Growth physiology and fate of diatoms in the ocean: a review

The index of the North Atlantic Oscillation, the dominant mode of climatic variability in the North Atlantic region, changed in the late 1980s (1987e1989) from a negative to a positive phase. This led to regime shifts in the ecology of the North Sea (NS) and the central Baltic Sea (CBS), which involved all trophic levels in the pelagial of these two neighbouring continental shelf seas. Increasing air and sea surface temperatures, which affected critical physical and biological processes, were the main direct and indirect driving forces. After 1987, phytoplankton biomass in both systems increased and the growing season was extended. The composition of phyto- and zooplankton communities in both seas changed conspicuously, e.g. dinoflagellate abundance increased and diatom abundance decreased in the CBS. Key copepod species that are essential in fish diets experienced pronounced changes in biomass. Abundance of Calanus finmarchicus (NS) and Pseudocalanus sp. (CBS) fell to low levels, whereas C. helgolandicus (NS) and Temora longicornis and Acartia spp. (CBS) were persistently abundant. These changes in biomass of different copepod species had dramatic consequences on biomass, fisheries, and landings of key fish species: North Sea cod declined, cod in the CBS remained at low levels, and CBS sprat reached unprecedented high biomass levels resulting in high yields. The synchronous regime shifts in NS and CBS resulted in profound changes in both marine ecosystems. However, the reaction of fish populations to the bottom-up mechanisms caused by the same climatic shift was very different for the three fish stocks.

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Synchronous ecological regime shifts in the central Baltic and the North Sea in the late 1980s

Wasmund, N., and Uhlig, S. 2003. Phytoplankton trends in the Baltic Sea. – ICES Journalof Marine Science, 60: 177–186.Monitoring data of phytoplankton abundance and biomass (1979–1999) and Chl. a (1979–2000) from surface samples (0–10m) of the Kattegat, Belt Sea and Baltic proper wereinvestigated for long-term trends. The Mann–Kendall test as well as the LOESS smootherwas applied for three taxonomic groups in spring, summer and autumn separately. Chl. atrends were analysed by linear regression. Downward trends were found for diatoms inspring and summer whereas dinoﬂagellates generally increased in the Baltic proper butdecreased in the Kattegat. In autumn, diatoms increased at some stations. For cyano-bacteria, downward trends were detected mainly in the Kattegat/Belt Sea area. Chl. aconcentrations showed a general decrease in the Kattegat/Belt Sea area but an increase inthe Baltic proper. Observed changes in trends during the two decades might indicate shiftsin the ecosystem.

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Phytoplankton trends in the Baltic Sea

The cellular silicon, nitrogen, and carbon content and the kinetics of silicic acid use were determined for Thalassiosira weissflogii grown under nutrient-replete, iron-deficient, and zinc-deficient conditions to assess the effect of metal deficiency on dlatom silicon metabolism. Iron- and zinc-deficient T weissflogi~ce lls contained 40 and 66% more silicon, respectively, than their nutrient replete counterparts. Low Zn and low Fe also increased cellular C and N content. Low Zn increased cellular carbon by 55 % and cellular N by 41 %. Low Fe increased cellular C and N by 68 and 45 %, respectively. Fe stress did not alter cellular Si/N ratios significantly, but Si/C ratios declined by 17%. In contrast, Zn stress increased SVC and Si/N ratios by 41 and 53%, respectively. Both Zn and Fe stress dramatically altered the kinetics of silica production by T weissflogii. Zn deficiency increased the half saturation constant (K,) 64 ",, and decreased the maximum specific uptake rate (V,,,) by 60%. In contrast, Fe stress did not affect the value of K,, but decreased V,,, by 66%, similar to the decrease observed under low Zn. The decrease in V,,,,, in Zn-deficient cells was almost entirely due to the higher biogenic silica content of the metal-deficient cells. The decline in V,,, under Fe stress resulted from both the increase in cellular silica content and a 50% decline in the cellular uptake rates for silicic acid. The results indicate that Fe and Zn availabihty can significantly alter silicification in diatoms and affect the number and efficiency of silicon transport molecules in the cell membrane.

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Effects of iron and zinc deficiency on elemental composition and silica production by diatoms

During the years 1993-1995 several research projects were carried out at the Tvärminne Zoological Station, SW Gulf of Finland, and at the southern part of the Gulf of Riga to study the structure and functioning of the pelagic ecosystem. These studies revealed several important aspects of the life-strategies of dominant species exemplified by the formation of deep resting populations and differential vertical migration of the micro­ organisms in the water column. A mesocosm experiment at the SW coast of Finland unveiled a high potential of Eu­ treptiella gymnastica (Euglenophyceae) to utilise pulsed nutrient inputs (e.g. through upwellings) and gain dominance in the phytoplankton community in the coastal waters. The success of the species was supported by high growth rate (1.03 divisions day-1) and vertical migration to deeper nutrient rich layers. However, the duration of Eutreptiella blooms is controlled by the high grazing losses (exponential grazing loss rate -0.60 day-1) by the ambient zooplankton populations, stressing the importance of the species in the food-chain. The potential for subsequent blooms of Eutreptiella gymnastica is apparently granted by the deposition of resting cysts to the benthic seed bank. It is hypothesised that blooms of E. gymnastica can have an impact on the vertical flux of particulate or­ ganic carbon (POC) through sedimentation of polysaccharide rich resting cysts and zooplankton fecal pellets packed with indigestible polysaccharide (paramylon) particles. Vertical migration of autotrophic micro-organisms was investigated on a diel basis in field conditions during two spring bloom periods (1994 at the SW coast of Finland; 1995 at the southern Gulf of Riga) and one midsummer period (1994 at the southern Gulf of Riga). The results indicate a significant vertical niche separation among the dominant species of vernal bloom dinoflagellates (Peridiniella catenata and Scrippsiella hangoei) in the Gulf of Finland. The temporal dynamics of vertical distribution of domi­ nant species in the Gulf of Riga revealed a species specific pattern which was modified by the environmental factors and a potential of several species to benefit from deep nu­ trient pools. The seasonal pattern in the vertical distribution of Mesodinium rubrum (an autotrophic ciliate) revealed strong surface accumulations during the peak of the vernal bloom, as opposed to nearly uniform occurrence down to 30 m depth during the post­ bloom period. This seasonal pattern was evident at both of the investigated areas. In 1993-1995 the phytoplankton community structure and sedimentation was studied on a seasonal basis in the southern part of the Gulf of Riga. The results indicate that phytoplankton sedimentation was low in midsummer and early autumn periods. In spring the sedimentation was dominated by the vertical flux of the dominant diatom species Thalassiosira baltica, which formed a deep resting population. The sedimentation of T. baltica was apparently not induced by nutrient depletion but rather by vertical stabili­ sation of the water column. It also demonstrated the different scenarios in the sedimen­ tation of phytoplankton, depending on whether the spring community is dominated by diatoms (high sedimentation) or dinoflagellates (low sedimentation).

C.O. Davis

Papers

Sinking as a factor affecting phytoplankton species succession: The use of selective loss semi-continuous cultures