Jorma Kuparinen

Succession and growth limitation of phytoplankton in the Gulf of Bothnia (Baltic Sea)

A one year field study of four stations in the Gulf of Bothnia during 1991 showed that the biomass was ca. two times, and primary productivity ca. four times, lower in the north (Bothnian Bay) than in the south (Bothnian Sea) during the summer. Nutrient addition experiments indicated phosphorus limitation of phytoplankton in the Bothanian Bay and the coastal areas in the northern Bothnian Sea, but nitrogen limitation in the open Bothanian Sea. A positive correlation between the phosphate concentration and the production/biomass ratio of phytoplankton was demonstrated, which partly explained the differences in the specific growth rate of the phytoplankton during the summer. Differences in photosynthetic active radiation between the stations also showed a covariation with the primary productivity. The relative importance of nutrient or light limitation for photosynthetic carbon fixation could not, however, the conclusively determined from this study. Marked differences in phytoplankton species composition from north to south were also observed. The number of dominating species was higher in the Bothnian Sea than in the Bothnian Bay. The distribution of some species could be explained as due to nutrient availability (e.g. Nodularia spumigena, Aphanizomenon sp.), while salinity probably limits the distribution of some limnic as well as marine species. The potentially toxic phytoplankton N. spumigena, Dinophysis acuminata and Chrysochromulina spp. were common in the Bothnian Sea but not in the Bothnian Bay. The pico- and nanoplankton biomass during late summer was higher than previously reported due to a revised carbon/volume ratio.

Growth and herbivory by heterotrophic dinoflagellates in the Southern ocean, studied by microcosm experiments

Growth and herbivory of heterotrophic dinoflagellates (Gymnodinium sp.) from the Weddell Sea and the Weddell/Scotia Confluence were studied in 1988 in 100-liter microcosms. The microcosms were screened through 200-µm or 20-µm mesh nets and incubated for 12 d at 1 °C under artificial light. Mean cell volume of dinoflagellates was 1 000 to 1 500µm3, and that of their phytoplankton prey 360 to 430µm3. Dinoflagellate growth rate followed a Holling type II functional response, with a maximum growth rate of 0.3 d−1 and half-saturation food concentrations of 1.0µg chlorophylla l−1, 50µg C l−1, or 1 500 cells ml−1. Carbon budgets based on14CO2 assimilation and biomasses of phytoplankton and heterotrophic dinoflagellates suggested a balance between phytoplankton grazing loss and dinoflagellate consumption, assuming a dinoflagellate carbon conversion efficiency of 40%. Applying this to the functional response yielded estimates of maximum ingestion rate (0.8µg Cµg−1 C d−1, or 6 pg C dinoflagellate−1 h−1) and maximum clearance (0.8 to 1.2 × 105 body volumes h−1, or 80 to 120 nl ind.−1 h−1). The microcosm experiments suggested that heterotrophic dinoflagellates may contribute significantly to maintenance of low phytoplankton biomass in the Southern Ocean.

Baltic Sea cyanobacterial bloom contains denitrification and nitrification genes, but has negligible denitrification activity

Abstract A cyanobacterial bloom in the Gulf of Finland, Baltic Sea, was sampled throughout the development and senescence of aggregates in August 1999. While conditions inside the aggregates were favourable for denitrification (rich in nitrogen and carbon, with anoxic microzones), essentially none was detected by a sensitive isotope pairing method. Polymerase chain reaction-based methods, targeting functional genes encoding the key enzymes of denitrification and nitrification processes (nirS, nirK, amoA), revealed that the non-aggregated filaments harboured amoA gene fragments with high similarity to Nitrosospira amoA sequences, as well as both types of nitrite reductase genes, nirS and nirK. Only the nirS-type nitrite reductase gene and no amoA was detected in aggregated filaments. Thus, despite optimal environmental conditions and genetic potential for denitrification, the blooms of filamentous nitrogen-fixing cyanobacteria must be seen solely as a source, and not as a sink of nitrogen in the Baltic Sea.

Bottom-up and top-down controls of the microbial food web in the Southern Ocean: experiments with manipulated microcosms

We have studied bottom-up and top-down control of the Southern Ocean microbial food web by microcosm experiments. Water from the Weddell Sea and Weddell Scotia Confluence were used for the experiments. Microcosms were manipulated by nutrients and light, and by size-selective screening. Incubation at the higher light level doubled phytoplankton growth rates from 0.12 to 0.24 day-1 in the Weddell experiment and from 0.15 to 0.30 day-1 in the Confluence experiment. Nutrient enrichment had no significant effect on growth rates in either of the experiments, indicating that phytoplankton growth was not nutrient-limited. In the microcosms where dinoflagellate growth rate was different, high dinoflagellate numbers were reflected as depressed nanoflagellate growth as well as depressed growth of phytoplankton, suggesting that dinoflagellates controlled both heterotrophic nanoflagellates and autotrophic nanoplankton. Only during short periods, when dinoflagellate numbers were low, could exponential growth of nanoflagellates be demonstrated. Bacterioplankton growth rates were, on average, 0.26 day-1 in the Weddell experiment and 0.22 day-1 in the Confluence experiment. Bacteria were controlled by heterotrophic nanoflagellates. Potential growth rates up to 0.75 day-1 were measured from batch cultures without predators. With the microcosm experiments, we could demonstrate a strong top-down control by dinoflagellates on phytoplankton and on heterotrophic nanoflagellates, and a control by heterotrophic nanoflagellates on bacteria. We could also demonstrate weak nutrient limitation on autotrophs and substrate limitation on heterotrophic bacteria. In the two study areas, biomass production and carbon flow were mediated mainly by organisms that passed through a 20 μm net and had growth rates in the order of 0.20 to 0.30 day-1.

Spatial distribution of bacterioplankton production across the Weddell-Scotia Confluence during early austral summer 1988-1989*

SummaryWe applied two methods to measure bacterio-plankton production, the [3H]-thymidine (TTI) and the [3H]-leucine (LEU) incorporation into cold trichloro-acetic acid precipitate. Both methods gave similar results of the distribution of production in time and space (r2=0.82, n=66). Using empirically determined conversion factors the TTI gave production values from 21 to 125 mg Cm−2 day−1, which are within the range reported earlier from the Southern Ocean. Highest production rates were associated with the open water in the Confluence area (59°S–60°S) and with the Scotia Sea front. Low production rates were recorded from the ice covered areas in the Weddell Sea and in the open Scotia Sea waters. Good correlation on an areal basis was found between bacterioplankton production and other measures of heterotrophy, including ETS (r2=0.93, n=9) and NH4(r2=0.50, n=21). Good correlation was also found between bacterioplankton and phytoplankton production (r2=0.63, n=19). Bacterioplankton production seems to be driven by products from photosynthesis and heterotrophic processes, most likely grazing, which are tightly coupled to autotrophy. Quantitatively, bacterioplankton production was on an average 11 % of net primary production, which is clearly a lower value than the 30% based on a review from temperate freshwater and marine ecosystems, but is comparable with values reported from the spring period in subarctic ecosystems. In comparison with the measurements of ETS, bacterioplankton contribution to community respiration was also lower than predicted from results from temperate ecosystems. We concluded from these results and the results obtained from microcosm experiments (Bjørnsen and Kuparinen 1991b) that the flux of organic matter to eucaryote heterotrophs via bacterioplankton during spring and early summer periods in the Southern Ocean is of considerable, but not of equivalent importance as in temperate waters.