A new biomonitoring protocol to determine the ecological health of impoundments using artificial substrates

The colonisation potential of macroinvertebrate families was measured using artificial substrates. These substrates were tested on three highveld impoundments. Bronkhorstspruit Dam was an amictic, mesotrophic impoundment, while Hartbeespoort and Roodeplaat Dams were both monomictic, eutrophic impoundments. Bronkhorstspruit Dam had the best water quality and the smallest algal population, comprising mainly diatom species. Hartbeespoort Dam had the most saline water (TDS 96–400 mg/l; NH₄ <0.004–0.218mg/l; NO₂+NO₃ 0.109–2.776mg/l; PO₄ <0.04–0.06mg/l), while Roodeplaat Dam had the highest nutrient concentrations (TDS 200–373mg/l; NH₄ <0.04–0.933mg/l; NO₂+NO₃ <0.04–2.310mg/l; PO₄ 0.041–0.472 mg/l). Both Hartbeespoort and Roodeplaat Dams had large algal populations that resulted in very alkaline water during summer. Microsystis dominated the phytoplankton population in these two impoundments during summer and autumn, while green algae and diatoms were dominant during winter and spring. Ecological health was determined using two different biotic indices. Two modifications of the Belgian Biotic Index and a modification of SASS4 (SASSD) were tested. Three health classes were suggested for SASSD scores and ASPT values. It is concluded that artificial substrates with SASSD (and ASPT) as a biotic index can be used to determine the biological health of impoundments, but that further refinement of the index is needed.     

A PRELIMINARY LIMNOLOGICAL STUDY OF BUFFELSPOORT DAM AND ITS CATCHMENT

SUMMARY

A limnological survey of the Buffelspoort Dam and its catchment between 1973 and 1975 showed that the water of the area contained extremely low concentrations of dissolved minerals and that there was no point source of nutrient supply to the dam. The anion and cation dominance in the catchment water may be summarized as HCO3 >Cl >SO₄NO₃ and Na⁺ >Mg⁺⁺ >Ca⁺⁺ >K⁺, respectively, and this water may be classified as soft bicarbonate water. The impoundment was warm monomictic and had an extensive anaerobic hypolimnion during summer stratification. Changes in water transparency were related to the seasonal fluctuation in water content and the hydrological cycle. Nutrient concentrations in the dam displayed seasonal trends which were related to stratification, hydrological events and phytoplankton activity. On the basis of its nutrient content and chlorophyll a concentrations the impoundment can be classified as mesotrophic. The further development of holiday resorts in the catchment poses a cutrophication hazard, especially if expansion results in the installation of sewage treatment plants.     

NITROGEN TRANFORMATIONS AND THE NITROGEN BUDGET OF A HYPERTROPHIC IMPOUNDMENT (HARTBEESPOORT DAM,SOUTH AFRICA)

SUMMARY

The major components of the nitrogen balance of hypertrophic Hartbeespoort Dam were studied at weekly intervals from October 1980 to September 1984. The Crocodile River contributed over 80Z of the hydraulic load and over 97X of the annual total nitrogen load to the impoundment. More than 802 of this nitrogen load was composed of nitrate. Monthly measurements of nitrification and denitrification rates carried out within the impoundment over one annual cycle revealed that the rates of both processes were highest at the ammonia-oxygen chemocline during summer stratification and at the sediment-water interface during winter isothermy. Mass balance calculations showed that between 35 and 49% of the annual total nitrogen load were lost via denitrification each year; the data are compared with those in the literature. Conditions in the lake will favour a change in phytoplankton species composition from Microcystis to a nitrogen-fixing species if the present trend of decreasing inorganic nitrogen concentrations and N: P ratios is maintained.     

Nitrogen limitation of phytoplankton in a Spanish karst lake with a deep chlorophyll maximum: a nutrient enrichment bioassay approach

An in vitro nutrient addition bioassay was performed to testthe relative inorganic nitrogen (N) and phosphorus (P) limitationof phytoplankton in a Spanish karst lake (El Tejo) during thelast part of the stratification period, when nutrient limitationis most pronounced. Nutrient deficiency was tested in samplesfrom three different layers of the lake: the epilimnion, metalimnionand oxic hypolimnion. Nitrogen additions, either without orcombined with P, increased phytoplankton growth in all threestrata, compared with controls or P treatments. This showedthat N was the nutrient limiting phytoplankton growth in latesummer–early fall. Since both hypolimnetic diffusion andgroundwater fluxes of N-rich waters into the lake are much reducedduring summer, N becomes the limiting nutrient as stratificationadvances. We suggest that in this Mediterranean area with lowatmospheric deposition of anthropogenic N and in lakes relativelyfree of surface run-off, nutrient supply by atmospheric depositionmight be a key factor in controlling nutrient deficiency forphytoplankton growth.     

Temporal and vertical dynamics of phytoplankton net growth in Castle Lake, California

The impact of nutrient additions, zooplankton grazing and light intensity on phytoplankton net growth with depth and season was studied with five microcosm experiments in meso-oligotrophic, subalpine Castle Lake, California, during the period of summer stratification in June-September 1994. The incubations (4 day) were performed at 5 m intervals from the surface to the bottom using natural phytoplankton and zooplankton assemblages, with enrichments of phosphorus and nitrogen. The phytoplankton community was only limited by nutrients in the upper 5 m (epilimnion), as indicated by change in chlorophyll concentration. Nutrient enrichments had the greatest effect on the phytoplankton net growth in June and July. High light inhibited the phytoplankton net growth at the surface. Low light intensities limited phytoplankton at 20 m and below, and at the end of the growing season already around 10-15 m. A deep chlorophyll maximum in the hypolimnion in June-August was not limited by either light or nutrients. The results showed variation in grazers' impact on phytoplankton. These results suggest the importance of nutrient limitation only in the epilimnion with light inhibition at the surface, light limitation in the hypolimnion, and varying impact of zooplankton grazing in influencing the development of the phytoplankton in Castle Lake.      

Seasonal changes in the chemistry and biology of a meromictic lake (Big Soda Lake,Nevada, U.S.A.)

Big Soda Lake is an alkaline, saline lake with a permanent chemocline at 34.5 m and a mixolimnion that undergoes seasonal changes in temperature structure. During the period of thermal stratification, from summer through fall, the epilimnion has low concentrations of dissolved inorganic nutrients (N, Si) and CH₄, and low biomass of phytoplankton (chlorophyll a ca. 1 mgm ^-3). Dissolved oxygen disappears near the compensation depth for algal photosynthesis (ca. 20 m). Surface water is transparent so that light is present in the anoxic hypolimnion, and a dense plate of purple sulfur photosynthetic bacteria (Ectothiorhodospira vacuolata) is present just below 20 m (Bchl a ca. 200 mgm^-3). Concentrations of N H₄ ⁺, Si, and CH₄ are higher in the hypolimnion than in the epilimnion. As the mixolimnion becomes isothermal in winter, oxygen is mixed down to 28 m. Nutrients (NH₄ ⁺, Si) and CH₄ are released from the hypolimnion and mix to the surface, and a diatom bloom develops in the upper 20 m (chlorophyll a > 40 mgm^-3). The deeper mixing of oxygen and enhanced light attenuation by phytoplankton uncouple the anoxic zone and photic zone, and the plate of photosynthetic bacteria disappears (Bchl a ca.10mgm^-3). Hence, seasonal changes in temperature distribution and mixing create conditions such that the primary producer community is alternately dominated by phytoplankton and photosynthetic bacteria: the phytoplankton may be nutrient-limited during periods of stratification and the photosynthetic bacteria are light-limited during periods of mixing.     

The Effect of Outflow Depth on Phosphorus Retention in a Small,Hypertrophic Temperate Reservoirwith Short Hydraulic Residence Time

Stratification and phosphorus fluxes (input, output, sedimentation, and release from sediments) were studied in České údolí Reservoir (49°43′N, 13°21′E; V – 2.65 × 10⁶ m³; A – 1.04 × 10⁶ m²; z_max – 5.5 m; surface altitude – 314 m a.s.l.) during two summer stratification periods which differed in outlet operation and in hydraulic residence time (1997: surface outlets and 14 days; 1998: bottom outlets and 23 days). Use of bottom outlets resulted in weaker thermal stratification, a less pronounced oxygen deficit in the hypolimnion, and significantly lower P retention (17%) in comparison with surface discharge (42%). Factors apparently contributing to lower retention of P during the use of bottom outlets were: (i) faster flushing of the hypolimnion which intensified longitudinal transport of particles, (ii) more intense P release from sediments due to a lower concentration of nitrate in the water column and to more frequent contact of sediments in the shallows with water of pH >9. During both periods, phytoplankton production was limited only by light, not by nutrients. Phytoplankton biomass was comparable in both years despite some differences in vertical distribution.     

Origin and succession of phytoplankton in a river-lake system (Spree,Germany)

The River Spree (Germany) flows through an impoundment and several shallow lakes in its middle and lower course. In this river-lake system, the seasonal and longitudinal dynamics of dominant phytoplankton populations were studied in relation to retention time of water, mixing conditions and nutrient supply from 1988–92. Some phytoplankton species populated the same river section for weeks or months each year at their season. Such stable populations have to origin from river zones functioning like mixed reactors. In the Spree system, centric diatoms originated from an impoundment and filamentous cyanobacteria from a flushed lake with longer retention time of water. Downstream, biomass and composition of phytoplankton altered nearly simultaneously along the system.The fate of planktonic organisms washed from mixed reactors into the flow depended on the conditions at the zones of origin. During spring, populations dominating phytoplankton communities of the well-mixed lakes grew further under river conditions. However the biomass of summer species, adapted to intermittent stratification, was halved along the river course. These seasonal differences were probably caused by lower maximum growth rates of summer species and enhanced losses (photorespiration, sedimentation or grazing of benthic filter feeders, but not of zooplankton) of algal populations under river conditions in summer.Phytoplankton assimilation, settlement of diatoms, or denitrification caused declining (probably growth limiting) concentrations of dissolved inorganic phosphorus (spring), silicon (early summer) or nitrogen (summer) along the river course, respectively. The minimum content of DRP was often followed by a clear-water phase. Reduced DSi supply selected against diatoms and additional DIN shortage favoured N₂-fixing cyanobacteria in the last lake of the system.R-strategists (sensu Reynolds) were selected in both the flushed, shallow lakes and the lowland river. In general, the biomass of cyanobacteria increased within the lakes and declined along the river course. Some diatom populations grew in the river, but were grazed or settled down in the lakes. Beside this general picture, different populations from the same phylogenetic group did not necessarily perform in similar ways.     

THE LIMNOLOGY OF SOME SOUTH AFRICAN IMPOUNDMENTS I. THE PHYSICO-CHEMICAL LIMNOLOGY OF HARTBEESPOORT DAM

SUMMARY

Hartbeespoort Dam is a eutrophic, warm monomictic lake with overturn occurring in April. The lake is stratified for about six months of the year and in late summer the stratification appears stable. Light penetration in the dam depends both on the inflow of turbid water as well as on the magnitude of the algal population. In summer the surface waters are often super-saturated with oxygen, whilst the hype limnion is anaerobic. The concentrations of nitrogen compounds, phosphate, Na, K, Ca, CI and SO₄ are higher in waters entering the dam via the Crocodile River. Nitrogen compounds arc the primary growth-limiting nutrients for algal growth and phosphate is the secondary growth-limiting nutrient. Phosphate concentrations in the water have increased approximately a hundredfold since 1928 and NO₃ ?N concentrations about threefold. The pH as well as the mineral content of the dam have also increased over the last 50 years.     

The effects of photosynthetically raised pH and light on some ciliated Protozoa in a eutrophic pond

Some ciliated Protozoa (e.g. Loxodes magnus, L. striatus, Spirostomum teres, S. ambiguum and Frontonia leucas) are abundant during summer in the hypolimnion of a eutrophic pond in north-west England but are absent from the epilimnion. The work described in this paper was begun with the aim of investigating the suggestion that high pH values caused by phytoplankton photosynthesis contributed to the exclusion of these ciliates from the epilimnion. In July 1973, phytoplankton photosynthesis and high pH were found only in the epilimnion, hence conditions were compatible with the above suggestion. Ciliates were, therefore, kept in the laboratory in hypolimnion water and were exposed to phytoplankton photosynthesis, both with and without pH increase. It was found that Loxodes died under both treatments hence there is no evidence that high pH is lethal to Loxodes. It seemed possible, therefore, that either light or toxins released by algae during photosynthesis are lethal to Loxodes. L. magnus was, therefore, exposed to light in the absence of phytoplankton (in filtered hypolimnion water) both in the laboratory and in the pond and it was found that light was lethal. High light intensities might, therefore, contribute to the exclusion of at least Loxodes species from the surface water of the pond, although other adverse factors are probably operative since Loxodes species do not migrate into the epilimnion at night.     

Nutrient cycling and productivity in a desert saline lake: observations from a dry,low-productivity year

Seasonal variability of nutrients and productivity were examined in Pyramid Lake, a hyposaline, N-deficient, terminal desert lake, during a dry period. River inflow and N-fixation during 1990 were minimal allowing internal nutrient cycling to be more closely studied. Nutrient cycling was strongly affected by seasonal thermal stratification that was typical for a warm monomictic lake. Concentrations of nitrate, phosphate, and silicate in surface waters were highest during winter mixing and decreased rapidly in the spring due to a diatom bloom. Maximum average chlorophyll concentration in surface waters was 2.7 ± 1.2 µg 1^–1 and occurred in April while surface nutrients were being depleted. In contrast to chlorophyll, maximum particulate carbon in surface waters occurred in July–August when areal productivity was highest (367–398 mg C m^–2 day^–1). Concurrent with spring nutrient depletion in surface waters was increasing N-deficiency in the plankton. After the spring bloom dissipated in May, particulate matter (POM) became increasingly N-deficient reaching maximum elemental C : N of > 18 during summer-fall. Profiles of the C : N ratio of POM were nearly constant with depth for individual sampling dates suggesting that the residence time of POM in the water column was short (< 1 month). While surface waters were nutrient depleted during summer stratification, nutrient concentrations of bottom waters progressively increased, presumably through the oxidation of POM sinking to the bottom (103 m). Converting the rate of oxygen depletion in bottom waters to carbon equivalents of POM suggests that 42 % of mean annual phytoplankton production in overlying waters during 1990 was mineralized in bottom waters.     

Bacterioplankton in a small polyhumic lake with an anoxic hypolimnion

Bacterioplankton biomass and dark fixation of inorganic carbon were measured in the highly humic (water colour up to 550 mg Pt l^?1) and acidic lake, Mekkojärvi. Strong thermal and chemical stratification developed in the water column early in spring and led rapidly to anoxia in the hypolimnion, which extended to less than 1.0 m from the surface. In the epilimnion only small bacteria were abundant. In the anoxic zone both the abundance and the mean size of bacteria were considerably higher than in the epilimnion. These differences are thought to be the result of different grazing pressure from zooplankton in the two zones. In late summer a high concentration of bacteriochlorophyll d in the upper hypolimnion indicated a high density of photosynthetic bacteria. Bacterial biomass was similar to that of phytoplankton in the epilimnion, but 23 times higher in the whole water column. In August, dark fixation of inorganic radiocarbon in the anaerobic zone was 51% of the total ¹⁴C-incorporation and the contribution of light fixation was only 5.4%. In the polyhumic Mekkojarvi, bacterioplankton was evidently a potentially significant carbon source for higher trophic levels, but bacterioplankton production could not be supported by phytoplankton alone. Allochthonous inputs of dissolved organic matter probably support most of the bacterial production.     

Hypolimnetic phosphorus retrieval by diel vertical migrations of lake phytoplankton

SUMMARY. I. Movement of ³³P from hypolimnion to epilimnion in a small, dystrophic lake was investigated using small-diameter experimental tubes enclosing thermally stratified water columns. This approach was made possible by the extremely sharp stratification found in such lakes, in which the euphotic zone closely coincides with the epilimnion.
2. The vertical distribution of inorganic phosphorus in the lake showed a sharp increase across the thermocline so that enhanced concentrations were available to phytoplankton just below the thermocline. Inorganic nitrogen concentrations did not show such a marked relation to thermal stratification.
3. One abundant motile alga ( Cryptomonas marssonii ) showed striking and regular vertical migrations in the lake, moving below the thermocline at night and returning to the surface waters in early morning. These migrations took cells across a 10°C temperature gradient. Non-motile phytoplankton showed constant vertical distributions.
4. In the experimental tubes an upward movement of phosphorus took place from hypolimnion to epilimnion which was only attributable to transport by phytoplankton cells undertaking active vertical migrations. No equivalent movement of phosphorus occurred in control tubes from which algae were absent.
5. The possible significance of such nutrient retrieval is discussed with reference to plankton phosphorus budgets and competition between phytoplankton species.     

Factors controlling the seasonal incidence of Pandorina morum (Mull.) Bory (Chlorophyta,Volvocales) in a small pond

Pandorina morum normally forms large summer populations in Abbot's Pond, a shallow (3.5 m maximum) but sheltered and thermally stratified pond. Restriction of growth until summer is shown not to be due to secretion of inhibitory substances by other organisms in winter and spring, nor to the prior production of stimulatory substances. Growth in February is prevented by low temperature, and between February and May 1972 was restricted by phosphate limitation. Development of summer populations was associated with direct thermal stratification and the decline of non-motile algal competitors for nutrients, and with the availability of phosphate from the anaerobic hypolimnion through diurnal migration of the coenobia.     

Migrations of haemoglobin-rich Daphnia longispina in a small,steeply stratified,humic lake with an anoxic hypolimnion

Migrations of Daphnia longispina were studied in a small humic lake with an exceptionally shallow oxic epilimnion. Horizontal distributions showed clear avoidance of the shoreline, which might be explained by the lower density of predators (Chaoborus sp. and Notonecta sp.) in the central parts of the lake. In early summer all size classes of D. longispina exhibited upward nocturnal vertical migration, descending to the upper hypolimnion in daytime. Later in summer, when the nocturnally migrating Chaoborus sp. had grown large enough to graze on small Daphnia, the latter seemed to shift towards twilight migration. However, large Daphnia individuals showed no synchronized migration; rather their bimodal vertical distributions suggested asynchronous vertical migration. Large individuals showed a particular tendency to concentrate near to the oxycline, close to the dense phytoplankton and bacteria populations in the upper part of the anoxic hypolimnion. According to vertical trap experiments, large D. longispina visited the anoxic hypolimnion and might harvest its abundant food resources. The high haemoglobin content of large individuals seems a specific adaptation to allow access to low oxygen water and hence to maximize grazing potential, in both epi- and hypolimnion, and minimize predation pressure. By staying predominantly in cooler water near the oxycline, Daphnia might also minimize its energy consumption to adjust to low food availability while sustaining a sufficiently high population density to exploit those unpredictable short periods with abundant food which are common in small headwater lakes. It is suggested that migrations of zooplankton are a complex behavioural adaptation which may not be explained by any single factor. In humic lakes with shallow stratification, vertical migrations seem to offer particularly high potential advantages, because of the short distances between dramatically different environments in the water column. In further studies more emphasis should be placed on migrations of individuals rather than populations, and migrations should be considered as a dynamic part of the structure and function of the whole planktonic ecosystem.     

Phytoplankton community succession in a lake subjected to artificial circulation

East Sidney Lake, a small, eutrophic bottom release impoundment in NY, has undergone artificial circulation for three seasons. The artificial circulation system resulted in an overall reduction in the physical stability of the water column, making the lake subject to alternating periods of weak chemical stratification and mixing. Phytoplankton community succession exhibited a high degree of regularity from year to year, culminating in mid summer dominance by heterocystous cyanophytes in all years. Changes in the physical structure of the water column, with attendant changes in Z _eu :Z _mix , were not important determinants of phytoplankton community makeup in East Sidney Lake. Seasonal patterns and community characteristics were not affected by artificially induced alterations in stability, but instead were most sensitive to surface temperatures, flushing rate and TN:TP. The timing of cyanophyte blooms was not affected by artificial circulation, nor was maximum seasonal phytoplankton biomass reduced.     

Studies on ciliated protozoa in eutrophic lakes: 1. Seasonal distribution in relation to thermal stratification and hypolimnetic anoxia

An investigation into the spatial and temporal distribution of ciliate populations in a small monomictic eutrophic lake was carried out over a one-year period. A community of large cilates dominated by Loxodes striatus, L. magnus, Spirostomum teres and Frontonia leucas occurred in the deep water sediments during the period of winter mixis. These cilates vacated the sediment during summer stratification, progressively colonising the hypolimnion in phase with the upwards spread of anoxic conditions, returning once more to the sediment following destratification. The possible significance of this ciliate community is discussed.     

The primary production of phytoplankton in Lake Vechten

The primary production of the phytoplankton of Lake Vechten (The Netherlands) (area, 4.7 ha; mean depth, 6 m), an unpolluted and stratified sandpit was investigated from 1969 to 1980 (except in 1971, 1975 and 1976) by the in situ ¹⁴C-technique. Other data collected include: solar radiation, transparency, oxygen and thermal structure. In winter and spring diatoms, Cryptophyceae and Chlorococcales were important algal groups, while in summer Dinophyceae and Chrysophyceae were important. The chlorophyll-a concentration was compared to the cellular biovolumes (= fresh weight) of the most abundant phytoplankton species. The primary production maxima occurred in winter, spring and during the summer stratification. The vertical profiles of photosynthesis exhibit light inhibition at surface to a maximum of 4 m. The maximum of zooplankton grazing in May–June caused a sharp decrease in the phytoplankton biomass and seston concentration accompanied by the highest transparency (clear water phase).The values for cellular C-fixation range from 10 to 1307 mg C · M^–2 · day^–1 (annual mean of 280 mg C · m^–2 · day^–1). High dark fixation (up to 100%) was encountered in the metalimnion and hypolimnion from July to October together with peaks of ¹⁴C-fixation due to a crowding of phytoplankton and phototrophic anoxic bacteria. Extracellular excretion by phytoplankton, investigated in 1977 to 1979, was 15% of the annual mean of the total C-fixation. The photosynthetic efficiency, turnover rates, and activity coefficients were low, particularly in the summer months when Ceratium hirundinella was predominant. The seasonal variations were controlled mainly by solar radiation and probably phosphate, the former being more important in the non-stratification period and the latter during the stratification period.     

THE SUMMER CONDITION OF THREE EASTERN TRANSVAAL RESERVOIRS AND SOME CONSIDERATIONS REGARDING THE ASSESSMENT OF TROPHIC STATUS

SUMMARY

Summer temperature, oxygen and light penetration profiles in the Da Gama, Klipkopje and Witklip Reservoirs are presented. Analysis of waters from different depths showed the presence of chemical stratification despite the absence of clear thermal stratification. An algal bioassay procedure also confirmed the presence of increasing quantities of available nutrients with depth in all three reservoirs. The significance of the results in assessing trophic status is discussed and it is concluded that for South African impoundments, an anaerobic hypolimnion is not exclusively associated with eutrophy.