Seasonal variations of contribution of large and fine fractions of phytoplankton to primary production and chlorophyll a in the Sevastopol Bay from April 1985 till March 1986

It has been shown that in the Sevastopol Bay during the year primary production and chlorophyll "a" created by picoplankton (0.45-2.5 µm) consisted on the average 20-44% of total production. It was approximately a half of the level for oligotrophic waters of the ocean. Picoplankton of waters studied is represented by eucaryotes, cell diameter of which is, as a rule, about 2-3 µm. Contribution of the finest fraction of phytoplankton (0.43-0.85 µm) to primary production and con¬tent of chlorophyll "a" was insignificant (0-4%).

(Table 1) Biovolume variations of cyanobacteria and total phytoplankton in the Frederiksborg Slotsso, Denmark

Several studies have shown that submerged macrophytes provide a refuge for zooplankton against fish predation, whereas the role of emergent and floating-leaved species, which are often dominant in eutrophic turbid lakes, is far less investigated. Zooplankton density in open water and amongst emergent and floating-leaved vegetation was monitored in a small, eutrophic lake (Frederiksborg Slotsso) in Denmark during July-October 2006. Emergent and floating-leaved macrophytes harboured significantly higher densities of pelagic as well as plant-associated zooplankton species, compared to the open water, even during periods where the predation pressure was presumably high (during the recruitment of 0+ fish fry). Zooplankton abundance in open water and among vegetation exhibited low values in July and peaked in August. Bosmina and Ceriodaphnia dominated the zooplankton community in the littoral vegetated areas (up to 4,400 ind/l among Phragmites australis and 11,000 ind/l between Polygonum amphibium stands), whereas the dominant species in the pelagic were Daphnia (up to 67 ind/l) and Cyclops (41 ind/l). The zooplankton density pattern observed was probably a consequence of concomitant modifications in the predation pressure, refuge availability and concentration of cyanobacteria in the lake. It is suggested that emergent and floating-leaved macrophytes may play an important role in enhancing water clarity due to increased grazing pressure by zooplankton migrating into the plant stands. As a consequence, especially in turbid lakes, the ecological role of these functional types of vegetation, and not merely that of submerged macrophyte species, should be taken into consideration.

(Table 1) Results of measurements of productive characteristics of phytoplankton and environmental parameters in the Barents Sea in September-October 1997

The described studies were carried out in the eastern part of the sea during the end of the summer seasonal succession from September 1 to October 12, 1997. Concentration of chlorophyll a in the surface layer varied from 0.09 to 1.24 mg/m**3; it tended to increase in the southern regions (<74°N). Primary production in the water column (P_p) varied from 24 to 214 mg C/m**2/day and was on average 91 mg C/m**2/day. The low level of P_p seems to result from combination of physical and chemical environmental factors unfavorable for photosynthesis (e.g. deficiency of nutrients and low values of insolation and temperature) and intensive grazing of phytoplankton by zooplankton. The lower boundary of the photosynthetic layer in open waters was located at depth 60-75 m; irradiance there was 0.1-0.5% of incident irradiance. In deep-water regions (>200 m) the subsurface maximum of chlorophyll occurred in the layer at 20-40 m; usually this maximum resulted in formation of additional maxima of primary production.

Siliceous phytoplankton in middle Eocene sediments of ODP Hole 207-1260A

The Middle Eocene diatom and silicoflagellate record of ODP Site 1260A (Demerara Rise) is studied quantitatively in order to throw light on the changes that siliceous phytoplankton communities experienced during a Middle Eocene warming event that occurred between 44.0 and 42.0 Ma. Both Pianka's overlap index, calculated per couple of successive samples, and cluster analysis, point to a number of significant turnover events highlighted by changes in the structure of floristic communities. The pre-warming flora, dominated by cosmopolitan species of the diatom genus Triceratium, is replaced during the warming interval by a new and more diverse assemblage, dominated by Paralia sulcata (an indicator of high productivity) and two endemic tropical species of the genus Hemiaulus. The critical warming interval is characterized by a steady increase in biogenic silica and a comparable increase in excess Ba, both reflecting an increase in productivity. In general, it appears that high productivity not only increased the flux of biogenic silica, but also sustained a higher diversity in the siliceous phytoplankton communities. The microflora preserved above the critical interval is once again of low diversity and dominated by various species of the diatom genus Hemiaulus. All assemblages in the studied material are characterized by the total absence of continental and benthic diatoms and the relative abundance of neritic forms, suggesting a transitional depositional environment between the neritic and the oceanic realms.

Water chemistry and iron speciation in samples taken during SWEDARP 97/98 campaign with S.A. Agulhas to the Southern Ocean

The distribution and speciation of iron was determined along a transect in the eastern Atlantic sector (6°E) of the Southern Ocean during a collaborative Scandinavian/South African Antarctic cruise conducted in late austral summer (December 1997/January 1998). Elevated concentrations of dissolved iron (>0.4 nM) were found at 60°S in the vicinity of the Spring Ice Edge (SIE) in tandem with a phytoplankton bloom, chiefly dominated by Phaeocystis sp. This bloom had developed rapidly after the loss of the seasonal sea ice cover. The iron that fuelled this bloom was mostly likely derived from sea ice melt. In the Winter Ice Edge (WIE), around 55°S, dissolved iron concentrations were low (<0.2 nM) and corresponded to lower biological productivity, biomass. In the Antarctic Polar Front, at approximately 50°S, a vertical profile of dissolved iron showed low concentrations (<0.2 nM); however, a surface survey showed higher concentrations (1-3 nM), and considerable patchiness in this dynamic frontal region. The chemical speciation of iron was dominated by organic complexation throughout the study region. Organic iron-complexing ligands ([L]) ranged from 0.9 to 3.0 nM Fe equivalents, with complex stability log K'(FeL) = 21.4-23.5. Estimated concentrations of inorganic iron (Fe') ranged from 0.03 to 0.79 pM, with the highest values found in the Phaeocystis bloom in the SIE. A vertical profile of iron-complexing ligands in the WIE showed a maximum consistent with a biological source for ligand production and near surface minimum possibly consistent with loss via photodecomposition. This work further confirms the role iron that has in the Southern Ocean in limiting primary productivity.