobacterial bloom species disappeared. In the microplankton size group, the abundances of Dinophysis norvegica Claparede & ...