The seasonality of phytoplankton in African lakes

Although some study of the subject began in 1899, wide-ranging information from African water-bodies has only become available since 1950. Important developments included the establishment of long-term centres of research, the adoption of improved methods for quantitative algal sampling, the more intensive study of environmental conditions, the beginnings of experimental testing, and the improvement of taxonomic knowledge.At higher latitudes (> 20 °) examples of pronounced algal seasonality are long-established; they are accompanied and influenced by marked changes in radiant energy income and so water temperature, and often by effects of seasonal water input. Illustrations are given from lakes in Morocco and South Africa.More generally in Africa, including the tropical belt, annual patterns of phytoplankton seasonality are usually either dominated by hydrological features (water input-output) or by hydrographic ones (water-column structure and circulation). Examples of both types are discussed, together with instances (e.g. L. Volta) of combined hydrological and hydrographic regulation. In both the seasonal abundance of diatoms is often distinct and complementary to that of blue-green algae, with differing relationships to vertical mixing and water retention.Horizontal variability in the seasonal cycle is especially pronounced in the larger or morphometrically subdivided lakes. Some inshore-offshore differentiation is also known to affect phytoplankton quantity (e.g. L. George) and species composition (e.g. L. Victoria). Longitudinal differentiation is common in elongate basins especially when with a massive or seasonal inflow at one end (e.g. L. Turkana, L. Nubia, L. Volta); occasional terminal upwelling can also be influential (e.g. southern L. Tanganyika). Such examples grade into the longitudinally differentiated seasonality of flowing river-reservoir systems, as studied on the Blue and White Niles.The annual amplitude of population density, expressed in orders of magnitude (=log₁₀ units), is one measure of seasonal variability. It can exceed 3 orders both in systems subject to hydrological wash-out (e.g. Nile reservoirs) and in the more variable species components of lakes of long retention (e.g. L. Victoria). Low amplitudes can be characteristic of some components (e.g. green algae in L. Victoria) or of total algal biomass (e.g. L. George, L. Sibaya).Seasonal changes may be subordinated to inter-annual ones, especially in shallow and hydrologically unstable lakes (e.g. L. Nakuru).     

The ‘shock period’: dynamics of phytoplankton during the spring–summer transition of a stratifying English lake

The spring to summer transition in a productive English lake is considered with respect to phytoplankton and its environmental conditions. Salient environmental changes include the onset of temperature/density stratification that is usually accompanied by a clear-water phase associated with a maximum of grazing Daphnia and a minimum of phytoplankton in the 0â€5 m zone. Below this zone, as thermal stratification progresses, a deep maximum of phytoplankton can develop under strong thermal/density gradients and enhanced light penetration. Examples are resolved by estimations of chlorophyll-a , beam attenuance in situ and cell counts. Attributed origins are by sedimentation of diatoms, migration of flagellates, and depth-adjusted buoyancy of a gas-vacuolate cyanophyte. The transition period involves a decline of spring-associated diatom populations and a rise of summer-associated species. The generally low algal abundance within the transition phase has at least four origins †prior nutrient (Si) depletion, sedimentation, grazing, and low inoculum levels of successor species. It can be augmented by the re-growth of species abundant in spring, by early extensions of normally summer species, by seasonally characteristic colonial chrysophytes, and by other phytoflagellates of small size that are seasonally less specific (opportunistic) and probably critical for Daphnia grazing with consequent generation of the clear-water phase.     

Selective recruitment and resurgence of tropical river phytoplankton: evidence from the Nile system of lakes,rivers, reservoirs and ponds

The recruitment of major species-components of phytoplankton is considered with reference to diverse water-bodies—rivers, headwater lakes, reservoirs and lateral waters—of the Nile system. The importance of massive ‘inocula’, carried in flowing and inter-communicating waters, is emphasised; the most direct evidence is from quantitative population dynamics in longitudinally sampled river-reservoir systems. Other indications of extraneous derivation are taken from geographical distribution and apparent invasions of newly recorded species. For five reservoirs, there are records of initial colonisation (in two) and of the annual colonisation of seasonally impounded water (in three). There was a selective recruitment of reservoir/river major species over 1 year to newly created ponds, from a massive inoculum of added river water, studied in relation to the novel and changing physical and chemical environment. Periods of predominant decomposition were there linked with the abundance of other species not observed in the original river water. Reservoir species can be carried to extend abundance far downstream, as observed in the Blue Nile where normal succession was altered by a new reservoir upstream; also in Egypt below the former Aswan Reservoir and the later High Dam Lake. The different and distinctive compositions of the phytoplankton communities in headwater lakes reflect large environmental differences that, with high downstream turbulence, probably make them insignificant for recruitment of most species typical of the downstream waters. There is evidence from hydrology and floristic similarities for recruitment from lateral standing waters along the river, especially those of one major swamp area.     

Seasonal dynamics of phytoplankton and phytobenthos,and associated chemical interactions,in a shallow upland lake (Malham Tarn,northern England)

Seasonal changes of phytoplankton were followed over 3 years (1985–87) in a shallow, unstratified and calcareous upland lake.The phytoplankton was of low to moderate abundance and generally dominated by phytoflagellates. Seasonality involved a winter minimum of abundance, a spring maximum of diatoms, and often brief increases in summer that included blue-greens, especially the colonial Gloeotrichia echinulata. Some components were of benthic origin. Seasonal growth of the main component of the phytobenthos, Chara globularisvar. virgata, caused a regular summer depletion in lake water of Ca²⁺ and HCO₃ ^- (alkalinity) by associated CaCO₃ deposition, and a more extreme (and unusual) depletion of K⁺. Chemical analysis of Chara biomass and of underlying sediments indicated a large benthic nutrient stock, much surpassing that represented by the phytoplankton. Growth in this biomass, and the magnitude of water-borne inputs, influenced the removals of Ca²⁺, K⁺ and inorganic N. The phytoplankton was probably limited by a low-P medium, to which co-precipitation of phosphate with CaCO₃ may have contributed. A vernal depletion of Si was probably limiting to diatom growth, and appeared to be mainly induced by benthic rather than planktonic diatoms. Examples of long-term change in composition of the phytoplankton and phytobenthos are noted and discussed in relation to the interaction of these components, nutrient enrichment, and possible alternative stable states.     

Past and contemporary trends and attitudes in work on primary productivity

Against a historical background, a personal commentary is givenon questionable traditions and quantitative operations embodiedin the study of the primary productivity of phytoplankton. Mostsuch problems appear to have arisen from over-simplificationof relationships in heterogeneous or otherwise complex systems. ^*This paper is the result of a study made at the Group for AquaticPrimary Productivity (GAP) First International Workshop heldat the Limnological Institute, University of Konstanz, in April1982.     

Nitrate reduction by Pseudomonas spp.: antagonism by fermentative bacteria

The reduction of nitrate by Pseudomonas denitrificans in a culture medium containing glycerol, yeast extract and 700 mg/1 NO₃–N, was antagonized by Aeromonas hydrophila, Escherichia coli and Enterobacter aerogenes. Nitrate reduction by Ps. denitrificans in mixed culture with a fermentative heterotroph was inhibited when 100–150 mg/1 NO₂–N had accumulated in the medium. The number of Ps. denitrificans declined concomitantly with the appearance of NO₂ in the culture medium, but there was only a slight increase in the numbers of fermentative hetero-trophs. The fermentative heterotrophs did not antagonize nitrate reduction by Ps. denitrificans , when the culture medium contained 140 mg/1 NO₃–N. When mixtures of equal parts of Ps. denitrificans and Esch. coli cultures were added to autoclaved river water relatively high concentrations of NO₂ were produced from the nitrate present in the water.     

Chemical and algal relationships in a salinity series of Ethiopian inland waters

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, seepage-out).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 HCO₃ ^- + CO _inf3 ^sup2- and Na⁺ predominant and Ca²⁺ and Mg²⁺ 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 SiO₂ l^-1 and > 1 mg PO₄-P l^-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 lauterborni), or high (e.g. Spirulina platensis). Phytoflagellates are most strongly represented in waters with higher concentrations of the bivalent cations Ca²⁺ and Mg²⁺. The common cyanophyte Microcystis aeruginosa can tolerate a wide salinity range, here as elsewhere.     

Phytoplankton–zooplankton seasonal timing and the 'clear-water phase' in some English lakes

SUMMARY 1. An examination is made of the relative seasonal timing of the postwinter increase of phytoplankton and zooplankton populations in four English lake basins. It centres upon weekly sampling over 20 years and rough counts of larger Crustacea, as copepods and cladocerans, from filtered samples that were used for chlorophyll a (Chl) estimation. 2. Typically, a spring maximum of phytoplankton, dominated by diatoms and earlier in the shallower lakes, is accompanied or followed by a maximum of copepods and then one of cladocerans dominated by the Daphnia hyalina–galeata complex. Regarding timing, the maximum of copepods has no apparent relation with phytoplankton abundance (Chl). The maximum of cladocerans appears to be largely independent of variation in the phytoplankton maximum, but is generally associated with a minimum in Chl. Evidence for some direct causality in this inverse correlation after the spring phytoplankton maximum is best displayed by the shallow Esthwaite Water in which the peaks of Chl and cladocerans are separated further than in the deep Windermere basins where phytoplankton growth is delayed. In Esthwaite Water, and possibly often in Windermere, a principal minimum in Chl is ascribable to grazing by Daphnia. 3. The typical inverse relationship of Chl and cladocerans is lost in some years when relatively inedible large phytoplankters (e.g. colonial chrysomonads, filamentous cyanophytes) are abundant and Chl minima are less pronounced, although maxima of cladocerans still occur. Conversely, available edible phytoplankters include various small forms grouped as μ‐algae and Cryptomonas spp.; their probable depletions by Daphnia appear to be sequential and may limit the latter's maxima, whose inception is temperature‐dependent. 4. The spring–summer maxima of cladocerans and minima of Chl are generally coincident with a main seasonal maximum of Secchi disc transparency and light penetration – to which removal of non‐phytoplankton particles by filtering cladocerans may contribute.     

Potassium dependence and phytoplankton ecology: an experimental study

SUMMARY 1. Unialgal cultures of three species common in the freshwater phytoplankton were used to test limitation of specific growth rate and final yield in defined media of low K⁺ concentration (range <0.3–6 μmol L⁻¹ or mmol m⁻³).
2. Growth rate of the diatom Asterionella formosa was independent of K⁺ concentration above 0.7 μmol L⁻¹. Final yield was dependent on initial concentration when accompanied by K⁺ depletion below this concentration, but not by lesser depletion with more residual K⁺. Analyses of particulate K in the biomass indicated a mean final cell content of 2.8 μmol K 10⁻⁸ cells, approximately 1.0% of the organic dry weight.
3. Less detailed work with the diatom Diatoma elongatum showed no dependence of growth rate or final yield upon the initial K⁺ concentration in the range 0.8–3.2 μmol L⁻¹. The phytoflagellate Plagioselmis nannoplanctica suffered net mortality in the lowest concentration tested, 0.8 μmol L⁻¹.
4. Comparison with the range of K⁺ concentration in natural fresh waters, including a depletion induced by an aquatic macrophyte, suggests that K⁺ is unlikely to limit growth of phytoplankton. Nevertheless, there can be correlation of K⁺ with lake trophy.