Kristiina Vuorio

Phytoplankton in Lake Tanganyika : vertical and horizontal distribution of in vivo fluorescence

Determinations of chlorophyll a and in vivo fluorescence of photosynthetic pigments were used to study vertical and horizontal distribution of phytoplankton in Lake Tanganyika (East Africa). Blue excited fluorescence (IVFb) was an approximate predictor of chlorophyll a at different depths and locations. Green excited fluorescence (IVFg), which reflects phycoerythrin in cyanobacteria, explained chlorophyll a variation equally well, and in combination with IVFb the degree of explanation was improved to 87% (n = 90). Particularly during the shallow stratification in March–May, the maxima of chlorophyll a, IVFb and IVFg were located within the thermocline. Such distribution may have resulted from the high penetration of UV light, often accentuated by very shallow daytime thermal stratification, leading to inhibition of phytoplankton near the surface. Because the decrease of chlorophyll a specific IVFb was less striking towards the surface, the decrease of IVFb was not caused by light inhibition only. In October–November, epilimnetic IVFb and chlorophyll a values seemed to be consistently higher than in April–May and often showed remarkable patchiness. The sometimes very dense phytoplankton blooms (Anabaena sp., Cyanobacteria) observed in the central and southern parts of the lake, suggest that local upwelling or mixing events may be important for the development of phytoplankton in Lake Tanganyika.

The stoichiometry of particulate nutrients in Lake Tanganyika — implications for nutrient limitation of phytoplankton

We studied the potential nutrient limitation of phytoplankton by means of seston nutrient stoichiometry and nutrient enrichment bioassays in the epilimnion of Lake Tanganyika. In most cases, the particulate carbon to phosphorus (C:P) ratio was high and indicated moderate P deficiency, while the respective C:N ratio mainly suggested moderate N deficiency. The N:P ratios of seston indicated rather balanced N and P supply. In three two-day enrichment bioassays in April–May 1995, a combined addition of P, N and organic carbon (glucose) always increased primary production in comparison to untreated controls. Primary production also slightly increased after the addition of phosphate-P, while the additions of single ammonium-N and glucose had no effect. Although the measured turnover time of P was short and our few nutrient enrichment experiments suggested that P may be the most limiting single nutrient, the particulate nutrient ratios and the strong stimulation of primary production after the combined addition of P and N mostly suggest that in the upper epilimnion of Lake Tanganyika plankton experience a restricted, but approximately balanced nutrient supply.

What factors control planktonic ciliates during summer in a highly eutrophic lake

A mesocosm experiment in 24 enclosures (6 m3) started at the end of June 1996 in a highly eutrophic shallow lake, Lake Köyliönjärvi (SW Finland). The original factorial design with nutrient, fish and macrophyte treatments was lost due to strong winds causing leakages. However, after the walls were made leak-proof again on July 11, the planktonic communities developed in divergent ways. On July 31 there was a tenfold variation in total crustacean biomass among the enclosures and the lake (40.2–417.5 μg C l−1), and chlorophyll a varied from 9.5 to 67.0 μg l−1. Here, the single-day data on the 25 planktonic communities is analysed by means of correlation and factor analysis in order to identify factors controlling the protozoans, with particular emphasis on ciliates. The data set comprised: total phosphorus, nitrogen, chlorophyll, bacteria, autotrophic picoplankton, heterotrophic flagellates, abundance and species composition of ciliates, phytoplankton and metazooplankton. The results indicate that although the total ciliate abundance (ranging from 16.2 to 95.0 ind l−1) was controlled by food resources, the observed differences in ciliate community structure could be attributed partly to differential predation by metazooplankton. The effect of Daphnia cucullata, the dominant daphnid cladoceran, was stronger than that of other metazoans.

Spatial Distribution of Phytoplankton and Picocyanobacteria in Lake Tanganyika in March and April 1998

The spatial distribution of phytoplankton and picocyanobacteria was studied in Lake Tanganyika in relation to environmental factors. Sampling was conducted within three weeks during the wet season of 1998 when the water column stratification was most stable. The secchi depth varied between 11 and 16 m, except off the river Malagarasi (3 m). The depth of the euphotic zone was 33 to 56 m. Altogether, 218 phytoplankton taxa were identified. Their total biomass varied between 13 and 88 mg m m 3 fresh weight. Picocyanobacteria were present at very high densities (10 4 to 6 2 10 5 cells ml m 1 ). The influence of the river Malagarasi appeared to spread along the water surface because of the lower density of the river water. The phytoplankton species number and biomass were particularly high near the river mouth. Excluding that area, there were no statistically significant differences in the species composition among study sites. Nevertheless, differences in water column mixing, indicated by variable temperature gradients, seemed to affect the horizontal distribution of at least some phytoplankton species.

Size-Fractionated δ 15 N and δ 13 C Isotope Ratios Elucidate the Role of the Microbial Food Web in the Pelagial of Lake Tanganyika

Food web structure of the pelagic community in Lake Tanganyika was studied using the stable nitrogen and carbon isotopes 15 N and 13 C. Size-fractionated seston, zooplankton, shrimps, medusae and fish were sampled in the northern part of Lake Tanganyika. Picoplankton fractions as well as cyanobacteria-dominated nano/microplankton fractions had very low nitrogen isotope signatures typical for nitrogen-fixing organisms. Fractions containing mainly dead organic matter (and associated bacteria) or nano/microalgae (chlorophytes and diatoms) had i 15 N 2 to 4 higher. The low i 15 N signatures of small cyclopoids and shrimps suggest they are feeding on nitrogen-fixing cyanobacteria (picoplankton or larger forms), while the higher i 15 N signature of larger copepods suggest mixed feeding on large algae and small zooplankton and/or cyanobacteria. Medusae were slightly enriched in i 15 N relative to large copepods. Among fish, the signatures of Stolothrissa and small Lates stappersi suggested feeding on large copepods, while Limnothrissa and larger Lates were slightly more enriched, indicating partial piscivory. The enrichment of 13 C between the putative trophic levels (2 to 3) was higher, while that of 15 N (2 to 3) was lower, than usual in isotope studies. Our results indicate that picocyanobacteria and possibly also larger cyanobacteria are important producers in the pelagic food web of Tanganyika.