The upper limit of photosynthetic productivity by phytoplankton: evidence from Ethiopian soda lakes

Photosynthetic production by phytoplankton was studied in two Ethiopian soda lakes, alkalinity 51-67 m-equiv./l, with abundant blue-green algae. The deeper lake, L. Aranguadi, contained extremely dense crops composed almost entirely of SpiruUna (Oscillatoria, Arthrospira) platensis. Measurements of the spectral attenuation of light showed that the most penetrating component was displaced to the red spectral region, and estimates of the chlorophyll a content in umt area of the euphotic zone were often similar to the highest values (～200-300 mg/m²) expected on theoretical grounds. The vertical distribution of photosynthetic activity per unit water volume was of a typical pattern for phytoplankton, with light-inhibition often present, although the euphotic zones did not exceed 0.6 m in depth. From these profiles, computed rates of gross photosynthesis per unit area of lake surface reached a magnitude of 1.4–2.4 g O₂/m².h in both lakes. They were obviously severely limited by self-shading behaviour i n the algal populations. An appreciable inverse relationship between photosynthetic capacity and population density was only found in the densest populations of L. Aranguadi, with chlorophyll a content > 2000 mg/m³, where depressed rates might result from experimental artefacts in closed bottles. In this lake, two independent estimates of gross production, based on the analysis of diurnal changes in the open water, were as high as 43 and 57 g O₂/m². day. Diurnal changes of stratification in the two lakes are described and related to the controlling temperature (density) stratification. They include occasionally complete nocturnal deoxygenation in the deeper lake, evidence of heavy respiratory uptake. The high photosynthetic productivity is interpreted as dependent upon the coupling of high algal contents in the euphotic zone (^S) with high values of photosynthetic capacity (^max)- It is probably favoured by the tropical situation affecting temperature and illumination, by a surplus of dissolved inorganic phosphate, and especially by the considerable reserves of carbon dioxide in these soda lakes.     

Diversity in the influence of temperature on the growth rates of freshwater algae, and its ecological relevance

1. Growth rates of seven species of planktonic algae were determined in culture over a range of temperature from 2 to 35 °C. Additional observations on growth and viability were made for 13 species in the temperature range 20–35 °C. 2. There was a wide range of growth rates between species at their optimal temperatures, from 1.7 divisions day^?1 (Asterionella formosa) to 0.3 divisions day^?1 (Ceratium furcoides). 3. There were considerable differences between species for growth at low and high temperature. Certain algae, including the diatom A. formosa and the flagellates Cryptomonas marssonii, Dinobryon divergens and Eudorina unicocca var. unicocca, had growth rates of 0.4 divisions day^?1 or more at 5 °C. The cyanophyte Tychonema (formerly Oscillatoria) bourrellyi, the xanthophyte Tribonema sp., the desmid Staurastrum cingulum and the large dinoflagellate C. furcoides grew poorly or not at all at this temperature. All 21 species tested could grow at 25 °C, but many – including most of the diatoms, some cyanophytes, and all the flagellates – failed to grow persistently at 30 °C. Only Aphanizomenon flos‐aquae survived with moderate increase at 35 °C, a lethal temperature for the other species. 4. The applicability was considered of proposed quantitative formulations of the rate‐temperature relationship. Simple exponential relationships applied only to very limited lower ranges of temperature. The relationship proposed by B?lehrádek was a better fit over a wider temperature range, but still excluded rate‐decline at high temperature. 5. The interspecific differences found are of potential significance for restrictions in natural distributions associated with season, altitude (especially above 500 m) and latitude.     

Comparative estimations of the energy content of fish tissue from bomb calorimetry, wet oxidation and proximate analysis

SUMMARY. Four methods were used to determine the energy content of somatic tissues of Perca fluviatilis . Two forms of direct calorimetry (both adiabatic and non-adiabatic) and wet (dichromate) oxidation gave similar results. When energy contents were calculated from proximate analysis using accepted conversion factors (9.45 cal mg⁻¹ for lipid, 5.65 cal mg⁻¹ for protein) results were higher than those from the other methods. The discrepancy was eliminated when a lower, directly determined energy content for the extracted lipid fraction was used.
Some historic and technical aspects of the comparison are discussed.     

The effect of sewage effluent on denitrification in Grasmere (English Lake District)

SUMMARY. Denitrification processes in Grasmere were studied using several techniques. Sediments were analysed for nitrate reductase activity and the accumulation of nitrogenous gases in samples incubated in the laboratory. Changes in the concentrations of inorganic nitrogen species in the water column and the release of gas bubbles into gas traps suspended above the sediment were also measured. Sewage effluent entered the lake via the main inflow and faecal bacteria were used as tracers to map the path of the river (and effluent) across the lake. The main river flow was west of the island in the centre of the lake and across the smaller of the two main basins. A preliminary study indicated that this western basin was the site of most active denitrification and this was confirmed in the following year when a seasonal survey showed that deoxygenation, nitrate reduction and ammonium accumulation was more advanced at this site. In addition, this basin was the only area where consistent discharge of nitrogen and methane from the profundal zone was observed. The nitrate reductase assay demonstrated that the sediment was the major site of denitrification, and that activity in the water column was higher in the anoxic hypolimnion than in the euphotic zone. Gas evolution from incubated sediment samples was analysed by gas liquid chromatography (GLC) and indicated that maximum denitrifying activity occurred below the mud surface in oxidized sediments. There was reasonable agreement between the enzyme assay and the GLC technique but neither predicted events in the field satisfactorily. Viable, most probable number (MPN) counts of denitrifying bacteria had similar limitations. A bimodal seasonal pattern of nitrate reductase activity was observed at four sites during thermal stratification, but this could not be explained fully. The major site of denitrification was identified and it was calculated that the nitrogen gas discharged was equivalent to c. 54% of the nitrate removed from the water column.     

Long-term and seasonal changes in the chemical composition of precipitation and surface waters of lakes and tarns in the English Lake District

SUMMARY. The composition of bulk (wet and dry) precipitation in 1975 and 1976 was similar to that found 22 years previously. In 1975, mean values for nine precipitation samplers at one site (Wraymires) ranged from pH 4.3 to 4.5; similar values were obtained in 1976. Samplers covered with fine-mesh plastic gauze caught substantially more Ca²⁺ and K⁺ than open samplers, but pH and SO^2-₄+ NO₃^- concentrations were similar in open and covered samplers. In precipitation, c. 50% of H⁺ was balanced by NO₃^- and c. 50% by SOi; 80% of the SO^2-₄ was balanced by Ca²⁺ and Mg²⁺. Conccntrations of major cations (H⁺, Na⁺, K⁺,Ca²⁺, Mg²⁺) and anions (CI^-, NO₃^- SO^2-₄ and alkalinity [Alk—largely HCO₃^-]) in upland water- bodies were similar to those found in precipitation, but pH levels were generally higher and above 6.0 m some tarns. At lower altitudes, on base-rich roeks and soils, Ca²⁺ and Alk become dominant. Results of a survey of lakes and tarns in 1974–78 are compared with a survey in 1953–56 and published data (chiefly for pH and Alk) for 1947–50, 1932 and 1928. Comparisons are also made with other measurements of Alk in three productive lakes (Blelham Tarn, Esthwaite Water and Windermere) for 1936–39 and 1945–80. Winter levels of NO₃-N, PO₄-P and Si are given for these lakes; although the first two have increased during the late 1960s and the 1970s there has been no significant change in the last. NO₃^- and probably some SO^2-₄. In productive lakes a substantial (c. 50%) rise in mean Alk occurred during the late 1960s and the 1970s, possibly related to increased winter levels of NO₃-N and PO₄-P derived from sewage and fertilizers. In this period the maximum pH levels reached in summer were notably high, sometimes exceeding pH 10. The rise in Alk, conductivity and pH of surface waters is influenced by climatic factors (a decade of drier years), sewage input and biological productivity within the lakes. Considerable seasonal fluctuations in the concentrations of major ions, a characteristic feature of surface waters in the English Lake District, are illustrated and some implications for cation-anion balance briefly discussed. Mid-winter concentrations are usually high forNa⁺, K⁺, Cl^-. NO₃^-and low for Ca²⁺, Mg²⁺, Alk. SO^2-₄. Alkalinity. pH and conductivity of Lake District tarns and lakes show no signs of acidification during the period 1928–80. On the contrary, productive lakes have become more alkaline and some unproductive low-alkalinity (< 100 μ-equiv. 1^-1) lakes also show signs of alkalization, with increased mean concentrations of Na⁺. Ca²⁺ and Mg²⁺, balanced by Alk.     

Physiological and environmental constraints in the ecology of the planktonic dinoflagellate Ceratium hirundinella

SUMMARY. Constraints upon the vertical distribution of a population of Ceratium hirundinella , in a productive English lake during the summer stratification of 1976, are considered. They are interpreted in relation to vertical gradients of temperature, dissolved oxygen and irradiance, and to rates of photosynthesis and respiration measured as oxygen exchange in long- and short-term exposures. The motile cells tended to aggregate at an intermediate depth in the epilimnion, associated with a relative irradiance level of c . 10% or c . 140 μ einsteins m⁻²s⁻¹ as measured with a horizontal PAR sensor. Higher irradiances, and conditions below the oxycline, were apparently unfavourable, but the intervening layer was severely compressed at the height of summer stratification, when concentrations of inorganic nitrogen were also minimal. Thus the population apparently passed through a critical period, at which the cellular content of chlorophyll-α was much reduced. The preferred irradiance level in the lake corresponded to that at which measured rates of net photosynthesis were maximal. Increased rates of oxygen evolution were measured at higher irradiances in very short exposures; this behaviour may be of importance to cells experiencing vertical movement in nature.     

The Relative Growth Rates of Three Plankton Diatoms in Relation to Underwater Radiation and Temperature

Relative growth rates of three freshwater plankton diatoms-Asterionellaformosa, Fragilaria crotonensis, and Tabellaria flocculosa var.asterionelloides-are described from cultures suspended at variousdepths and during several seasons in the lake Windermere. Seasonalvariation in rates recorded near the surface (i m. depth) isinterpreted in terms of seasonal changes in temperature anddaylength. Rates recorded for Asterionella and Fragilaria aregenerally similar, but are approximately twice the rates obtainedwith Tabellaria. Depth profiles of relative growth rates areof similar form in all species, and normally show lightsaturationnear the surface. The shape of profiles for Asterionella isin good agreement with estimates of photosynthesis integratedover the growth periods. The parallelism between photosyntheticand relative growth rates of Asterionella is further illustratedfrom laboratory experiments: an approximate interconversion,under certain conditions, is given.