Ilppo Vuorinen

Are Drifting Algal Mats Conquering the Bottom of the Archipelago Sea, SW Finland?

Abstract The occurrences of drifting algal masses were mapped in the Archipelago Sea, Northern Baltic Sea, during the summers of 1996–1997 using an UW-video and SCUBA-diving. The study covers depths from the shore down to 50 meters. The algal masses were described and classified according to the thickness of the mass and possible occurrence of anoxia (smell of hydrogen sulfide, black colored sediment), i.e. indicators of the masses potential different ecological impacts. Three different types of drifting algal masses were distinguished: cover, mat, and mattress. The succession of the masses was studied during one growing season. First, the algae aggregate on the sea floor as a thin cover. Later they start to drift downwards along the sloping bottom, developing into partly anaerobic mats and further into totally anaerobic mattresses. The phenomenon was most severe in the outer archipelago of the SE corner of the study area. Factors contributing to the growth of ephemeral algae, which result in drifting algal mats, include high nutrient loading, good water transparency, and appropriate bottom substrate. During the study we became convinced that the UW-video system is a practical tool for studying underwater loose-lying algal masses.

Selective predation and habitat shift in a copepod species — support for the predation hypothesis

SummaryIn studying the relevance of the predation hypothesis as an explanation for the vertical migration of zooplankton two predictions were tested:1)The egg sacs of Eurytemora are sufficient to cause different predation pressure in food selection experiments with visually hunting planktivores as predators and ovigerous vs. non-ovigerous Eurytemora females as prey.2)If copepods avoid predation by vertical migration, there should occur differences in the vertical distribution pattern of ovigerous and non-ovigerous females according to selective predation. The results showed that, when ovigerous and non-ovigerous females were presented to predators, a significant preference for ovigerous females was found in high densities of prey. No preference was found in experiments with males vs. non-ovigerous females and at low copepod densities. In a field study we found that non-ovigerous females distributed throughout the water column preferred the deeper parts. Ovigerous females were also abundant in deeper waters but almost totally avoided the surface layer above 20 m. Thus our results support the predation hypothesis when the adaptive value of vertical migration is considered.On the basis of our results we deduced testable predictions on the evolutionary effects of predation:1)In heavily foraged communities there is strong coevolution among the prey to resemble each other in terms of which are critical in selective predation.2)If the carrying of an egg sac is considered as parental care, it is likely that parental care occurs more seldom in environments with high predation.

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.

Modification of life-history parameters of Daphnia pulex Leydig and D. magna Straus by the presence of Chaoborus sp.

Contamination of the culture water by high densities of phantom midge larvae, Chaoborus sp. resulted in size diminution, retarded reproduction and decrease in the clutch size of D. pulex, and size diminution of D. magna. Furthermore, D. pulex suffered heavy mortality. The results are discussed in the context of the hypothesis that energy is expended in the formation of defensive cyclomorphic spines. Other hypotheses are also discussed.

Seasonal dynamics of Acartia bifilosa and Eurytemora affinis (Copepoda: Calanoida) in relation to abiotic factors in the northern Baltic Sea

The seasonal response of the two most abundant calanoid copepods of the northern Baltic Sea, Acartia bifilosa Giesbrecht and Eurytemora affinis (Poppe), to abiotic factors was studied in Tvarminne, the SW coast of Finland, using long-term data from the years 1973 to 1984. The relationship between copepods and hydrography during the four seasons was evaluated by calculating correlations between deseasonalized and detrended time series of calanoid abundances, temperature and salinity. The dependence of the copepods on temperature and salinity was not the same during the different seasons, nor for copepods of different age. In general, A. bifilosa preferred waters with a salinity of between ca 4 and 7‰, and E. affinis avoided salinities above 6.5‰. The correlations between copepods and temperature were usually positive, but large season- and stage-specific variation occurred. We suggest that the different copepod generations were seasonally adapted to the prevailing hydrographical conditions and therefore responded to changes in a different way.

A field study of prey selection in planktivorous fish larvae

SummaryThe food selection of larval fish was studied from field samples collected in two areas that differ in productivity. In the area where planktonic primary and secondary production was high the fish larvae showed a tendency to specialise and they selected the largest prey species available as food; in the area of lower production fish were generalists and they fed equally on all size classes.Abundance of prey was found to be one of the decisive factors in the prey selection of planktivorous fish larvae. Large prey species (calanoids) were selected when their absolute abundance was high but when their abundance was low, small-sized cladocerans were preferred. Visibility and stage of life history were also assumed to affect the mode of selection.

Littoral fish communities in the Archipelago Sea, SW Finland: a preliminary study of changes over 20 years

A collapse of the littoral fish populations was found in the middle archipelago zone near the Island of Seili (northern Baltic Sea) when fish populations were studied in summer 1996 in a comparable way with earlier studies done in the early 1970s and 1980s. A similar comparison was made in the outer archipelago (the islands off the island of Utö), but no evident change was found. Reasons for the decline in littoral fish stocks are discussed, but no single-cause effect can be pointed out. Among possible explanations is a substantial increase of eutrophication in the middle archipelago together with increasing ferry traffic which both cause structural changes in the littoral environment of the study area.

Spatial and temporal changes in copepod zooplankton communities of Lake Tanganyika

Pelagic Copepoda zooplankton in Lake Tanganyika were sampled over two years at three localities. When the data from both years were pooled, post-naupliar cyclopoids formed 76% of the pelagic post-naupliar Copepoda off Bujumbura, 83% off Kigoma and 62% off Mpulungu. During both sampling years, dry weight biomass of post-naupliar copepod stages decreased at sampling sites in the order Bujumbura > Kigoma > Mpulungu. Cyclopoids off Kigoma showed a clear seasonal pattern with one abundance peak in October/November and another at the end of the wet season in April/May. At other locations, neither cyclopoids nor calanoids showed clear seasonal pattern. Correlation of seasonal biomass was low between the first and second dry seasons, and between the first and second wet seasons in all the sampling localities and in all the zooplankton groups. The yearly mean post-naupliar copepod dry weight biomass was estimated as 3.1 g dw m-2 off Bujumbura, 2.1 g dw m-2 off Kigoma and 1.5 g dw m-2 off Mpulungu.

Vertical distribution and migration of pelagic Copepoda in Lake Tanganyika

The diel vertical migration and distribution of planktonic copepods were investigated at three localities in Lake Tanganyika. During the day, the surface zone was usually totally devoid of crustacean zooplankton. Even naupliar stages of Copepoda, were often absent in the surface zone in daytime, although they were numerous at night. There were clear differences in vertical distribution between species and localities, as well as between the seasons. Cyclopoida were generally found nearer the surface, as compared to the calanoid Tropodiaptomus simplex (especially in the southern arm of the lake). The maximum abundance of Cyclopoida was found both day and night at depths between 20 and 50 m. Occasionally Cyclopoids were found below 50 m and then, only in the southern end of the lake, in the Mpulungu area. Calanoids, in comparison, utilised a broader depth range. Maximum numbers were found below 100 m, but they were also very common down to 140 m. The maximum depth of their distribution was 220 m, but only in the southern part of the lake. The lower limit of vertical distribution and migration of crustaceans was evidently related to the low oxygen concentration in deep water. Thus the seasonal differences in the vertical distribution of Copepoda seem closely to follow the general mixing patterns of epilimnion. The different mixing patterns in the northern and southern arms of Lake Tanganyika also suggest differences in the biological components of the ecosystem. The northern arm of the lake is characterized by stratification and the predominance of Cyclopoida with short-range vertical migration, while the southern end is characterized by more mixing and the predominance of Calanoida, with extended vertical migration.

Geochemical record on early diagenesis of recent Baltic Sea sediments

Abstract Crust-freezing technique was used to sample undisturbed short cores from two sample stations representing oxidized and reduced top sediments in the Baltic Sea. The structures and chemical properties of laminated and unlaminated loose surface sediments are described. Laminations are formed by accumulation of organic rich matter and associated formation of sulfides during winters, and precipitation of authigenic manganese-carbonate from the watermass above during summers. We observed that the laminations are not any more visible in recent Gotland Deep (F-81) sediments possibly due to accelerated sulfate reduction and methanogenesis close to the sediment surface. Early diagenetic processes, associated with sulfur and carbon, were found to control the occurrence of many elements. The distribution patterns of iron, manganese, molybdenum, copper, lead, carbon and sulfur were found to be changed because they are dissolved to interstitial water. In anoxic sediments the temporal framework is not always useful for evaluating metal trends, but shifting chemical environments and associated diagenetic processes affect the shape of metal profiles. In oxidized conditions, the postdepositional changes due to mineralization of carbon and sulfur compounds occur at greater sediment depths, and they do not affect the chemical properties of surface sediments.