Summary• Analyses were made to determine which changes in a Lake Zürich population of Planktothrix rubescens were dependent on light-and temperature-dependent growth rates, and when growth was limited by the mixing depth.• Changes in vertical distribution of the cyanobacterium, determined weekly from August 1998 to September 1999, were related to growth increments calculated at 1-h time and 1-m depth intervals from values of irradiance, attenuance, temperature and biomass in the lake, using algorithms based on growth rates in culture.• Population biovolume varied annually from 1.2 to 120 cm 3 m −2 . During summer, modelled growth in the metalimnion matched the 50-fold population increase. Modelled growth exceeded the observed increase when Planktothrix was mixed into the nutrient-depleted epilimnion, suggesting nutrient limitation. The measured increase ceased when the mixed depth exceeded the critical depth for growth in autumn (Sverdrup's principle). Light limitation explained the gradual decrease of the population in winter. The steep decline in spring had other causes.• Population changes were largely determined by interactions of light and depth distribution; decreases in nutrient loading have had little impact on Planktothrix growth in Lake Zürich.

No abstract

The vertical distribution of the cyanobacterium Planktothrir (Oscillazoria) rubescens in Lake Zürich was investigated from March 1993 to June 1995 by collecting filaments on filters and measuring them by epifluorescence microscopy and computer image analysis. The initial population, which began to stratify in April, decreased by up to 99% by June. During the summer, the population peaked at depths of 8-15 m; it reached a maximum areal filament-volume concentration of -60 cm −3 of lake surface in early September and was then entrained in the deepening surface layer. It became mixed progressively deeper, to the lake bottom in the cold winter of 1993-94, but less completely in the milder winter of 1994-95. Most of the filaments remained viable during the winter. At the end of the mild winter of 1994-5, 70% of filaments in the water column retained buoyancy, but after the cold winter of 1996-7 only 22% were buoyant. Few remained buoyant below 80 m, where the hydrostatic pressure caused gas vesicle collapse. The proportion that remain buoyant decreases with the depth and duration of winter mixing, and increases with the critical collapse pressure (Pc) of the gas vesicles, which provide buoyancy. Strains of P.rubescens isolated from Lake Zürich differed in mean (Pc) of their gas vesicles, from 0.9 to 1.1 MPa, the highest values in freshwater cyanobacteria. Allowing for a turgor pressure of 0.2 MPa. these strains would remain buoyant at depths down to 70 and 90 m, respectively. Natural selection for gas vesicles of high (Pc) will operate by increasing the proportion of filaments that remain buoyant in the upper parts of the water column after circulation to various depths during the winter because only buoyant filaments will form the inoculum for the following season

It has been suggested that the populations of planktonic cyanobacteria that occupy the metalimnion of stratified lakes during the summer months may be aestivating between the main periods of growth during entrainment in the epilimnion in spring and summer. We determined the vertical distribution of the biomass and daily integral of photosynthesis of the population of Planktothrix (Oscillatoria) rubescens in Lake Zu$ rich for 136 d from July to November 1995. The population showed an 80-fold increase during the stratified period but it only doubled over the subsequent period of entrainment. During the first eight days, part of the increase was attributed to recruitment of filaments floating up from greater depths but all of the subsequent production could be accounted for by photoautotrophic growth. On sunny days the biomass-specific photosynthesis of this population reached some of the highest values over the whole period despite its depth ( 13 m). On very cloudy days, however, primary productivity was very low and on 4 days, when the mean depth of the population exceeded 15 m, there was no net production. Over the whole period of the study, the accumulated photosynthetic production exceeded the increase in biomass of the population by a factor of 9n5. Although much of this production occurred during the period of entrainment only a small proportion was translated into growth of the population. It is concluded that the growth that takes place in the period of stratification in the metalimnion is essential to subsequent production.

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. 15 m) 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 a in Lake Lucerne (August: 0.03, September: 0.02 mg C mg -1 chl a h -1 mmol -1 m 2 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 -1 mmol -1 m 2 s). Lake Zürich showed the lowest AN max (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 Cassimilation were found (highest slopes of inhibition b, 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.

Summary• Buoyancy changes of the cyanobacterium Planktothrix rubescens -the Burgundyblood alga -were modelled from its buoyancy response to light and irradiance changes in Lake Zürich during autumnal mixing.• The daily insolation received by filaments at fixed depths and circulating to different depths was calculated from the measured light attenuation and surface irradiance. The active mixing depth , z a5 , was determined from the vertical turbulent diffusion coefficient, K z , calculated from the wind speed, heat flux and temperature gradients. The fixed depth resulting in filament buoyancy, z n , decreased from 13 to 2 m between August and December 1998; the critical depth for buoyancy , z q , to which filaments must be circulated to become buoyant, decreased from > 60 m in the summer to < 10 m in winter.• When z a5 first exceeded z n , in September, P. rubescens was mixed into the epilimnion. In October, z q > z a5 : circulating filaments would have lost buoyancy in the high insolation. Often in November and December, after deeper mixing and lower insolation, z a5 > z q : filaments would have become buoyant but would have floated to the lake surface (the Burgundy-blood phenomenon) only under subsequent calm conditions, when K z was low.• The model explains the Burgundy-blood phenomenon in deeper lakes; waterblooms near shallow leeward shores arise from populations floating up in deeper regions of the lake.

In late summer and autumn, before the vertical circulation reaches the thermocline, the phytoplankton population of Lake Zürich is dominated by the red-coloured filamentous cyanobacterium Planktothrix rubescens, which stratifies in the metalimnion at depths close to the photosynthetic compensation point. The filament volume concentration reached a maximum of 12 cm 3 m -3 ; the depth of the maximum varied from 10.5 to 12.5 m. Changes in the depth distribution were attributed to a combination of (1) seiche movements, which raised or lowered the thermocline by up to 2 m over 36 h, and (2) flotation by the buoyant filaments relative to the isotherms, by up 0.4 m d -1 . These changes caused a 2-fold change in insolation at the Planktothrix peak. Estimates were made of the daily integral of photosynthetic O 2 -production, SS(NP), by the population of P. rubescens over a period of four cloudless days. The estimates were calculated from measurements of surface irradiance (at 5-min intervals), vertical light attenuation, temperature, filament volume concentration and the photosynthesis/irradiance (P/I) curves of filaments concentrated from the metalimnion. Despite the similar, high insolation on each of the four days, the calculated values of SS(NP) varied from 9 to 53 mmol m -2 d -1 , owing to the changing depth distribution of the filaments. Measurements of P/I curves of lakewater samples incubated at a depth of 11 m showed changes in the photosynthetic coefficients during the day. These also generated large changes in calculated values of SS(NP). The computer spreadsheet used to calculate SS(NP) was modified to incorporate timebased changes in the photosynthetic coefficients and vertical distribution of the organism. These refinements provide a more accurate description of photosynthesis by the deep-living P. rubescens, which adjusts its position by buoyancy regulation to exploit the light field in the metalimnion, where it outcompetes other phytoplankton.

An investigation was untertaken to evaluate the nutrient status of the River Rhine (two stations) and eight of its tributaries (total of ten samplings). Determinations of the following inorganic substances were made: PO:--P; NOj-N; NOs-N; NH$-N and Cll. In addition, pH and carbonate alkalinity were measured. Bioassays to obtain the algal growth potential (AGP) were carried out using periphyton from the River Rhine. A linear relationship could be established between NOj-N and the AGP, while the AGP showed a non-linear dependence on the PO&P concentration. The critical N/P ratio for N or P limitation of the algal growth in bioassays was evaluated graphically and by calculation. The results of the two methods are in good agreement: N is the limiting factor at NOj-N/ PO&P ratios less than 10, while P is limiting at ratios greater than 20. At values between 10 and 20 neither N nor P can be supposed with certainty to be limiting.