Peter Bossard

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.

Present and past bio-available phosphorus budget in the ultra-oligotrophic Lake Brienz

Abstract.A detailed budget of the fluxes of bio-available phosphorus (bio-P) was established for the ultra-oligotrophic Lake Brienz (Switzerland) and its catchment. Lake Brienz is a cold, deep oligotrophic peri-alpine lake that receives an annual load of approximately 300 kt of suspended sediments, mainly from two glacier-influenced rivers. The challenge was to overcome the associated high background of mineral-bound inorganic phosphorus (IP) of ~200 t yr−1 that is mostly inaccessible to algae growth. The application of six complementary, independent datasets allowed a consistent balance of bio-P to be obtained. We made use of data on (a) the load imported by the contributing rivers, (b) net sedimentation from cores, (c) export of bio-P from catchment land to the surface waters estimated by a GIS model, (d) the downward flux of bio-P through the water column from sediment traps, (e) primary production, and (f) the mineralization rate of organic material from the consumption of oxidants in the uppermost sediment of the lake. The average bio-P load estimated from import measurements and net sedimentation is 7.0 t yr−1 with an error of about 10%: An estimated 5.4 t yr−1 enters by way of the two main rivers (including 0.9 t yr−1 from sewage treatment plants), 1.2 t yr−1 from the remaining catchment (including 0.4 t yr−1 from sewage treatment plants that are diverted directly into the lake), and ~1 t yr−1 from atmospheric deposition. Approximately 2 t of bio-P are retained annually in the sediments of the upstream dams and thereby withheld from downstream Lake Brienz. The maximum eutrophication of the lake in the late 1970s and the subsequent re-oligotrophication can be attributed to the loads of urban wastewater. The drop in biological productivity since the late 1970s is consistent with the decrease of bio-P fluxes archived in the sediment, the record of the sewage treatment plant outflows and the few occasional in-situ observations.

Quantifying the response of phytoplankton photosynthesis to ultraviolet radiation : Biological weighting functions versus in situ measurements in two Swiss lakes

Abstract: The sensitivity of photosynthesis to ultraviolet radiation (UV) was assessed for phytoplankton assemblages in two Swiss lakes, pre-alpine Lake Lucerne (Vierwaldstättersee) and alpine Lake Cadagno, using both in situ and laboratory incubations. Biological weighting functions for UV inhibition of photosynthesis (BWFs) were determined in the laboratory using polychromatic exposures in a Xe-lamp based incubator. Samples were concurrently incubated in situ under UV exposed and protected bottles (profiles 0-5 m), while additional spectral treatments were carried out at the 50 % UV-B penetration depth: full spectrum, UV-A only (Mylar protected) and UV protected quartz tubes. Both particulate (> 0.2 μm) and total organic carbon incorporation were measured. Measured attenuation coefficients and incident UV spectral irradiance data was used to evaluate a BWF/photosynthesis-irradiance model (BWF/P-I) for in situ exposure conditions and compared with measurements. The BWFs showed sensitivity across the UV spectrum at similar, though somewhat lower, levels than an average BWF for marine assemblages. Relative photosynthesis in situ (UV exposed/UV excluded) was about 40 % at the surface and about 60 % at the 50 % UV-B penetration depth. Similar inhibition was predicted by the BWF/P-I model. Generally, full spectrum (UV-B and UV-A) exposure had little additional effect compared to UV-A only exposure. Reciprocal transfer of samples between lakes showed enhancement of UV effects in L. Cadagno compared to incubation of the same sample in L. Lucerne, consistent with increased UV sensitivity due to the 5°C cooler water temperature in L. Cadagno. Similarly, BWF prediction of in situ response in L. Cadagno was improved by increasing UV sensitivity according to a Q10 of 2. Full profile calculations using the BWF/P-I model suggest stronger effects of UV on L. Lucerne compared to L. Cadagno phytoplankton due to greater sensitivity of the assemblage combined with higher overall transparency to UV relative to PAR in L. Lucerne. The BWF/P-I model was a good overall predictor of UV-dependent photosynthetic performance in these lakes.

Incident and in situ irradiance in Lakes Cadagno and Lucerne: A comparison of methods and models

Abstract: Incident surface irradiance of photosynthetically available radiation (PAR), and ultraviolet radiation (UV-A and UV-B) was monitored during the GAP Workshop (8-15 September 1999) at the field stations Kastanienbaum at Lake Lucerne (434 m a.s.l.) and Piora at Lake Cadagno (1923 m a.s.l.), Switzerland, with broad band sensors (LiCOR and Macam) and multifilter spectral radiometers (Smithsonian UV-B radiometer SR18, Satlantic and Biospherical Profilers). Calculations of clear sky spectral irradiance were performed using computer implementations of atmospheric radiative transfer models STAR and FASTRT as well as a simple two-stream model. Clear sky conditions prevailed at Lucerne so that detailed comparisons could be made between measured irradiance and model predictions. Good correspondence was found for the measurements and predictions. The best agreement was with the STAR model. Spectral measurements were consistent with measurements of the broad band UV-A and UV-B sensors after accounting for the manufacturer specified spectral response. The effective center wavelength of the broad band UV-B sensor is 320 nm, despite the fact that the maximum sensitivity of the sensor is at 310 nm. A daily UV index was estimated from the midday SR18 data. An analysis of the UV-A and UV-B data at the two sites of different altitude indicates that the so called altitude effect (AE) is variable during the course of the day and usually higher for UV-B than for UV-A. Lakes Lucerne and Cadagno have moderate transparency to UV and PAR with 50 % of 320 nm irradiance penetrating to 1 m in Lake Lucerne and 0.3 m in Lake Cadagno.

Large lakes and their sustainable development

This contribution has been edited from a working document, prepared by invited scientists attending a workshop in Konstanz, Germany, on the importance of external perturbations for short- and long-term changes in large lakes ecosystems, held from 21 to 26 October, 1991. It tries to assess our current understanding of the most important processes involved in the functioning of large lakes and to identify the currently most urgent research priorities in the fields of land-water interactions, physical processes, biogeochemistry and nutrient relations, remote sensing, biological interactions in food webs, and long-term monitoring programs.

Measurements and significance of bio-optical parameters for remote sensing in two subalpine lakes of different trophic state

Bio-optical measurements were carried out on eutrophic Lake Zug and oligotrophic Lake Lucerne, to provide data and models for thein situ calibration of multispectral imagery collected in 1991, during an AVIRIS flight over Central Switzerland.The results indicate that Secchi depth, chlorophyll and total suspended solid concentrations, vertical extinction coefficient, absorption coefficient and irradiance reflectance in the PAR region can be used to discriminate between the two lakes. Dissolved organics concentration, scattering and total attenuation coefficients estimates, as well as backscattering probability, are less acceptable, because of instrumental limitations.Relationships between optical and water quality parameters were investigated and found to behave according to accepted theoretical frameworks. Both lakes exhibited vertical and horizontal heterogeneities in chlorophyll and turbidity, and had contrasting mean bio-optical characteristics: although suspended solid concentrations were similar, transparency was lower in Lake Zug, but reflectance was substantially higher in Lake Lucerne. Water colour determined by chromaticity analysis was blue-green in Lake Lucerne and green in Lake Zug.Reflectance spectra simulated through a three-component optochemical model did not completely match those derived fromin situ measurements, because of lack of site-specific optical cross-sections for suspended minerals and dissolved organics.The monitoring of water quality in subalpine lakes by remote sensing, both with present and future technology, is discussed and considered as possible, provided that their optical behaviour is known, and preferably expressed by a valid optochemical model.

Die quantitative Erfassung von Methan im Seewasser

A preparation method for the quantitative analysis of methane in lake water samples has been developed. The method is based on the equilibrium reactions of gases in a closed two-phase (headspace-water) system, in which the headspace gas is analyzed gaschromatographically.The method shows a standard deviation of ±5%, including sampling and sample preparation and can also be used for the quantitative determination of other dissolved gases.

Large Lakes and Their Sustainable Development

This contribution has been edited from a working document, prepared by invited scientists attending a workshop in Konstanz, Germany, on the importance of external perturbations for short- and long-term changes in large lakes ecosystems, held from 21 to 26 October, 1991. It tries to assess our current understanding of the most important processes involved in the functioning of large lakes and to identify the currently most urgent research priorities in the fields of land-water interactions, physical processes, biogeochemistry and nutrient relations, remote sensing, biological interactions in food webs, and long-term monitoring programs.