Superficial ecosystem similarities vs autecological stripping: the "twin species" Mesocyclops leuckarti (Claus) and Thermocyclops oithonoides (Sars) - seasonal habitat utilisation and life history traits
Jens Petter NILSSEN and Svein Birger WÆRVÅGEN

Mesocyclops leuckarti and Thermocyclops oithonoides, among the most common European species of cyclopoid copepods, immigrated to central Europe from eastern refuges after the last glaciation. M. leuckarti arrived prior to T. oithonoides. In a border region of T. oithonoides in southern Norway, the species was found exclusively below the highest postglacial marine limit, whereas it had spread to other neighbouring lakes above the former marine limit close to its more central region of distribution in eastern Norway. The habitat of M. leuckarti is characteristically both littoral/profundal and planktonic, whereas T. oithonoides is a true planktonic species. The egg sacs of the larger species M. leuckarti protrude from its genital segment, likely increasing water friction. M. leuckarti has probably developed strategies to reduce predation on eggbearing females, such as staying in littoral, profundal or oxygen boundary regions where fish are either absent or experience hunting difficulties. We hypothesise that the numerical suppression of M. leuckarti, its sex ratio, the habitat distribution of adult females, and its life cycles in many eutrophic lakes, is strongly affected by fish predation. M. leuckarti is considerably larger than T. oithonoides; total body length: 1.0-1.3 mm vs 0.7-1.0, respectively. The negligibly coloured and smaller adult T. oithonoides may be outside the prey range for many fish species. In the lowland region, both species completed several numbers of reproductive cycles annually. There were various patterns of diannual and triannual life cycles. Some populations exhibited a conspicuously delayed revival from sediment diapause, others in eutrophic lakes developed slowly during the summer (probably due to naupliar competition from cladocerans), or stayed in the plankton during prolonged periods during autumn. At higher altitudes and in large cold lakes, one generation a year was recorded. In its northern range, M. leuckarti showed sediment diapause in all types of localities, even the deepest lakes, usually in the upper littoral region. In more shallow lakes, deeper diapause sites were observed. T. oithonoides diapaused in either the lower littoral, or the profundal regions. M. leuckarti showed different life cycles in localities within the same geographical region, especially in its southern range. In the shallow part of Bodensee in Germany it entered sediment diapause, whereas in the much deeper main basin it showed plankton diapause (also called "active diapause"). The period of diapause for M. leuckarti (especially in the sediment) decreased from north to south. At about 45º N, sediment and plankton diapause were non-existent, and the species exhibited continuous development, even with relatively low winter temperatures (in Lago Maggiore). T. oithonoides, whose southern distribution in western Europe extends to about 50º N, showed winter sediment diapause throughout its distribution, but frequently with a fraction of the local population in plankton diapause. The combined effects of these different abiotic and biotic parameters help explain the variations of life histories observed in the field.

Recent trends in chemistry and mass budget of a high altitude lake in the southern Alps (Laghetto Inferiore, Canton Ticino, Switzerland)
Alberto BARBIERI and Rosario MOSELLO

Since 1980, surveys have been performed every three years on about 50 small high-altitude Alpine lakes located in the Swiss part of the Lago Maggiore watershed (Southern Central Alps). Since 1996 one of these lakes, Laghetto Inferiore (LI) has been included in the Mountain Lakes Research (MOLAR) program, an extensive European co-operative research project with 23 partners. Laghetto Inferiore, situated at 2074 m a.s.l., has a watershed mainly composed of crystalline silicic rocks, a maximum depth of 33 m and a theoretical renewal time of 41 days. The watershed includes a second lake, Laghetto Superiore (LS), located at 2128 m a.s.l. The surface water pH of LI is around 6.6, while alkalinity is about 30 µeq l^-1, increasing to 90 µeq l^-1 at the maximum depth. Calculated inputs from atmospheric deposition and the main tributary streams were compared with changes in the lake concentration, and outflow fluxes on a monthly basis. Weathering ranged between 90-130 meq m^-2 of alkalinity using a mass budget approach, of which 50% was consumed by the actual (H⁺) and potential (NH₄⁺) atmospheric acidity. Chemical trends over the period 1985-1998 show an increase in alkalinity and a decrease in sulphate, due to a reduction in the deposition of atmospheric acidity, and a decrease in nitrate, probably because of increased lake productivity.

Changes in lake level and trophy at Lake Vrana, a large karstic lake on the Island of Cres (Croatia), with respect to palaeoclimate and anthropogenic impacts during the last approx. 16,000 years
Roland SCHMIDT, Jens MÜLLER, Ruth DRESCHER-SCHNEIDER, Robert KRISAI, Krystyna SZEROCZYNSKA, Ante BARIC

A multi-proxy approach study (cladocerans, diatoms, geochemistry, plant macrofossils, pollen), was performed on a sediment core from Lake Vrana (Vransko Jezero), a large and deep karstic lake on the northern Adriatic island of Cres, Croatia. Considerable lake-level changes occurred during the last approx. 16,000 years. The stratigraphic evidence suggests that periods of enhanced precipitation and the post-LGM rise in sea level were the main driving forces. The lake records indicate early human impacts. Sediment echo-sounding indicated that >25 m of lake sediments lies within the site, from which 5 m have been cored. Shallow lake stages occurred from 14.4 ¹⁴C ky BP to early Holocene. Prior to Alleröd, interglacial sediments were redeposited, reflecting the influences of rising sea-level (forming a local groundwater barrier), a temporary increase in precipitation, and lake-level changes. There appears to be a hiatus in the sequence, for no sediments assignable to the Alleröd chronozone could be found. A discordance in the echo profile at the appropriate horizon in the sequence supports this interpretation. Groundwater level increased again at 10.6 ky BP (during Younger Dryas chronozone), a swamp vegetation formed, which gave way to a shallow lake. During the Preboreal chronozone, this freshwater lake persisted with fluctuating levels. The establishment and subsequent persistence of the present deep water lake at about 8.5 ky BP, correspond with findings of a pluvial period at the Dalmatian coast, which lasted from 8.4 to 6 ky BP. First human catchment disturbances were related to settlements of Neolithic or Bronze Age. The increase in summer drought, coupled with forest clearance during Illyrian times, are assumed to be responsible for the change towards present evergreen oak vegetation in the lake catchment. The intensification in land-use during Roman and post-Roman settlements caused a slight increase in the lake trophic level.

Contribution to the knowledge of algae of Nigeria. I. Desmids from the Warri/Forcados Estuaries. Part II. The elongate baculiform desmids
Fred. Idiem' OPUTE

The freshwater zones of the Warri/Forcados Estuaries contain a rich and diverse flora of tropical desmids, which compare favourably and show strong affinities with desmids from other parts of Africa, tropical South America, Southeast Asia, tropical Australia and the Indian sub-continent. In this study, ninety (90) taxa belonging to seventeen (18) elongate baculiform and colonial genera are described. Fifty (50) of the taxa have been described for the first time for Nigeria, while nineteen (19) of them are new to Africa. Part 1 in this series of studies was devoted to the desmid genera Euastrum and Micrasterias. Based on new findings, a new taxon, Pleurotaenium subcoronulatum (Turner) W. West & G.S. West var. spinulosum Opute var. nova. has been proposed.

Inland dissolved salt chemistry: statistical evaluation of bivariate and ternary diagram models for surface and subsurface waters
Robert M. BACA and Stephen T. THRELKELD

We compared the use of ternary and bivariate diagrams to distinguish the effects of atmospheric precipitation, rock weathering, and evaporation on inland surface and subsurface water chemistry. The three processes could not be statistically differentiated using bivariate models even if large water bodies were evaluated separate from small water bodies. Atmospheric precipitation effects were identified using ternary diagrams in water with total dissolved salts (TDS) <25 mg l^-1 dominated by SO₄^-2, Ca⁺², and (Na⁺ + K⁺), though water whose inorganic chemistry was dominated by atmospheric precipitation were few. Waters affected by weathering had TDS of 15 to 1,000 mg l^-1 and were dominated by (HCO₃^- + CO₃^-2) and Ca⁺². The effects of rock weathering on ion chemistry were the dominant mechanism influencing water chemistry. The contribution of silicates, carbonates, and evaporites to ions in weathering were distinguished using ternary diagrams. Weathering of silicates was evident in low TDS waters, while weathering of carbonates and evaporites was evident in moderate and high TDS waters, respectively. Evaporation effects were first obvious in water around 1,000 mg l^-1 TDS as a shift towards higher SO₄^-2, Cl^-, and (Na⁺ + K⁺). At higher TDS, Cl^- became the dominant anion while (Na⁺ + K⁺) remained the dominant cation. The general patterns were consistent in lakes, rivers, and subsurface water bodies, although subsurface waters did not show an influence due to ions from atmospheric precipitation. While several of the TDS size classes separated statistically into distinct groups, there was wide variation in the pattern of inorganic ions within a TDS size class, especially when TDS >1000 mg l^-1. A principal components analysis showed that the variability in the relative proportions of the major ions was related to atmospheric precipitation, weathering, and evaporation. About half of the variation in the distribution of inorganic ions was related to rock weathering. By considering most of the important inorganic ions, ternary diagrams are able to distinguish the contributions of atmospheric precipitation, rock weathering, and evaporation to inland water chemistry.

The occurrence of Thelohania contejeani Henneguy, a microsporidian parasite of the crayfish Austropotamobius pallipes (Lereboullet), in Liguria Region (NW Italy)
Mario MORI and Sebastiano SALVIDIO

The presence of the microsporidian parasite Thelohania contejeani Henneguy, responsible for porcelain disease (thelohaniasis), is reported in several crayfish populations inhabiting the Liguria Region. The infestation rate of this parasite was low, ranging from 0.17 to 3.7 per cent, and in two water courses it remained constant through years. However, we recommend performing periodically sanitary controls on the infected populations of Liguria and extending such controls to other populations not yet monitored. In fact, the virulence of the pathogens could increase owing to environmental stresses such as pollution, increase in crayfish density, paucity of food etc. We underline the importance, already suggested by other authors, of creating a central disease register to check the diffusion of the pathogens and facilitate the adoption of countermeasures.

Concerning calculation methods and limitations of proxy-estimates of Proteins, Carbohydrates and Lipids in crustacean zooplankton from CHN analyses
Richard A. VOLLENWEIDER

Comparison of different stoichiometric methods for the estimation of proximate biochemical composition of crustacean zooplankton and some considerations on energy transfer to planktophagous fish
Nicoletta RICCARDI