H. R. Bürgi

Limnological description of the Lakes Zürich, Lucerne, and Cadagno

Abstract: This introductory article of the special GAP issue gives an overview on general limnological characteristics of the prealpine Lakes Zürich and Lucerne and the alpine Lake Cadagno and reports on the specific situation of primary production parameters during the international GAP Workshop in mid September 1999. Furthermore, it describes methods used for water analysis and fieldwork in these lakes.¶A comparison of data related to primary production in the three lakes in September 1999 during stratification shows that (i) phytoplankton community structure varied considerably between the lakes. The dominating algae were Planktothrix rubescens in Lake Zürich, various chrysophytes and diatoms in Lake Lucerne, and Echinocoleum elegans in Lake Cadagno, (ii) the euphotic zone in Lake Lucerne was considerably deeper (app. 15m) than in the other two lakes (app. 10 m), (iii) chlorophyll a standing crop was highest in mesotrophic Lake Zürich (August: 121 mg m-2), followed by oligotrophic Lake Lucerne (August: 75, September: 34 mg m-2) and mesotrophic Lake Cadagno (August: 33, September: 25 and 14 mg m-2), and (iv) areal primary production was highest in Lake Zürich (August: 105, September: 124 mg C m-2 h-1), followed by Lake Cadagno (August: 102, September: 52 mg C m-2 h-1) and Lake Lucerne (August: 90, September: 52 mg C m-2 h-1). Physiological parameters, determined in situ from P versus I relationships, showed a lower initial slope α in Lake Lucerne (August: 0.03, September: 0.02 mg C mg-1 chl a h-1μmol-1 m2 s) than in the other two lakes (Lake Zürich in August: 0.05, in September: 0.11; Lake Cadagno in August: 0.05, in September: 0.11 and 0.28 mg C mg-1 chl a h-1μmol-1 m2 s). Lake Zürich showed the lowest ANmax (August: 2.6, September: 3.2 mg C mg-1 chl a h-1, as compared to 5.9 - 7.4 mg C mg-1 chl a h-1 in the Lakes Lucerne and Cadagno), while in Lake Cadagno the highest inhibitory effects of C-assimilation were found (highest slopes of inhibition β, 0.007-0.011, as compared to 0.0003-0.0026 in the other two lakes), due to a higher UV-exposure in this alpine lake.

Identifying functional groups of phytoplankton using data from three lakes of different trophic state

Abstract.There is tremendous diversity in species of phytoplankton. Yet one may expect some degree of commonality in the response of similar species to similar conditions. Functional groups are those sets of species that respond similarly to environmental conditions because they have similar properties. The identification of such functional groups can assist model-based prediction of the abundance of phytoplankton as a function of time, space, and environmental conditions. Functional groups can also assist limnologists in the analysis and presentation of field data. We identified functional groups of phytoplankton using a combination of prior knowledge (based on taxonomic divisions and measurable properties) and statistical cluster analysis of long-term, species-level data from three Swiss lakes of different trophic state. For this task, we used the taxonomic division as the basic unit of analysis. Each taxonomic group was subdivided into several further groups by analysing the occurrence pattern of each species of the group and grouping together species with similar patterns. The reasons for the occurrence pattern for each species within a group were then analysed based on the main properties of the species. The results of this analysis were used to merge groups that had similar occurrence for similar reasons across taxonomic boundaries. Groups with different occurrence patterns but similar properties were also merged. This led to suggestions for functional groups at multiple levels of aggregation. The resulting groups were used in a subsequent study for modelling phytoplankton in the three lakes used for this analysis. The general methodology of combining prior knowledge on properties with empirical evidence on occurrence should be useful for finding functional groups of phytoplankton in other lakes as well. Comparisons of studies across lakes can then contribute to the identification of universal functional groups of phytoplankton applicable to a broad class of waters.

Change of phytoplankton composition and biodiversity in Lake Sempach before and during restoration

Lake Sempach, located in the central part of Switzerland, has a surface area of 14 km2, a maximum depth of 87 m and a water residence time of 15 years. Restoration measures to correct historic eutrophication, including artificial mixing and oxygenation of the hypolimnion, were implemented in 1984. By means of the combination of external and internal load reductions, total phosphorus concentrations decreased in the period 1984–2000 from 160 to 42 mg P m−3. Starting from 1997, hypolimnion oxygenation with pure oxygen was replaced by aeration with fine air bubbles. The reaction of the plankton has been investigated as part of a long-term monitoring program. Taxa numbers, evenness and biodiversity of phytoplankton increased significantly during the last 15 years, concomitant with a marked decline of phosphorus concentration in the lake. Seasonal development of phytoplankton seems to be strongly influenced by the artificial mixing during winter and spring and by changes of the trophic state. Dominance of nitrogen fixing cyanobacteria (Aphanizomenon sp.), causing a severe fish kill in 1984, has been correlated with lower N/P-ratio in the epilimnion. Buoyant algae such as Planktothrix rubescens (syn. Oscillatoriarubescens) increased in abundance due to enlargement of the trophogenic layer and extended mixing depth during winter. The interactions between zoo- and phytoplankton seemed to be depressed as a result of restoration measures. Zooplankton composition changed to more carnivorous and less herbivorous species. Oxygenation of the hypolimnion induced bioturbation of sediments, mainly by oligochaetae worms, and stimulated germination of spores and cysts and hatching of resting eggs.

Eine neue Netzgarnitur mit Kipp-Schliessmechanismus für quantitative Zooplanktonfänge in Seen

A new set of nets in twin arrangement for quantitative layer-sampling of crustaceans is presented and compared to a volume sampler, as regards capturing characteristics. Due to a favorable geometry and clearly defined opening of the net, there is no loss due to flight reaction nor water backup.

The cysts ofCeratium hirundinella: Their dynamics and role within a eutrophic (Lake Sempach, Switzerland)

The dynamics of theCeratium hirundinella population and the abundance of dinocysts in the plankton and sediments were studied in Lake Sempach in 1988. In 1987, a rich population ofCeratium (380 cells ml−1) accompanied byPeridinium spp. developed in the lake. The dinocysts were found entrapped in a kind of flocs, in the deepest part of the lake, in the upper flocculent layer. The number of viable cysts ofCeratium in the sediments decreased gradually from April to July 1988. TheCeratium population increased slowly starting in April, and reached a maximum number in August (31 cells ml−1).Peridinium willei reached 100 cells ml−1. Newly formed cysts ofCeratium were recorded in the plankton and sediments at the end of July — beginning of August. They appear in the sediments as separate cells. Their number increased gradually, reaching a maximum of 600 cysts l−1 at the end of October.Ceratium formed more cysts than didPeridinium, but the rate of survival of theCeratium cysts appears to be lower than that ofPeridinium cysts. In addition to their biological functions, the cysts also have an impact on the ecosystem as carriers of nutrients from down to up and from up to down.

Der Einfluss experimentell variierter Zooplanktondichte auf die Produktion und Sedimentation im hocheutrophen See

Size-selective enrichment or exclusion of planktonic crustaceans significantly alters the succession of phytoplankton and smaller zooplankton. The cell density of algae (particularly of smaller forms) and the rate of production (life expectancy) of zooplankton increase sharply in the test units poor in zooplankton. In this primary production phase, the phosphate content of the water decreases within 10 days to 5%, of its original value. Large herbivorous populations reduce the sedimentation rate of the organic substance, while increasing the P:C ratio of the sediment.

Investigations on fecundity of Bythotrephes longimanus in Lake Lucerne (Switzerland) and on Niche Segregation of Leptodora kindti and Bythotrephes longimanus in Swiss lakes

In Lake Lucerne, Switzerland, the predaceous cladocerans Leptodora kindti and Bythotrephes longimanus segregate along spatial and temporal dimensions. In spring (April–May/June), Bythotrephes longimanus occurs below 0–20 m, while Leptodora is absent. In summer and early autumn (July–September/October), when Leptodora dominates during daytime in the 0–20 m depth, Bythotrephes longimanus also lives in deeper zones. Food competition and fish predation pressure may be the cause of differences in ecology of Leptodora and Bythotrephes acquired during evolution. Due to its transparency and tolerance of higher temperature, Leptodora could avoid fish predation and, therefore, competes with Bythotrephes longimanus successfully. In addition, the differences between the two species may account for the spatial and temporal niche segregation in oligotrophic Swiss Lakes. But spatial niche segregation is less important in mesotrophic lakes with high prey density than in oligotrophic lakes with low prey density. In small, eutrophic lakes importance of temporal niche segregation also decreases, and Bythotrephes is seldom or not present. The preference of Bythotrephes to live in deeper water to avoid fish predation during summer may be the cause of its difficulties to establish itself in small and eutrophic lakes with high prey densities, where the hypolimnion is missing or anoxic.In the spring, Bythotrephes exhibits r-strategy (smaller body size and a higher fecundity), the female is already fertile after the first molt. In the summer, a K-strategy prevails (larger body length and lower fecundity than in the spring), and female Bythotrephes are fertile only after the second molt. Shortage of prey (biomass of Bosmina and Daphniadecreased after June especially in the surface layers) and the maximum fish predation pressure in summer may change the life strategy of Bythotrephes: while fecundity decreases from generation to generation, body length increases. Enhanced prey densities (e.g. during mesotrophic conditions in L. Lucerne) lead to larger individuals in summer and autumn.

Crustaceen-Plankton des Vierwaldstättersees. Grössenfraktionierung und Abundanzdynamik in den Jahren 1976 bis 1979

In order to create a common comparative biomass basis for the biological interactions between zooplankton and phytoplankton, zooplankton was quantitavely sampled at close intervals, counted and measured at determined development stages. The volumetric values calculated according to stereometric formulas do not indicate an important variability over the years. Significant volume increases of adult crustaceans were observed only from March till May. Comparisons with past plankton analyses revealed a rapid increase of the herbivorous crustaceans, i.e. by a factor of five; while simultaneously, the favourite food (nannoplankton) only increased from 3.1 g/m2 to 5.5 g/m2 wet weight.

The drift of zooplankton in a lake-outlet (Glatt) in a day-night-rhythm depending from the water level

To give an indication on the influence of the dynamics and behavior of lake plankton on the seston composition in the outlet, four 24-hour sampling sessions were made in the Greifensee (situated near Zürich, Switzerland) and its outlet (River Glatt).The most important crustacean species studied wereEudiaptomus gracilis, Cyclops abyssorum, Cyclops vicinus, Mesocyclops leuckarti andDaphnia spp.Leptodora kindtii was found during one sampling only. No evidence of shore avoidance was found. The vertical migration in the lake especially in the upper layer (0–6 m) is believed to have an influence on the amount of plankton which is flushed into the outlet. Although the composition and concentration of the plankton at the deepest point of the lake have been known, no accurate predictions on the quality of the plankton close to shore or in the outlet were possible.ZusammenfassungUm den Einfluß der Dynamik und des Verhaltens von Zooplankton auf die Partikel-Zusammensetzung im Abluß des Sees festzuhalten, wurden vier 24-Stunden-Aufnahmen im Greifensee (Kt. Zürich, Schweiz) und seinem Abfluß (Glatt) durchgeführt. Drei Probenahmestellen befanden sich im Abfluß, drei weitere in Ufernähe und eine an der tiefsten Stelle des Sees.Je nach Plankton-Spezies, Jahres- und Tageszeit konnten unterschiedliche Verteilungen beobachtet werden. Nur ein geringer Teil der planktischen Crustaceen führt im Greifensee eine ausgedehnte Vertikalwanderung durch. Hingegen sind Feinzonierungen, die sich nach der Stabilisierung der Thermokline in den obersten 6 Metern ausbilden, von großer Bedeutung. Eigentliche Uferformen finden sich äußerst selten im Abfluß. Durch ihr Verhalten (Aufenthalt in einem durch Bewuchs gekammerten System, Festhalten an Wasserpflanzen, Meidung des offenen Wassers) entziehen sie sich weitgehend der Abdrift.Die lichtgesteuerte tagesperiodische Wanderung wirkt sich auf die Dichte und Zusammensetzung des Zooplanktons im Abfluß aus. Während die aus der Oberfläche geschöpften Proben in Ufernähe und im Abfluß weitgehend übereinstimmen, ergeben sich infolge der diurnalen Migration (Meidung der obersten Wasserschichten während des Tages) große Unterschiede gegenüber den Netzzügen aus 0–6 m. Während der Nacht verwischen sich diese Differenzen. Durch die Feinzonierung der obersten Wasserschichten bedingt, wirkt sich der Wasserstand auf die Zooplanktondichte im Abfluß aus: Bei Hochwasser entspricht die abfließende Zooplanktonkonzentration den ufernahen Stellen, bei Niedrigwasser fließt nur zooplanktonarmes Oberflächenwasser ab.