Jouko Sarvala

Trophic structure of Lake Tanganyika: carbon flows in the pelagic food web

The sources of carbon for the pelagic fish production in Lake Tanganyika, East Africa, were evaluated in a comprehensive multi-year study. Phytoplankton production was assessed from seasonal in situ 14C and simulated in situ results, using on-board incubator measurements and knowledge of the vertical distributions of chlorophyll and irradiance. Bacterioplankton production was measured on two cruises with the leucine incorporation method. Zooplankton production was calculated from seasonal population samples, the carbon contents of different developmental stages and growth rates derived from published sources. Fish production estimates were based on hydroacoustic assessment of pelagic fish biomass and data on growth rates obtained from length frequency analyses and checked against daily increment rings of fish otoliths. Estimates for primary production (426–662 g C m-2 a-1) were 47–128% higher than previously published values. Bacterioplankton production amounted to about 20% of the primary production. Zooplankton biomass (1 g C m-2) and production (23 g C m-2 a-1) were 50% lower than earlier reported, suggesting that the carbon transfer efficiency from phytoplankton to zooplankton was low, in contrast to earlier speculations. Planktivorous fish biomass (0.4 g C m-2) and production (1.4–1.7 g C m-2 a-1) likewise indicated a low carbon transfer efficiency from zooplankton into planktivorous fish production. Relatively low transfer efficiencies are not unexpected in a deep tropical lake, because of the generally high metabolic losses due to the high temperatures and presumably high costs of predator avoidance. The total fisheries yield in Lake Tanganyika in the mid-1990s was 0.08–0.14% of pelagic primary production, i.e. within the range of typical values in lakes. Thus, no special mechanisms need be invoked to explain the productivity of fisheries in Lake Tanganyika.

Interannual variation of growth and recruitment in Pontoporeia affinis (Lindström) (Crustacea: Amphipoda) in relation to abundance fluctuations

Marked and regular changes in growth and recruitment were found to accompany abundance fluctuations of the amphipod Pontoporeia affinis (Lindstrom) in a northern Baltic coastal area. Seasonal series of quantitative macrobenthos samples were collected in 1967–1970 from different depths and archipelago zones near Tvarminne Zoological Station, southwestern Finland. Strong interannual fluctuations in the abundance of P. affinis, consistent with earlier long-term studies in the area, were observed at a 34-m soft-bottom site: the numbers declined from 1967 to a minimum of 1800 ind. · m−2 in 1968, and then increased to a maximum of 27600 ind. · m−2 in 1970. During the decline, relative length growth rates were low and population mortalities high, while the maximum growth rate accompanied by low mortality coincided with the population minimum in 1968. Differences in recruitment were even more pronounced. Although the age group due to reproduce was only 1.4 times more abundant in autumn 1969 than in autumn 1967, the population of young released by these animals in spring 1970 was 10.8 times that released in spring 1968; recruitment in spring 1970 was close to the maximum possible. These variations of recruitment were not correlated with temperature or primary production. A model is proposed for the regulation of P. affinis abundance. At high abundance levels, intraspecific competition for food may lead to overgrazing of the microbial food and sediment organic matter. The ensuing impaired growth and reproduction in turn lead to low numbers, which again allows recovery of the food source, good growth and successful reproduction. Thus, density-dependent success of recruitment may cause the cyclic changes of P. affinis abundance.

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.

Effect of Wind Induced Water Movements on Nutrients, Chlorophyll- a , and Primary Production in Lake Tanganyika

We determined the spatial and temporal distribution of major plant nutrients, dissolved oxygen, chlorophyll- a and primary production, at three deep-water locations in Lake Tanganyika to infer patterns of water displacements and to interpret the impact of wind-generated changes in nutrients on the timing and magnitude of phytoplankton production. In the southern end of the lake, the onset of cooler, windier conditions during May-September caused stratification weakening, tilting of isotherms and upwelling of dissolved inorganic nitrogen and soluble reactive phosphorus from nutrient-rich deeper waters. These factors coincided with an increase in chlorophyll- a concentrations but not with clear evidence of increased primary production. An internal seiche at that time seemed associated with similar but less pronounced events at the other stations after cessation of the winds. We suggest that internal seiching can spread kinetic energy throughout the lake, triggering upward nutrient fluxes through mixing elsewhere. The response to water displacements in terms of nutrient supply to the epilimnion seems to be governed by differences in the strength of vertical temperature, and by nutrient and dissolved oxygen gradients. At the most northern station, these gradients were strongest and closest to each other and throughout the year primary production rates, phosphorus and chlorophyll- a concentrations in the euphotic zone appeared generally higher than at the other stations, probably also because of increased nutrient input from the Rusizi River. We emphasise that the intra- and interannual differences between the strength of lake stratification and wind regime are of major importance in determining the extent of upwelling events upwind as well as internal seiching strength and duration. That consequently affects the extent of internal nutrient loading throughout the lake and ultimately plays an important role in the lake trophic status.

Long-term management of Pyhäjärvi (southwest Finland): eutrophication, restoration – recovery?

Abstract Pyhäjärvi, located in the centre of an intensive agricultural area in southwest Finland, is an example of a lake suffering from eutrophication. The lake has been intensively studied for decades and was the object of comprehensive restoration activities both in the catchment and in the lake since the 1990s. During the last 20 years the quality and general usability of water in Pyhäjärvi has deteriorated due to increased algal blooms but has shown some signs of recovery during recent years. These changes have been driven by both a variety of human activities and natural climate related factors such as dry years. Pyhäjärvi has been the object of intensive biomanipulation for decades, carried out by commercial fishermen, whose annual harvest rate approaches the total production of vendace (Coregonus albula), the main planktivore in Pyhäjärvi. The restoration project has also subsidized the harvest of commercially unwanted fish since 1995. In 2002–2006, the EU provided funds for this fishing, which was especially intensive in 2002–2004. The main goal of the future management is to maintain and ensure the current levels of moderately low algal biomass considered acceptable by the financiers and local users of the lake. Achieving the level of water quality during the 1980s is not currently realistic due to current intensive agricultural use of the catchment, lack of cost-effective tools for load reduction from the agriculture, and climate change threats.

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.

Internal phosphorus load estimation during biomanipulation in a large polymictic and mesotrophic lake

Abstract We quantified internal phosphorus (P) load for 26 years in the polymictic, large (155 km2) and shallow (mean depth 5.5 m) Lake Säkylän Pyhäjärvi, which was heavily biomanipulated by fish removal. Internal load was estimated as (1) partially net estimates from in situ P summer increases, (2) net estimates from P budgets (mass balance approach), and (3) gross estimates from predicted active sediment release area and sediment P release, dependent on August lake temperature. Long-term averages of these estimates were similar and large at about 60% of average external load (105 mg m−2 yr−1) and were larger than external load in years with high water temperature and low water load. Regression analysis revealed that external load is decreasing but internal load is increasing over time. Internal load was negatively correlated with annual water load and positively correlated with lake water temperature. Long-term average annual or summer P concentrations are adequately predicted by a P mass balance model that includes external load, internal load (Method 3), and sedimentation as independently predicted retention. Predictability was poor for individual years, however, partially due to the poor correlation of observed lake and outflow P concentrations, the variable abundance of planktivorous fish, and reflecting the violation of the steady state assumption when individual years are modeled. Scenario modeling shows that biomanipulation cancels out the effects of internal load and forecasts a rapid increase of internal load and P concentration due to climate change; therefore, measures that further decrease external and internal P load and strengthen biomanipulation are recommended.

Recent history of sedimentation and biotic communities in Lake Pyhäjärvi, SW Finland

The sedimentary record covering the last 150 years was studied in the productive clear water lake Pyhäjärvi in SW Finland. The lake has faced significant human-induced changes: (1) the water level was lowered by almost 2 metres in the early 1850s; (2) planktivorous coregonid fishes were successively introduced, commencing in 1908; and (3) nutrient input from intensified agriculture has increased during this century.Sediments were sampled from the 25 m deep depression of the otherwise shallow lake by freeze-corer and were date by 210Pb-chronology and pollen stratigraphy. According to litho-, chemo-, 210Pb- and pollen stratigraphies, the sedimentary sequence consists of five different sedimentary facies, each representing a different depositional environment resulting both from the lowering of water level and different stages of final deposition. The sediments in the depression are believed to have been deposited orderly, but, as a result of resuspension, they have a substantial littoral sediment input.After the lowering of the lake level, oxygen content in the depression is believed to have decreased on the basis of black coloration (sulfides) of the sediment from 1870–1880 onwards. The oxygen deficit worsened after the 1940s when e.g. iron, zinc, calcium and phosphorus were increasingly liberated into the water body. In contrast, diatoms, chironomids and cladoceran communities were notably stable, with the most important biotic changes being: 1) the decreased body size of the cladoceran Bosmina coregoni, apparently due to intensive selective predation by the introduced whitefish; and 2) increased abundances of the diatom Fragilaria crotonensis, and the profundal chironomid Chironomus plumosus f. semirectus after the 1950s, suggesting an increase in the trophic status of the lake. Eutrophication was probably in response to increased nutrient supply due to intensified cultivation and use of industrial fertilizers in the lakes drainage.