Contribution a l'etude limnologique du lac Pavin (France). II: Relations entre parametres abiotiques-phyto et zooplancton dans la zone 0–20 metres

At 1197 m in the French Massif Central, lake Pavin has a volcanic origin. It is a meromictic lake of 44 ha area and 98 meters depth.Zooplancton samples (250 liters) were collected bimonthly with a water pump from May 1973 to April 1974 at six depths from surface to 20 meters.Countings were made to study the dynamics of two Cladocera: (Daphnia longispina and Ceriodaphnia quadrangula), two copepode populations, the calanoide Acanthodiaptomus denticornis and the cyclopoide Cyclops abyssorum. For the last two species, males, females, ovigerous females, Nauplius larvae and each of the five copepodite stages were differentiated.If Cladocera have a period of development limited from June to September for Ceriodaphnia and from June to December for Daphnia, copepods are presents all around the year with two principals periods of reproduction. The first during spring (March–April), the second during fall (October–November). If this second generation presents an important development for Cyclops abyssorum, that is not the same for Acanthodiaptomus denticornis, in which species nauplius larvae died in winter. The synthetical mathematical model realized by Principal Components Analysis illustrates the relationships between phyto-and zooplancton and bio-physico-chemicals parameters and specially the fact that, in this lake, the highest values of phytoplanctonic primary production and activity coefficient (Production/Standing crop) are found when herbivorous zooplancton presents the more important development and nutrients their lowest concentrations.

     

Phytoplankton Primary Productivity and Population Efficiency Studies in a Prairie-Parkland Lake Near Edmonton,Alberta, Canada

The standing crop and primary productivity of a small eutrophic, prairie-parkland lake were measured. In general, both standing crops and primary productivity were large, 29.4 and 73.09 mg chlorophyll a m⁻³ and m⁻² and 78.71 and 196.77 mg C hr ⁻¹m⁻³ and m⁻² respectively. Productivity decreased with increasing depth, therefore, decreasing light intensity. Relations between productivity and chlorophyll a content, productivity and light intensity, phytoplankton productivity efficiency and light intensity, productivity and water temperature were investigated, as was the photosynthetic index. Experiments designed to determine the photosynthetic capacity of the phytoplankton distinguished between actively growing and senescent populations. The latter were present during the winter ice cover.     

Importance quantitative de différentes classes de taille phytoplanctoniques en milieu lacustre eutrophe

The distribution of three phytoplanktonic size fractions was studied in an eutrophic lake. Size fractionation experiments were performed using cell enumeration by inverted microscopy and chlorophyll a estimation. The results were also compared with the ATP content in the analysed algal fractions.The (1–12 µm) fraction represented only on average 5 % of the total biomass, when estimated by the cell enumeration method, but made up 26 % of the total chlorophyll a and 33 % of the total ATP. This discrepancy confirms that cell enumeration with an inverted microscope seriously underestimates the nanoplankton biomass.The (12–45 µm) fraction made up to 50% of the total biomass regardless the method of analysis used. The chlorophyll a and biovolume estimation in this fraction were positively correlated.The contribution of algal species with large cell size (45–160 µm) to the total algal biomass was higher (39%) when based on their biovolume estimation than when on the chlorophyll a (27%). The low ATP contribution of this fraction (17%) may be attributable to high percentage of dead cells.

     

Deontostoma species from subantarctic coasts (Nematoda,Leptosomatidae)

The major cyanobacteria in Crawford Lake are benthic mat forming Lyngbya and Oscillatoria and not phytoplankton. The eutrophication of the lake has resulted in a decline in the mat forming cyanobacteria as inferred from palaeopigment analyses of the core from this lake. In previous palaeolimnological studies the concentrations of oxillaxanthin and myxoxanthophyll have been used as correlates with lake trophic levels. High concentration of oscillaxanthin and myxoxanthophyll were interpreted as indicating eutrophic conditions prevailed. Our results indicated that when Crawford Lake was most eutrophic little oscillaxanthin and myxoxanthophyll was produced. High values of ¹³C at the depth of 34–48 cm (1500–1760 A. D.) were related to a dense population of benthic Oscillatoria and Lyngbya living on the bottom of the lake during that period. The Oscillatoria and Lyngbya utilize bicarbonate as a source of inorganic carbon. Carbonate has a high ¹³C value. Very low ¹³C values were found at 0–7 cm (1955–1992 A. D.) in the Crawford sediment core at the time when phyto- plankton dominated the core. Phytoplankton are enriched in ¹²C by photosynthetic assimilation of CO₂.     

Limnological Aspects of Two Tropical Crater Lakes (Lago Biao and Lago Loreto) on the Island of Bioko (Equatorial Guinea)

The limnology of Lago Biao and Lago Loreto, two tropical crater lakes on the island of Bioko, was investigated for the first time in January of 2002. Lago Biao fills a caldera at 1750 m altitude and is 18.5 m deep. This lake was characterized by low conductivity (11.6-13.3 S cm^–1), low pH (5.9-6.2), an oxycline between 6 and 10 m depth (90-27% saturation), and a Secchi depth of 5.5 m. Dinoflagellates, desmids, and green algae dominated the phytoplankton community. The rotifers Anuraeopsis fissa, Polyarthra indica, Trichocerca simoneae, and T. similis were most abundant above or in the thermocline. During the day Tropocyclops prasinus tjeefi and Chaoborus cf. ceratopogones concentrated in the oxycline, whereas Daphnia laevis was most abundant between 0 and 4 m depth. In contrast, L. Loreto is 82 m deep and probably fills an explosion crater (maar) at 1050 m altitude. The lake had a turbid, yellowish appearance and a Secchi depth of only 0.6 m. Oxygen content dropped from 121% at the surface to 6% in 10 m depth. The pH declined from 9.15 at the surface to 7.4 in only 2.5 m depth and became acidic at approximately 15 m depth. One species of cyanobacterium, one dinoflagellate and one green alga dominated the phytoplankton and together reached densities of 12 100 cells ml^–1. Only one rotifer (Brachionus angularis) and one copepod (Tropocyclops prasinus tjeefi) species colonized the open water. Intact algal cells and zooplankton specimens were encountered near the bottom.     

The interaction of components controlling net phytoplankton photosynthesis in a well-mixed lake (Lough Neagh, Northern Ireland)

The homogeneous distribution of the phytoplankton in a shallow (mean depth 8·6 m) unstratified lake, L. Neagh, Northern Ireland, facilitated the study of the interaction of components controlling gross photosynthesis per unit area. These included the photosynthetic capacity, the phytoplankton content of the euphotic zone, and a logarithmic function describing the effective radiation input. These factors were analysed for two sites, the open lake and Kinnego Bay, which respectively had standing crops of up to 90 and 300 mg chlorophyll a m^?3 and maximum daily rates of gross integral photosynthesis of 11·7 and 15·6 g O₂ m^?2 day^?1. Values are reduced by the high contribution to light attenuation by non-algal sources, which increases at low standing crops particularly in winter, when values of integral photosynthesis decrease to 0·5 g O₂ m^?2 day^?1. This relative change is the result of self-shading behaviour of the phytoplankton altering the crop content of the euphotic zone at different population densities. Changes in the irradiance function, incorporating day length, are largely responsible for the changes in daily rates of integral gross photosynthesis; as daily irradiance is also a determinant of water temperature, it exerts further influence through the photosynthetic capacity which was strongly correlated with temperature. Much of the gain in gross photosynthesis resulting from higher photosynthetic capacity may not be reflected in a higher net column photosynthesis, because of the greater proportional rise in respiration with temperature. The balance in the water column between respiration losses and photosynthetic input may frequently alter since the ratio of illuminated to dark zones is between 1/4 to 1/5 in the open lake, and small shifts in any of the controlling features may result in conditions unfavourable for growth. This is analysed especially for the increase of diatoms in spring, when small modifications of the underwater light field can delay growth.     

Chemical limnology of two artificial lakes used for both stormwater management and recreation

The aims of this study were to document the mainly chemical behaviour of two linked artificial lakes used for both stormwater management and recreation in the new town of Craigavon. Further, the understanding of their behaviour should help in their management and the design of other similar lakes.The lake mean total phosphorus (73 µg P l^–1), nitrate (0.50 mg N l^–1) and chlorophyll a (25 µg l^–1) concentrations, Secchi depth (1.2 m) and the estimated total phosphorus loading (1.98 g m^–2 a^–1) all classify the main lake as eutrophic. An important source of the phosphorus load on the lakes is the urban area of Craigavon (52% of the total load). The interrelationships between total phosphorus, chlorophyll a and Secchi depth in the main lake are similar to those in natural ones. In addition, the lake follows the total phosphorus load — trophic state relationships (lake total phosphorus and chlorophyll a concentrations and Secchi depth) found to apply elsewhere. These two points indicate that the artificial lakes in Craigavon behave similarly to natural ones.     

Biomass and photosynthesis of size-fractionated phytoplankton in Canadian Shield lakes

Both in situ primary production and biomass (chlorophyll ) of fractionated phytoplankton (<64,µ, <25 µm and < 10 µm) were studied in 10 Canadian Shield lakes to elucidate the spatial and temporal variability of the contribution of size fractions to the biomass and primary production of the phytoplankton community. Mean summer biomass and production of each size fraction varied significantly between lakes. Within lakes, temporal variation was low for biomass but great for production. However, temporal variation can be considered of minor importance during the sampling period, as compared to the spatial variation between lakes. Algae from the < 10 µm size fraction were the most important in biomass (41–65 %) and production (23–69%). The temporal trends for both phytoplankton variables thus generally followed closely that of the < 10 µm size fraction. Among the physical, chemical and morphometric variables of the studied lakes, water transparency (Secchi disk), total phosphorus, lake volume, lake area, and mean depth gave the best correlations with phytoplankton variables.Contribution number 354 from the Groupe de recherches en Ecologie des Eaux douces, Limnological Research Group, Université de Montréal.     

Caractéristiques du microphytobenthos d'un lac volcanique. Characterization of the Microphytobenthos in a Volcanic Meromictic Lake (Lake Pavin,France). II. Productivity and Relationships

The collection of microphytobenthos for primary production studies in Lake Pavin was performed with a Jenkin sampler. This corer permits sampling at different levels below the surface of the lake. Five levels (10, 20, 30, 40, and 60 m below the surface) were regularly investigated for more than one year. Each was inhabited by a characteristic population of dominant diatoms. The primary production, which was very large during the summer, decreases with depth. During the winter, the ¹⁴C assimilation occurs at a very low rate in clear incubators, but in dark incubators, the rate is high. Therefore, the ¹⁴C fixation in darkness is greater than the photosynthetic assimilation. This phenomenon together with the very important data recorded for photosynthetic efficiency (especially at the lowest levels, 40 and 60 m deep) suggest that microphytes on the bottom of the lake assimilate organic compounds during much of the year.     

Chlorophyll-nutrient relationships in North Island lakes (New Zealand)

A model for the prediction of chlorophyll a in the near surface waters (1 m) of North Island lakes was developed using data from the literature and our own study of 12 North Island lakes. Annual geometric mean concentrations of chlorophyll a, total nitrogen and total phosphorus were used since no distinct growing season was discernable. Annual mean ratios of total nitrogen to total phosphorus in the near surface waters ranged between 10 and 59 (by weight). Strong correlations were obtained between log-transformed values of chlorophyll a and total nitrogen (r² = 0.53, n = 16), chlorophyll a and total phosphorus (r² = 0.71, n = 21), and between total nitrogen and total phosphorus (r² = 0.69, n = 16). However, after correcting for the high interdependency between total nitrogen and total phosphorus, only total phosphorus was found to be important in predicting chlorophyll concentrations. Much of the variance in the chlorophyll-phosphorus relation was attributable to differences in mean lake depth. Lakes with mean depths less than 11 m had significantly more chlorophyll a per unit of total phosphorus ( = 0.54 µg · µg^–1, SE = 0.05, n = 6) than lakes of greater mean depth ( = 0.17 µg · µg^–1, SE = 0.02, n = 14). When the effect of mean depth was taken into account, 89% of the variance in chlorophyll a was explained compared with 71% for the simple linear regression on total phosphorus alone.     

Photoecology of the coral Leptoseris fragilis in the Red Sea twilight zone (an experimental study by submersible)

Summary Depth-dependent photoadaptational responses of the Red Sea zooxanthellate coral (Leptoseris fragilis) were studied down to 160 m from the research submersible GEO. Light saturation curves for photosynthesis revealed, with I _C=1-2, I _K=10.9 and I _sat=20 E·cm^–2·sec^–1, the lowest values of photokinetic parameters ever reported for a symbiotic coral. In summer, positive net production occurs only around noon at approx. 100m depth. Biomass parameters of corals at 100–135 m are negatively correlated with depth in algal cell density, protein, chlorophyll and carotenoid but not in pigment ratios or cell based pigment content. Coral size decreased with depth. Corals transplanted from 110–120 m original depth to 40, 70, 90 and 160 m showed high survival after one year. O₂-production and dark O₂-uptake increased with decreasing transplantation depth. After one year, transplants at 70 and 90 m but not at 40 m had higher algae density and pigment concentrations. The host light-harvesting systems described by Schlichter, Fricke and Weber (1986) are partially destroyed in 40 m but not in 70 and 90 m transplants. Different light exposures alter P-I-responses (P _max, I _C, I _K, I _sat) but not biomass parameters, indicating molecular or biochemical adaptation. The coraal's optimal light fields lie between 70 to 90 m. Its exceptional bathymetric distribution is linked with the newly discovered host light-harvesting systems which probably enhance photosynthetic performance in a dim environment.     

Wirkungen von Rohöl-Emulgatorgemischen auf marine Fischbrut und deren Nährtiere

Zusammenfassung 1. Es wurde die Einwirkung eines Rohöl-Emulgatorgemisches (Irak-Öl/Moltoclar) auf die Larven vonClupea harengus L. undAgonus cataphractus L. sowie auf Wildplankton untersucht.2. Bei Emulgatorkonzentrationen von 2,5 bis 5,0 mg/l war die Letalitätsgrenze erreicht.3. Subletale Schädigungen ließen sich bis zu einer Konzentration von 0,5 mg/l deutlich nachweisen.4. Irak-Rohöl hatte keine schädigenden Wirkungen auf Heringslarven.

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Effects of crude oil-emulsifier mixtures on marine fish fry and their food animals

The effects of crude oil-emulsifier mixtures on the larvae ofClupea harengus L. andAgonus cataphractus L. are described. The herring larvae tested had total lengths of 20 to 26 mm; the larvae ofAgonus cataphractus were investigated shortly after their first food intake. Samples of wild plankton from daily catches (used as food during rearing experiments) were also tested. The different dilutions were obtained from the following initial mixture: 20 ml emulsifier Moltoclar, 80 ml Irak crude oil, 900 ml stale sea water (32 S ). Dilutions with the following content of the emulsifier were used: 50 mg/l, 25 mg/l, 5 mg/l, 2.5 mg/l, 0.5 mg/l, 50µg/l, 5µg/l, 0.5µg/l. Lethal concentrations ranged from 2.5 to 5.0 mg/l. Sublethal damages were clearly ascertained down to a concentration of 0.5 mg/l. Irak crude oil alone did not cause damage to herring larvae during the observation period of 4 days.

     

Oligochaeta of Lake Taimyr: a preliminary survey

Lake Taimyr in Siberia is northernmost among the world's large lakes: 73°40–75°20N, 99–106°E. The lake area is up to 4650 km² in summer, with a maximum depth of 26 m and a mean depth of only 2.8 m. The ice-free period lasts about three months. The water level sinks 5.5–6 m during winter, so that 85% of the bottom surface is frozen into ice for some time and subjected to negative temperatures, probably down to –20 °C. In artificially melted sediment samples, 75–92% of animals survived. The average summer biomass of zoobenthos is about 1 g m^–2 wet weight, a half of this being formed by Oligochaeta. Altogether 76 samples with 3742 oligochaete specimens collected by V. N. Grëze in 1943–1944 were studied. At least 14 taxa of Tubificidae, Lumbriculidae, and Enchytraeidae were found in the lake, and some more enchytraeids in an adjacent river. Many immature animals could not be identified to species. Naididae were completely lacking probably due to the absence of macrovegetation. The shallow freezing zone is inhabited mostly by Alexandrovia ringulata. The profundal fauna is dominated by Lamprodrilus isoporus, Stylodrilus sp., and Isochaetides sp.     

Irradiance Regulation of Photosynthesis and Respiration in Modern Marine Microbialites Built by Benthic Cyanobacteria in a Tropical Lagoon (New Caledonia)

Microbialites are organosedimentary deposits that have built up as a result of the growth and binding of detrital sediment by a benthic microbial community. This study focuses on microbialites built by monospecific populations of cyanobacteria in the south-west lagoon of New Caledonia, where they have been observed down to 20–25 m depth. The aim was to study their photosynthetic and respiratory responses to various light intensities. The Phormidium sp. TK1 microbialite was collected at 19 m depth and the P. crosbyanum (Tilden) microbialite was collected at 0.5 and 13 m depth. Phormidium sp. TK1 showed all the characteristic features of a low-light adapted species. The initial slope of the Photosynthesis versus Irradiance curve for this microbialite was close to the maximum quantum yield indicating an efficient light absorption and utilization at low light. The photosynthesis maximum was located 0.2–0.4 mm below the surface and did not shift with changing light intensity. Respiration rates were low and not enhanced by light; photoinhibition was observed at higher light intensities. In Phormidium crosbyanum (Tilden) microbialites, the photosynthesis maximum shifted downward to lower depths with increasing light, probably as a result of phototactic migration of cyanobacterial filaments, and light-enhanced respiration was observed at light intensities above light saturation. The photosynthetic para- meters measured in P. crosbyanum indicate that P. crosbyanum is capable of photo-acclimation at high light intensities. The gross productivity of the different microbialites was comparable to values measured in cyanobacterial stromatolites observed in other shallow environments. However, the microbialites studied here were characterized by a lower respiration / production ratio which indicates a higher growth efficiency.     

Seasonal succession,standing crop and determinants of primary productivity of the phytoplankton of Ministik Lake,Alberta, Canada

Ministik Lake (longitude 113°01; latitude 53°21), a well-mixed, shallow (mean depth 1.83 m), eutrophic lake is characterized by eutrophic chlorococcalean and cyanophycean phytoplankton associations, and little change in standing crop size with increasing depth. Standing crops and primary productivity are low during the winter but pronounced spring and late summer/early autumn maxima occur. Mean yearly areal standing crop (B) and primary productivity (A) were 199.2 mg m^–2 chlorophyll a and 319.5 mg C hr^–1 m^–2 respectively. Annual productivity was estimated at 1399,6 g C m^–2yr^–1. The mean increase in the extinction coefficient () per unit increase in standing crop (B) was 0.03 In units m^–1. High non-algal light attenuation (q) occurred averaging 46% which prevented the ratio B from attaining more than 74.2% of the theoretical maximum except once when selfshading occurred. Insignificant relationships existed between B (mg m^–3 chlorophyll a) and A_max (mg C hr^–1 m^–3), A and B and A and B. Close correlations existed between A and A_max/ and A and I₀ (W m^–2). The depth of the euphotic zone (Z_eu) varied between 0.6 and 2.0 m; the average relationship between z_eu and was z_eu = 3.78/_te, and the mean standing crop in the euphotic zone represented 58.3% of the theoritical maximum. The high _q values made the model of Talling (1957) inapplicable to this lake. The Q₁₀ value for the lake was 1.20. _max (mg C chlorophyll a ^–1 hr^–1) was closely related to both temperature and irradiance, and depressed by high pH values.Growth of the cyanophycean algae was correlated with temperature, chlorophycean algae with phosphate-phosphorus and temperature and the diatoms with dissolved silica and phosphate-phosphorus, but only in the case of Chaetoceros elmorii did any nutrient appear limiting. Indirect evidence that free CO₂ limited photosynthetic rates is provided by the _max:pH relationship.     

The application of continuous monitoring microdensitometry to an analysis of NAD+ binding and 3 beta-hydroxy-delta 5-steroid dehydrogenase activity in the regressing corpus luteum of the pro-oestrous rat ovary

Summary A technique which allows for the continuous microdensitometric monitoring (at 3.3 s intervals) of an enzyme reaction has been applied to a study of 3-hydroxy-⁵-steroid dehydrogenase activity in regressing corpora lutea cells of unfixed tissue sections (5µm thick) of the pro-oestrous rat ovary. Initial reaction rates for NAD⁺ reduction by the activity of the enzyme were maintained for less than 3 min at all the concentrations (0-500µmol/l) of co-factor employed. The relationship between NAD⁺ concentration and enzyme activity under initial velocity rate conditions was hyperbolic and maximum enzyme activity was seen when the NAD⁺ concentration was greater than 250µmol/l. The apparent Michaelis—Menten constant (K _m) was 29µmol/l and V_max 0.63µmol NAD⁺ reduced/min/cm³ corpora lutea.     

Primary production of epipsammic algal communities in Lake Balaton (Hungary)

Benthic algal communities can play an important role in matter and energy flux of shallow lakes. Their contribution to total primary production of lakes has been largely unexplored. The aim of this study was to estimate the primary production of the epipsammic algal communities at different water depths in Lake Balaton (Hungary) with photosynthetic measurements performed in laboratory. The photosynthesis of the benthic algae of different origin was studied at nine different irradiance levels, in three replicates. The maximum photosynthetic rate (P _max) was always higher in samples from the shallow parts than those from the deeper regions of the lake. Along the west–east longitudinal axis of the lake P _max decreased in the southern part and increased in the middle of the lake as a consequence of differences in the chlorophyll-a concentrations. Knowing P _max, I _k, global radiation and extinction coefficient, the primary production (mg C m⁻² day⁻¹) of the epipsammic algal community was calculated at different water depths. In the shallow regions at 0.5 and 1 m water depth 75–95% and 60–85% of the production was attributable to the epipsammon. The percentage contribution of epipsammon was at 2 m water depth 20–65%. In the deeper pelagic region (>3 m) more than 85% of the primary production originated from the phytoplankton.     

Seasonal habitat selection by lake trout (<Emphasis Type="Italic">Salvelinus namaycush</Emphasis>) in a small Canadian shield lake: constraints imposed by winter conditions

The need for cold, well-oxygenated waters significantly reduces the habitat available for lake trout (Salvelinus namaycush) during stratification of small temperate lakes. We examined the spatial and pelagic distribution of lake trout over two consecutive summers and winters and tested whether winter increased habitat availability and access to littoral regions in a boreal shield lake in which pelagic prey fish are absent. In winter, lake trout had a narrowly defined pelagic distribution that was skewed to the upper 3 m of the water column and spatially situated in the central region of the lake. Individual core areas of use (50% Kernel utilization distributions) in winter were much reduced (75%) and spatially non-overlapping compared to summer areas, but activity levels were similar between seasons. Winter habitat selection is in contrast to observations from the stratified season, when lake trout were consistently located in much deeper waters (>6 m) and widely distributed throughout the lake. Winter distribution of lake trout appeared to be strongly influenced by ambient light levels; snow depth and day length accounted for up to 69% of the variation in daily median fish depth. More restricted habitat use during winter than summer was in contrast to our original prediction and illustrates that a different suite of factors influence lake trout distribution between these seasons.     

Sources of water used by riparian Eucalyptus camaldulensis overlying highly saline groundwater

Water sources of Eucalyptus camaldulensis Dehn. trees were investigated on a semiarid floodplain in south-eastern Australia. The trees investigated ranged in distance from 0.5 to 40 m from a stream, with electrical conductivity 0.8 dSm^–1, and grew over groundwater with electrical conductivity ranging from 30 to 50 dSm^–1. The sources of water being used by the trees were investigated using the naturally occurring stable isotopes of water and measurements of soil water potential. Xylem water potential and leaf conductance were also examined to identify the trees' response to using these sources of water. Trees at distances greater than about 15 m from the stream used no stream water. The trees used groundwater in summer and a combination of groundwater and rain-derived surface-soil water (0.05–0.15 m depth) in winter. In doing so they suffered water stress at electrical conductivities higher than approximately 40 dSm^–1 (equivalent to approximately –1.4 MPa). Trees adjacent to the stream used stream water directly in summer, but may have used stream water from the soil profile in winter, after the stream had risen and recharged the soil water. E. camaldulensis appeared to be partially opportunistic in the sources of water they used.     

Optimum growth conditions and light utilization efficiency of Spirulina platensis (= Arthrospira fusiformis) (Cyanophyta) from Lake Chitu, Ethiopia

Spirulina platensis (= Arthrospira fusiformis) was isolated from Lake Chitu, a saline, alkaline lake in Ethiopia, where it forms an almost unialgal population. Optimum growth conditions were studied in a turbidostat. Cultures grown in modified Zarrouk's medium and exposed to a range of light intensities (20–500 µmol photons m^–2s^–1) showed a maximum specific growth rate (µ_max) of 1.78 d^–1. Quantum yield for growth (µ) was 3.8% at the optimum light for growth of 330 µmol photons m^–2s^–1, and ranged from 2.8 to 9.4%. With increase in irradiance, the chlorophyll a concentration decreased, and the carotenoids/chlorophyll a ratio increased by a factor of 2.4. The phosphorus to carbon ratio (P/C) showed some variation, while the nitrogen to carbon ratio (N/C) remained relatively constant, thus causing fluctuations in the N:P ratio (7–11) of cells. An optimum N:P ratio of about 7 was attained in cells growing at the optimum light for growth. Results from the continuous culture experiments agreed well with maximum values of photosynthetic efficiency given in the literature for natural populations of S. platensis in the soda lakes of East Africa, Lake Arenguade (Ethiopia), and Lake Simbi (Kenya).