Chlorophyll differences in Mono Lake (California) observable on Landsat imagery

In Mono Lake (California), a large saline lake, chlorophyll concentrations in the euphotic zone increased from 4 to 45 µg l^–1 between July and October 1979. These seasonal changes in chlorophyll are detectable on imagery obtained with the multispectral scanner on Landsat. Computer-compatible tapes of Landsat images were normalized for solar zenith and corrected for atmospheric scatter and absorption to obtain Landsat band 4 emittances (W m^–2 str^–1) of 13.4 ± 0.5 when chlorophyll was 4 µg l^–1 and 4.6 ± 0.3 when chlorophyll was 45 µg l^–1. Lake wide, spatial heterogeneity of chlorophyll of 2 µg l^–1 in July and 8 µg l^–1 in October was not detectable on the Landsat imagery.     

Heterotrophic glucose uptake and respiration in Lake Kinneret

The turnover times of glucose, averaged for 0–10 m in the upper waters of Lake Kinneret and measured by the addition of single or multiple concentrations of substrate, ranged from 23 to 188 hours and 1 to 87 hours respectively. Potential uptake rates (estimated as V_max) ranged from 0.095 to 1.94 µg glucose l^–1h^–1, while measured uptake rates varied from 0.09 to 1.1 µg glucose l^–1h^–1. Concentrations of dissolved carbohydrates and glucose averaged 0.71 mg glucose equivalents l^–1 and 39 µg glucose l^–1 respectively. No evident relationships between glucose cycling and any fractions of dissolved organic matter, phytoplankton biomass or primary productivity were found. Turnover times were generally most rapid immediately after the decline of the spring Peridinium bloom. The respiration percentage of incorporated glucose ranged from 25% to 61% with highest values during the summer months. Respiration may be influenced by the nature of the indigenous bacterial population as well as by temperature. Daily heterotrophic glucose carbon uptake was about 9% of the photosynthetic incorporation and could provide a bacterial yield of about 7 × 10⁴ ml^–1d^–1.     

Total organochlorine and organobromine in Finnish water courses

Methods were developed for analyzing total organochlorine (TOCl) and total organobromine (TOBr) in different kinds of water, sediment and biological samples. Sediment samples were extracted with alkaline propanone and biological samples with a neutral propanone + water solution. Neutron activation analysis (NAA) was used for the halogen determination. The detection limits for TOCl and TOBr are respectively 5 and 0.5 µg l^–1 for water (sample size 100 ml) and 0.3 and 0.1 µg g^–1 dw (sample size 1 g) for sediment and biological samples. Recoveries for common organochlorine compounds were usually over 90%.TOCl concentrations measured in waters in a natural condition were below 15 µg l^–1; and in polluted waters, from 100 to 500 µg l^–1, and occasionally to 2000 µg l^–1. TOBr concentrations were generally well below 10 µg 1^–1.     

Evaluation of the meiobenthic copepod Bryocamptus zschokkei (Schmeil) as an ecologically-relevant test organism for lotic freshwaters

Despite proven utility as ecotoxicological testorganisms in marine systems, harpacticoidcopepods have not been used in freshwaterbiomonitoring. Here we assess the value ofBryocamptus zschokkei, a common member ofmeiofaunal stream communities, as a testorganism for lotic systems by measuring theeffects of copper on acute (survival ofovigerous females) and sub-lethal (offspringproduction, juvenile [i.e. naupliar andcopepodite] development times) end points. Allovigerous females survived exposure atconcentrations <180 µg Cu l^–1, butat >180 µg Cu l^–1, mortalityincreased with exposure times up to 72 h;seventy-two and ninety-six hour LC50s wereidentical (290 µg Cu l^–1). After sixtydays, total offspring production per femaledeclined with increasing copperconcentration; no offspring were producedat 150 µg Cu l^–1 and significantlyfewer offspring were produced at 100 µg Cul^–1 compared with the control. In thejuvenile development tests, nauplii andcopepodites did not survive at 150 µg Cul^–1, but there was no significant effecton development times at lower Cu concentrations(0–100 µg Cu l^–1). Comparison oflaboratory-derived, toxicity end points withdata for B. zschokkei populations incopper-contaminated streams in south-westEngland, demonstrated that significant declinesin the number of surviving offspring closelymatched concentrations of copper across whichdeclines in field populations occurred. Theseresults indicate that B. zschokkei haspotential as an ecologically-relevant testorganism for lotic systems.     

Physiological background for using freshwater mussels in monitoring copper and lead pollution

In studying the effect of copper (10 ± 0.57 µg Cu l^–1 and 100 ± 3.01 µg Cu l^–1) and lead (50 ± 1.12 µg Pb l^–1 and 500 ± 12.5 µg Pb l^–1) on the filtration activity of Anodonta cygnea L. it was found that both heavy metals resulted in significant shortening of the active periods, but little change occurred in the length of the rest periods. The concentrations of copper and lead were measured in the gill, foot, mantle, adductor muscle and kidney for 840 hours of exposure to 10.9 ± 5 µg Cu l^–1 and 57.0 ± 19 µg Pb l^–1 as well as during subsequent depuration. Uptake was observed after 72 hours of exposure. The highest copper concentration (59.1 ± 16.2 µg Cu g^–1) was measured at 672 h in the mantle, and the highest lead value (143 ± 26.1 µg Pb^–1) was obtained in the kidney. Depuration of copper was fastest from the foot, and from the adductor muscle for lead. The gill had the longest half-depuration time (> 840 h for copper and > 672 h for lead).     

Seasonal dynamics of rotifer and crustacean zooplankton populations in a eutrophic,monomictic lake with a note on rotifer sampling techniques

The abundances, biomass, and seasonal succession of rotifer and crustacean zooplankton were examined in a man-made, eutrophic lake, Lake Oglethorpe, over a 13 month period. There was an inverse correlation between the abundance of rotifers and crustaceans. Rotifers were most abundant and dominated (>69%) the rotifer-crustacean biomass during summer months (June–September) while crustacean zooplankton dominated during the remainder of the year (>89%). Peak biomasses of crustaceans were observed in the fall (151 µg dry wt l^–1 in October) and spring (89.66 µg dry wt l^–1 in May). Mean annual biomass levels were 46.99 µg dry wt l^–1 for crustaceans and 19.26 µg dry wt l^–1 for rotifers. Trichocerca rousseleti, Polyarthra sp., Keratella cochlearis and Kellicottia bostoniensis were the most abundant rotifers in the lake. Diaptomus siciloides and Daphnia parvula were the most abundant crustaceans. Lake Oglethorpe is distinct in having an unusually high abundance of rotifers (range 217–7980 l^–1). These high densities can be attributed not only to the eutrophic conditions of the lake but also to the detailed sampling methods employed in this study.The research was supported by National Science Foundation grants DEB 7725354 and DEB 8005582 to Dr. K. G. Porter. It is lake Oglethorpe Limnological Association Contribution No. 25 and Contribution No. 371 of the Harbor Branch Foundation, Inc.     

Sample size in the monitoring of benthic macrofauna in the profundal of lakes: evaluation of the precision of estimates

We discuss here the influence of sample size (number of replicates) on the accuracy and precision of the results when sampling profundal benthos with an Ekman grab according to the Finnish standard, SFS 5076, which is equivalent to the Swedish and Norwegian standards. The aim was to find criteria for choosing a sample size which would avoid any powerful influence of chance on the results without entailing an unreasonable amount of work for monitoring purposes.Lake Haukivesi (area 620 km², total phosphorus 13 µg l^–1 and colour 35 Pt mg l^–1), Lake Paasivesi (116 km², 5 µg l^–1 and 35 Pt mg l^–1) and Lake Puruvesi (322 km², 4 µg l^–1 and 5 Pt mg l^–1) were sampled randomly in June and October 1991. 25 Replicate samples were taken on each occasion from the deep profundal area of each lake, defined here as 60-100% of the maximum depth. The sedimentation areas studied were fairly homogeneous, since the animal communities were not markedly affected by the variations in depth. Distribution estimates for the statistics studied, such as number of individuals, expected number of species, diversity and benthic quality indices, were calculated for a large set of random samples taken from the empirical data by computer (bootstrap sampling). The sample variance, s ², correlated with the mean animal density, m (ind. m^–2), according to the equation s ² = 31.77 m ^1.247. The sample size required to achieve the desired precision in mean animal density (D, expressed as the ratio standard error/mean) can thus be estimated as n = 31.77 m ^–0.753 D ^–2. The number of replicate samples needed to achieve a standard error of 20% of the mean density was 10 in Lake Haukivesi, seven in Lake Paasivesi and 11 in Lake Puruvesi. The accuracy and precision of the estimated number of species, Shannon's diversity and Benthic Quality Index improved markedly as the sample size was increased to 10 replicates. As a compromise between work load and statistical reliability, a figure of 10 replicate Ekman samples is proposed here for the monitoring of profundal benthos. The proposed sample size usually produces individual numbers which are high enough for practical purposes, probably at least 100 individuals, which is recommended as a minimum in the standard. The lower number of replicate samples recommended in recent Finnish handbook, 3–5, usually produces inadequate data, and this may detract from the comparability of the results and leave the changes in profundal communities undetected.     

Effects of a plume front on the distribution of inorganic and organic matter off the Rhône River

Hydrological and chemical structures off the Rhône River estuary resulting from the introduction of the river flow into the Mediterranean Sea are described. The effect of the fresh-water/sea-water interface on the distribution of inorganic and organic matter off the Rhône river is investigated. Strong vertical gradients of inorganic and dissolved organic matter such as lipids characterized the first few meters in this area (from 83.7 to 0.6 N-NO₃ µgat l^–1, from 6.39 to 0.92 N-NH₄ µgat l^–1 and from 299 to 73 µg l^–1 of total dissolved lipids). At the interface, substantial increases of particulate organic (PON: from 45 µg l^–1 at surface to 118 µg l^–1 at the interface, POC: from 462 to 876 µg l^–1, total particulate lipids: from 33 to 648 µg l^–1) and suspended matter in general (from 18 to 22.2 mg l^–1) were observed. High phytoplanktonic production may account for some of this enrichment, although passive accumulation might also be involved.     

Primary production of phytoplankton and standing crops of zooplankton and zoobenthos in hypertrophic Lake Teganuma

Primary production of phytoplankton and standing crops of zooplankton and zoobenthos were intensively surveyed in Lake Teganuma during May 1983–April 1984. The annual mean chlorophyll a concentrations were as high as 304 µg · l^–1–383 µg · l^–1. The daily gross primary production of phytoplankton was high throughout the year. The peak production rate was recorded in August and September, when blue-green algae bloomed. The annual gross primary production was estimated as 1450 g C · m^–2 · y^–1, extremely high as compared with other temperate eutrophic lakes. Zooplankton was predominantly composed of rotifers. The annual mean standing crop of zooplankton was 0.182 g C · m^–2 around the middle between the inlets and the outlet and was lower than in most other temperate eutrophic lakes. Zoobenthos was mostly composed of Oligochaeta and chironomids. The annual mean standing crop of zoobenthos ranged from 0.052 g C · m^–2 to 0.265 g C · m^–2, the lowest values among temperate eutrophic lakes, which is in contrast to the high primary production.     

Effects of agricultural production on phosphorus losses from paddy soils: a case study in the Taihu Lake Region of China

A field plot experiment was conducted on two types of paddy soils in the Taihu Lake Region of China from June 2000 through 2002 to assess phosphorus (P) losses by runoff and drainage flow and the effectiveness of rice–wheat double cropping on reducing P losses from paddy soils. Commercial NPK compound fertilizer and single superphosphate fertilizer were applied to furnish 0, 30, 150, and 300 kg P ha^–1 for rice season trials, and 0, 20, 80, and 160 kg P ha^–1 for wheat season trials. The experiments consisted of four replicates (plots of 5 × 6 m in a randomized block design) of each treatment in Argic stagnic anthrosols (Anzhen site) and six replicates in Cumulic stagnic anthrosols (Changshu site). P30 and P20 treatments (30 and 20 kg P ha^–1 in rice and wheat seasons, respectively) were considered as conventional P application rates in this area. Higher P treatments, such as P150 and P300 for rice and P80 and P160 for wheat, were intended to simulate the status of soil P in ~10–20 years with an application of P30 or P20 kg P ha^–1 each season. Results revealed that the average concentration of total P (TP) in runoff samples was 0.870 mg P l^–1 from P30 plots during the rice season, and 0.763 mg P l^–1 from P20 plots during the wheat season in both years at the Anzhen site, while it was 0.703 and 1.292 mg P l^–1, respectively, at the Changshu site. Average TP load (mass loss) at the Anzhen site with conventional P application rates was 220.9 and 439.5 g P ha^–1 during rice season in 2000/2001 and 2001/2002, respectively, but was 382.3 and 709.4g P ha^–1 during wheat season, respectively. Mass loss at the Changshu site was 140.4 and 165.7 g P ha^–1 during the rice season and 539.1 and 1184.6 g P ha^–1 during the wheat season, respectively. P losses from paddy soils were significantly greater during the wheat season, especially at the Changshu site, indicating that planting rice reduced P. Phosphate fertilizer levels significantly affected P concentrations and P loads in runoff both seasons. Both mean concentrations and average seasonal P loads from the P150/P80 plots were lower than that from the P300/P160 plots, but significantly higher than that from the P30/P20 and P0 plots. This implied that runoff P loads would be greatly increased in 10–20 years as a result of the accumulation of soil P if 50 kg P ha^–1 (rice season plus wheat season) is applied each year.     

A laboratory study of feeding and assimilation in Euchlanis dilatata lucksiana

Feeding of Euchlanis dilatata lucksiana, a brachionid rotifer isolated from the Loosdrecht Lakes (The Netherlands), was examined in the laboratory using ¹⁴C-labelled food. The gut-filling time at a food concentration of 9.6 µg C ml^–1 was about 15 minutes. Animals which were fed on 3 size fractions of the lake seston (< 7 µm, 7–15 µm, and 15–33 µm) showed a clear preference (Jacobs' selectivity index) for the largest size fraction. This fraction was composed predominantly of filamentous cyanobacteria. For animals weighing 0.37–0.49 µm C ind.^–1 the daily ration (daily food consumption per unit body weight × 100) ranged from 50 to 100% at food levels of 2 mg C l^–1 and below, but increased to 150–250% at food concentrations of 5 mg C l^–1 and above. The assimilation efficiency was 100% up to 5 mg C l^–1 of food, but decreased to about 80% at higher food levels.     

Nutrient and hydrologic budgets of a great lakes coastal freshwater wetland during a drought year

A coastal wetland along Lake Erie (Ohio, U.S.A.) was studied to determine hydrologic and phosphorus budgets and spatial and temporal variation of phosphorus and related chemical parameters. The wetland was influenced by changing Lake Erie water levels, seiches, shifting shoreline sediments, and watershed inflow during a year of severe drought. The water budget for a 7-month period (March – September, 1988) had average inflow of 15 200 m³ day^–1 from the watershed and 3.5 m³ day^–1 from Lake Erie. The wetland increased in volume by 700 m³ day^–1 despite a drought that resulted in 80% more evapotranspiration than rainfall as a barrier beach isolated the wetland from Lake Erie for 77% of the study period. Conductivity decreased by 34% as water flowed through the wetland and turbidity and total suspended solids were variable and statistically similar at inflow and outflow. Average total phosphorus concentrations in the inflow and outflow were also similar (247 and 248 µg P l^–1 respectively) although total soluble phosphorus and soluble reactive phosphorus decreased significantly (=0.05) from inflow to outflow (averages 94 to 45 µg P l^–1 and 7.5 to 4.0 µg P l^–1 respectively). Nutrient budgets from field data estimate a retention of 36% of the phosphorus, presumably in the sediments (0.8 mg P m^–2 day^–1). A general nutrient retention model, an estimated deposition rate from a sediment core and a simulation model predicted higher mass retention of phosphorus but similar percentage retention.SommaireUn marecage qui côtoie le lac Erie (USA) a servi de site expérimental pour en déterminer les budgets d'eau et de phosphore, de même que pour la variation spatiale et temporelle du phosphore et d'autres facteurs chimiques. Le marécage a été influencé par: niveaux d'eau qui changeaient; seiches; sédiments mouvants du littoral; et afflux de la ligne de partage des eaux dans une année de grande sécheresse. Le budget d'eau dans une période de 7 mois (mars–septembre 1988) montre un afflux de 15 200 m³ jour^–1 de la ligne de partage, et 3.5 m³ jour^–1 du lac Erie. Le volume du marécage a augmenté par 700 m³ jour^–1 malgré une sécheresse qui a produit plus d'évapotranspiration (80%) que de pluie pendant qu'une plage-obstacle a isolé le marecage du lac Erie pendant 77% de la période d'observation. La conductivité a diminué par 34% pendant que l'eau coulait, et la turbidité et les TSS ont varié, tout en démontrant des statistiques similaires à l'afflux et au déversement. Les moyennes pour les concentrations du total du phosphore à l'afflux et au déversement ont été similaires (247 and 248 µg P l^–1), quoique le TSP et le SRP ont diminué (=0.05) de l'afflux au déversement (donant des moyennes de 94 à 45 µg P l^–1 et de 7.5 à 4.0 µg P l'^–1). Les budgets de substances nutritives pour les données suggèrent une reténtion de 36% du phosphore, évidemment dans les sédiments (0.8 mg P m^–2 jour^–1). Un modèle pour la rétention des nutrients, un taux de déposition, estimé par un noyau de sédiments, et une simulation avaient prédit un plus grand taux de rétention de phosphore, mais un pourcentage similaire pour la rétention.From a paper presented at the Third International Wetlands Conference, 19–23 September, 1988, University of Rennes, France.     

Arsenic concentrations in water and fish from Chautauqua Lake,New York

Synopsis Arsenic persists in Chautauqua Lake, New York waters 13 years after cessation of herbicide (sodium arsenite) application and continues to cycle within the lake. Arsenic concentrations in lake water ranged from 22.4–114.81 g l^–1, = 49.0 ag l^–1. Well water samples generally contained less than 10 g l^–1 arsenic. Arsenic concentrations in lake water exceeded U.S. Public Health Service recommended maximum concentrations (10 g l^–1) and many samples exceeded the maximum permissible limit (50 g l^–1). Fish accumulated arsenic from water but did not magnify it. Fish to water arsenic ratios ranged from 0.4–41.6. Black crappie (Pomoxis nigromaculatus) contained the highest arsenic concentrations (0.14–2.04 g g^–1 ), X = 0.7 g g^–1) while perch (Perca flavescens), muskellunge (Esox masquinongy) and largemouth bass (Micropterus salmoides) contained the lowest concentrations (0.02–0.13 g g^–1). Arsenic concentrations in fish do not appear to pose a health hazard for human consumers.     

Phytoplankton and zooplankton community structure after nutrient additions to the oligotrophic Lake Hecklan,Sweden

Lake Hecklan, in central Sweden, was fertilized with phosphorus and nitrogen during thermal stratification (late May-early Oct) 1984–1987. The nutrient additions were relatively small and raised the total phosphorus concentrations from 6 to 10 µg l^–1. The working hypothesis was that this moderate increase in the phosphorus concentration could increase the phytoplankton biomass without adverse changes in the planktonic community structure. The fertilization increased the phytoplankton biomass from 0.1 to a maximum of 2 mm³ l^–1. Chrysophyceae and Cryptophyceae dominated throughout the experimental period. Thus, the phytoplankton composition remained typical for a Swedish forest lake and provided a potential for increased zooplankton growth. An increased growth of zooplankton was indicated by increased biomass of Cladocera and Copepoda in 1984 and 1985, and by increased fecundity of herbivorous zooplankton.     

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.     

Evaluation of xylitol production from corn cob hemicellulose hydrolysate by Candida parapsilosis

Candida parapsilosis was grown for 59 h in a medium containing corn cob hydrolysate consisting of 50 g xylose l^–1, 3.0 g glucose l^–1, 2.0 g arabinose l^–1, and 0.9 g acetic acid l^–1. A biomass of 9.1 g l^–1 was produced with 36 g xylitol l^–1 and 2.5 g ethanol l^–1. In a medium containing 50 g xylose l^–1 instead of corn cob hydrolysate, the concentrations of cells, xylitol, and ethanol were 8.6 g l^–1, 33 g l^–1, and 0.2 g l^–1, respectively. The differences between two cultures were due to the glucose and arabinose in the corn cob hydrolysate stimulating growth and the low concentration of acetic acid stimulating xylitol production.     

Effects of aflatoxin B1 on in vitro cultures ofNicotiana tabacum var. Samsun

Calli ofNicotiana tabacum (tobacco) were treated with two dose ranges of aflatoxin B₁ (0.1–2.0 µg ml^–1 - low does; 5–25 µg ml^–1 aflatoxin B₁). The ability of calli to recover following 3 weeks of toxin exposure was also investigated. The I₅₀ (50% inhibition) value for fresh mass accumulation was approximately 2 µg ml^–1 AFB₁. Fresh mass accumulation was significantly lower than the control value from 0.5 µg ml^–1 AFB₁. Following 3 weeks growth without a toxin source, the growth of calli up to and including 10 µg ml^–1 AFB₁, was significantly greater than control calli, indicating reversibility of the toxic effects. With increasing toxin concentration, chlorophyll content of callus was inhibited from 0.5 µg ml^–1. Transfer to a toxin-free medium resulted in a degree of recovery (up to 0.5 µg ml^–1). In the dose range 5–25 µg ml^–1, the levels of chlorophyll were drastically reduced, with no recovery following AFB₁ removal. Electron microscopy revealed a disruption of chloroplast structure as an early deteriorative event in AFB₁ exposure of callus cells. Protein levels were less sensitive, with inhibition manifested only in the high dose range. Shoot development occurred at all concentrations, but was significantly inhibited from 5 µg ml^–1 AFB₁. Recovery following toxin removal was minimal at these higher AFB₁ concentrations. The number of necrotic calli increased progressively from 5 µg ml^–1 as toxin levels increased.     

Plankton and hydrochemistry of Lake Futalaufquen (Patagonia,Argentina) during the growing season

Plankton communities and hydrochemistry of an oligotrophic lake occupying a glacial valley in Argentinian Patagonia (42 °49S; 71 °43W) were studied. Monthly samples at three stations integrated from 0 to 50 m and stratified samples at the site of maximum depth, were taken during the growing season. Transparency was always controlled by glacial silt, and not by phytoplankton. Lake water belongs to the calcium-bicarbonate type, with low conductivity (24 µS cm^–1), and poor buffering capacity. Forty-five phytoplankton taxa were found. Mean phytoplankton density was 49 cells ml^–1 and mean biomass 69 µg l^–1. N:P relationships, inorganic nitrogen exhaustion in the photic layer, and correlations between nutrients and phytoplankton density suggests nitrogen as the main limiting factor. Fifteen zooplankton species were found. Mean zooplankton density was 12.2 ind. l^–1 and mean biomass 22.9 µg l^–1. Diatoms and Boeckellidae were the dominant planktonic groups. Morphometry and hydrological factors were responsible for horizontal heterogeneity in phytoplankton and chemical variables.     

The macroinvertebrate communities of streams draining a small granitic catchment exposed to acidic precipitations (Vosges Mountains,northeastern France)

The physico-chemical characteristics and the macroinvertebrate communities of six streams draining a small granitic catchment located in the Vosges mountains (northeastern France) have been studied. Among the streams, five were affected by acidification and characterised by low pH (mean pH: 4.90 to 5.90), low HCO _inf3 ^sup– concentrations (mean concentration: 0–16 µeq l^–1) and elevated aluminium concentrations (mean concentration: 154–278 µg l^–1). Only one stream was typical of waters draining granitic bedrock (mean pH: 6.90; mean aluminium concentration: 79 µg 1^–1; mean HCO _inf3 ^sup– concentration: 152 µeq l^–1).The macroinvertebrates communities of the acidified streams were strongly affected compared to the non-acidified one. Richness, abundance and diversity (Shannon index) were significantly lower when acidification occurred. All the major taxonomic groups were affected, but Molluscs and Ephemeroptera were the most acid-sensitive or organisms, whereas Plecoptera and Oligochaetes were dominant. However, recolonization of acidified streams was possible during summer low flow when the global water quality slightly increased.

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Résumé Les caractéristiques physico-chimiques et les peuplements de macroinvertébrés benthiques de six ruisseaux drainant un bassin versant granitique du massif vosgien ont été étudiés. Parmi les cours d'eau, cinq sont caractérisés par des faibles valeurs moyennes de pH (4.90 à 5.90), et d'alcalinité (HCO _inf3 ^sup– : 0–16 µeq l^–1) et par des concentrations moyennes en aluminium élevées (154–278 µg l^–1) Seul un ruisseau apparaît typique du massif (pH: 6.90; aluminium: 79 g l^–1 et HCO _inf3 ^sup– : 152 µeq l^–1).Les peuplements de macroinvertébrés des ruisseaux acidifiés sont sévèrement affectés. La richesse, l'abondance et la diversité (indice de Shannon) sont alors significativement plus faibles que dans le cours d'eau non acidifié. Tous les groupes faunistiques majeurs sont touchés, mais les Mollusques et les Ephémèroptères sont les organismes les plus sensibles à l'acidification, tandis que les Plécoptères et les Oligochètes sont dominants. Toutefois, il a été mis en évidence une recolonisation possible des rivières acidifiées par certaines espèces, durant la période d'étiage estivale quand la qualité de l'eau s'améliore.

     

Interactions between phytoplankton,zooplankton and fish in a shallow,hypertrophic lake: a study of phytoplankton collapses in Lake Søbygård,Denmark

Since 1983 severe phytoplankton collapses have occurred 1–4 times every summer in the shallow and hypertrophic Lake Søbygård, which is recovering after a ten-fold decrease of the external phosphorus loading in 1982. In July 1985, for example, chlorophyll a changed from 650 µg l^–1 to about 12 µg 1^–1 within 3–5 days. Simultaneously, oxygen concentration dropped from 20–25 mg O₂l^–1 to less than 1 mg O₂l^–1, and pH decreased from 10.7 to 8.9. Less than 10 days later the phytoplankton biomass had fully recovered. During all phytoplankton collapses the density of filter-feeding zooplankton increased markedly, and a clear-water period followed. Due to marked changes in age structure of the fish stock, different zooplankton species were responsible for the density increase in different years, and consequently different collapse patterns and frequencies were observed.The sudden increase in density of filter-feeding zooplankton from a generally low summer level to extremely high levels during algae collapses, which occurred three times from July 1984 to June 1986, could neither be explained by changes in regulation from below (food) nor from above (predation). The density increase was found after a period with high N/P ratios in phytoplankton or nitrate depletion in the lake. During that period phytoplankton biomass, primary production and thus pH decreased, the latter from 10.8–11.0 to 10.5. We hypothesize that direct or indirect effects of high pH are important in controlling the filter-feeding zooplankton in this hypertrophic lake. Secondarily, this situation affects the trophic interactions in the lake water and the net internal loading of nutrients. Consequently, not only a high content of planktivorous fish but also a high pH may promote uncoupling of the grazing food-web in highly eutrophic shallow lakes, and thereby enhance eutrophication.A tentative model is presented for the occurrence of collapses, and their pattern in hypertrophic lakes with various fish densities.