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/m''^) 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 Oa/m^.h in both lakes. They were obviously severely limited by self-shading behaviour in 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/m^, 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 O-zjm^. 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
Examples of stratification from nine tropical upland lakes, two in Uganda and seven in Ethiopia, arc described. Strong thermal gradients were often developed from diurnal warming in the 0–5‐m layer. In some productive lakes, these gradients frequently trapped photosynthetic oxygen to form well‐defined maxima. They sometimes also delimit deeper, oxygen‐poor layers in some highly productive lakes. One lake showed nocturnal mixing with complete deoxygenation, followed by a superficial restratification and photosynthetic reoxygenation by day. Deeper thermal gradients often determined the course of oxygen depletion at lower levels, and in some lakes also determined other aspects of chemical stratification, including the deep accumulation of Ca++ and HCO3− ions. Considerable variation of the stratification was found in the Ethiopian Lake Awassa, both in time and horizontally over the lake basin, with some evidence that the deeper layer was formed by down‐streaming from locally cooled margins. Comparison is made with other lakes at similar altitudes in equatorial Africa. Although seasonal changes arc inadequately known, the incidence of complete mixing probably varies from very frequent (polymictic lakes) to very infrequent (oligomictic or meromictic lakes), and is determined by both depth and shape of the lake basin.
Most Ethiopian lakes are parts of closed drainage systems and collectively form an extensive salinity series, here treated comparatively for geographical, chemical and algal characteristics. Chemical data are presented for 28 lakes and numerous inflows, including original analyses for 15 lakes, in which total ionic concentration and electrical conductivity vary over 4 orders of magnitude. The principal determinant of a lake's position in the series is the open or closed nature of its individual drainage. At present there are three major closed systems (Awash R. -Afar drainage, northern rift lakes, southern rift lakes), numerous crater lakes with seepage -in and -out, and two cryptodepressions with marine inputs. Salinity is primarily determined by evaporative concentration, enhanced in lakes associated with past marine influence or recent volcanic activity by readily soluble materials in the catchment, and by some thermal-reflux pathways. However, anomalously dilute closed lakes exist, indicative of other processes of solute loss (e.g. past basin overflow, 'reverse weathering', seepageout).There are strong positive correlations between increasing salinity and the concentrations of Na + , alkalinity and Cl-. The last is used, in conjunction with other analyses of atmospheric precipitation, to estimate the marine and denudative contributions and the evaporative concentration factor, and to distinguish trends of ionic species during evaporative concentration. With several exceptions, affected by past penetration of sea water into the Danakil and L. Assal cryptodepressions, the most saline lakes are soda lakes with HCO + CO]-and Na + predominant and Ca 2+ and Mg 2 + largely eliminated. Soluble reactive silicate and phosphate tend to increase in concentration along the salinity series, but the unknown dynamics of algal growth are likely to introduce variance. Concentrations in some lakes are extremely high, e.g. > 40 mg SiO 2 1 -1 and > I mg PO 4 -P 1-'.Phytoplankters recorded from individual lakes are tabulated and where available the community biomass concentration as chlorophyll a is given. Lakes of high salinity-alkalinity are typically very productive in terms of phytoplankton biomass and photosynthetic rates (exceptions: the very deep L. Shala and the very saline L. Abhe), supported in part by relatively high concentrations of phosphorus and inorganic carbon. Many species are of restricted salinity-alkalinity range, being characteristic of waters where levels are low (e.g. desmids, Melosira spp.), intermediate (e.g. Planctonema lauterborn), or high (e.g. Spirulinaplatensis). Phytoflagellates are most strongly represented in waters with higher concentrations of the bivalent cations Ca 2 + and Mg 2 + . The common cyanophyte Microcystis aeruginosa can tolerate a wide salinity range, here as elsewhere.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.