In this study we documented the instar densities and life cycle of Telmatogeton torrenticola Terry (Chironomidae : Telmatogetoninae) from Kinihapai Stream, Maui, Hawaii. Greatest larval densities of this midge are found on substrates of high velocity, shallow flows of cascades, and splash zones of waterfalls, with lower densities in riffles. In the summer of 1994 we compared the effects of two microhabitats (termed optimal and suboptimal) on inter-instar density and relative abundance. In a second year (1995), we evaluated the effect of long-term reduced stream flow on these variables only in optimal microhabitats. A significant reduction in stream flow from 1994 to 1995 was correlated with a similar reduction in larval densities that precluded larval colonization of suboptimal habitats in 1995, thereby preventing sampling in this microhabitat during that summer. Depth of optimal habitats of 1995 were significantly shallower than both habitats of 1994, with suboptimal habitats of 1994 the deepest. Total larval density was significantly higher in 1994 optimal habitats, while 1994 suboptimal and 1995 optimal habitats were statistically similar. Individual instar densities showed variable differences among habitats and years, with the first three instars always highest in 1994 optimal habitats. All instars and pupae were collected on most sampling dates in both years ; however, pupae were only collected on a two dates in suboptimal habitats of 1994. Based on larval size frequency histograms, T. torrenticola has a multivoltine, asynchronous life cycle, with continuous reproduction, which is variable among microhabitats characterized by different flow velocity and depth, and between years of differing stream discharge. Reduced stream flow during the summer of 1995 had effects of reducing densities and changing life cycle features similar to those found in 1994 (a year of higher stream flow) suboptimal habitats.

As euglenophytes were found to be especially abundant in the Siemianówka Reservoir, Poland, in July 1994, a detailed floristic survey was made. Among the 40 euglenophyta taxa, Trachelomonas was most frequent, with 29 species. The other taxa were Phacus (5), Euglena (5), Lepocinclis (4) and Monomorphina (1). Nine taxa were first recorded for Poland: Trachelomonas curta var. minima, T. intermedia var. papillata, T. neotropica, T. globularis fo. punctata, T. oblonga var. australica, T. planctonica fo. longicollis, fo. ornata, and var. flexicollis, T. silvatica. Details of ultrastructure are illustrated for 29 taxa. T. volvocina var. coranata and var. papilato-punctata are documented with SEM for the first time.

La méthode PICT (Pollution-Induced Community Tolerance) a été initialement proposée par Blank et al. (1988) afin de caractériserun écosystème aquatique soumis à un polluant suspecté. Le concept du PICT est fondé sur le fait qu'une communautéalgale présente différents «composants» ayant une sensibilité variable vis-à-vis du xénobiotique étudié. Ainsi, les organismesles plus sensibles, exposés à une concentration et pendant une durée suffisantes au toxique, ne sont plus concurrentiels et sontremplacés par des organismes plus tolérants. La communauté présente alors une tolérance supérieure vis-à-vis du polluant ou dutype de polluant étudié, en comparaison à une communauté similaire n'ayant pas connu de pression de sélection par le toxique.En conciliant deux approches méthodologiques complémentaires (un test global rapide de réponse physiologique et une étudeprécise des peuplements de la communauté), il est possible de comparer des communautés selon leur tolérance plus ou moinsinduite par le polluant étudié.Cette méthode a été appliquée à la comparaison de deux systèmes limniques très différents : le lac Léman (Haute-Savoie),mésotrophe, peu pollué par les herbicides inhibiteurs du Photosystème II (P.S. II) et la retenue de Villaumur (Ille-et-Vilaine), trèseutrophe et polluée par ce type d'herbicides. Les deux plans d'eau semblent effectivement présenter des communautés phytoplanctoniquesdifférentes, du point de vue structure des peuplements et tolérance à l'Atrazine.Afin de confirmer cette sélection de la communauté phytoplanctonique par les herbicides inhibiteurs du PS. II présents dansla retenue de Villaumur, nous avons étudié expérimentalement l'effet des polluants sur la structure d'une communauté phytoplanctoniqueprovenant du plan d'eau le moins pollué (Léman) : une série de microcosmes était contaminée par de l'eau filtréedu plan d'eau le plus pollué (Villaumur) alors qu'une autre série était contaminée par de l'Atrazine (10 µg/l). Certaines espècesd'algues tolérantes ont été sélectionnées dans les deux types de microcosmes contaminés, après trois semaines de traitement.Enfin, une comparaison expérimentale de l'effet de l'Atrazine sur deux souches d'une même espèce isolées de chaque plan d'eau(calcul et comparaison de CE50), suggère une probable sélection au niveau génotypique par les herbicides inhibiteurs de la photosynthèse.La méthode PICT présente un intérêt indéniable comme indice de pollution spécifique d'un plan d'eau. Notre approche expérimentalea bien mis en évidence l'effet sélection des communautés par les polluants, mais les premiers résultats de mesures insitu montrent que pour valider cette méthode, il est nécessaire de poursuivre l'amélioration du protocole et de l'appliquer sur dessystèmes variés, contrôlés et/ou connus.

Every month 2 samples of the Rhine and 1 sample of the Rhone are analysed for about 20 chemical and physical variables at 7 stations by two monitoring agencies.This paper presents some of our conclusions on the accuracy of these programmes and on the solubility of the calcium/carbonate/sulphate/phosphate system. In total we have analysed 916 data sets ; they were screened for imprecision before being used.At all stations the water is saturated with CaCO3 , but the "apparent" solubility product depends on the pH. The regression line can be described by IPca = 8.73 x 10-15. pH15.667.In both the Rhine and the Rhone an acidification is seen and an increase of the sulphate concentration when going downstream. We have shown that the increase in sulphate concentration is due to the disposal of CaSO4. This addition of sulphate influences the quotient Ca/HCO3. The data fit very well the theoretical regression line Ca/HCO3 = 0,5(SO4) + 0.5.The acidification originates from the mineralization of organic matter. For 6 stations dowstream from the great lakeswe found that the two rivers are saturated with hydroxy-apatite, the -log of the ionic product being 49.9 (Rhine ; σ = 0.6 ;N = 259) and 50.0 (Rhone ; σ = 0.3 ; N = 479). The o-phosphate concentration does not therefore depend on the phosphate loading of the rivers, but on the Ca concentration, the temperature and the pH. This has great influence on the transport of phosphate through the delta's of the two rivers. The disturbance of acidity and buffering capacity influences heavy metal behaviour as well.

Des investigations malacologiques ont été réalisées en 1994-1995 le long de deux secteurs de rivière pour dénombrer les gîtes du mollusque Lymnaea truncatula et déterminer leurs caractéristiques. Les deux secteurs étudiés concernent a) la Creuse sur 10 km, à côté d'Argenton-sur-Creuse (Indre) et b) la Vienne sur 25 km, entre Saint-Victurnien (Haute-Vienne) et Etagnac (Charente). Le mollusque est présent dans 27 stations sur la Creuse mais on note de grandes variations dans l'abondance de la limnée : 6 gîtes avec moins de 10 L. truncatula en juillet, 9 avec 11 à 20 limnées, 6 avec 21 à 50 mollusques, et 6 avec plus de 51 individus. La longueur des gîtes ne dépasse pas 5 m dans 15 cas, 10 m dans 6 cas, 15 m dans 4 cas et 35 m dans les 2 autres stations. Neuf gîtes à L. truncatula ont été retrouvés sur les 25 km de la Vienne. Un habitat est fort de 101 limnées tandis que les autres ont de faibles effectifs avec moins de 50 limnées par gîte. Le gîte s'étend sur 5 mètres de rive dans huit cas et sur 30 m dans l'autre cas. De cette étude, il ressort que ces berges de rivière ne constituent pas un milieu favorable pour le développement de l'espèce.

Les proliférations de macrophytes atteignent désormais les eaux courantes et entraînent de nombreuses nuisances. La compréhension de ce phénomène nécessite une étude approfondie des peuplements végétaux en place et des paramètres d'environnement qui les conditionnent, afin de déterminer les phytocénoses de référence et leurs séquences de dégradation. Lors de cette étude, vingt et un cours d'eau, situés en tête de bassin versant et représentatifs de la diversité géologique de la Lorraine, ont été prospectés. Le choix des stations d'étude a été réalisé en fonction d'un découpage abiotique, définissant des zones de cours d'eau homogènes au niveau de la géologie, la pente et la largeur. La pertinence de l'utilisation de la géologie (classes de perméabilité) comme premier critère hiérarchique de classification des cours d'eau est confirmée par les résultats de cette étude. Les analyses chimiques et les relevés morphométriques effectués sur chaque station d'étude ont conduit à individualiser quatre grands types de cours d'eau (sur granites, sur grès, sur calcaires et sur marnes), dont la végétation aquatique a été inventoriée. Des successions végétales typiques en fonction de la position amont-aval ont été observées. Les notions de milieux naturellement fragiles et d'espèces « à risque de prolifération » ont été affinées. Cette étude constitue la base d'une typologie des cours d'eau par la végétation aquatique.

Les Naididae (27 espèces) et les Tubificidae (21 espèces) prédominent parmi les 64 espèces récoltées de 1974 à 1976. Limnodrilus hoffmeisteri est l'espèce à la fois la plus constante et la plus abondante (environ 25 % de l'effectif total). Les populations les plus diversifiées appartiennent au Haut Rhône et au cours inférieur du fleuve en aval de la confluence de l'Isère.

Les populations des secteurs amont et aval du Rhône, du lac Léman et du Danube présentent un degré élevé de similitude. La corrélation significative existant entre la constance et l'abondance relative des espèces recencées témoigne de l'importance de la pollution dans la répartition du potamobenthos.

Streams and rivers on the Vanuatu islands of Espiritu Santo, Pentecost, Éfate and Tanna were sampled for gastropods and thephysical and chemical characteristics of the water were investigated between 25/11/95 and 13/12/95. Twenty three species of neritidand forteen of thiarid prosobranch gastropods were found, together with the pulmonate Physastra nasuta and an opisthobranchspecies Strubellia. Six thiarid, two Clithon and one Neritina species, which are present in the Solomon Islands and SouthEast Asia, but which are absent further south on New Caledonia and the Fiji islands, were found on the Vanuatu islands. Onthe other hand, Septaria bougainvillei and Clithon pritchardi, which are present on the Fiji islands and New Caledonia, werefound no further north than the southern Vanuatu island of Tanna. This suggests that Vanuatu is a transition zone for both northernand southern freshwater snails. Éfate had many more species (26) than the other islands (16-19-. This was partly explainedby its central geographical position and the higher ion content of its streams.

The role of both natural and artificial ponds in supporting biodiversity and as an infrequent habitat for aquatic organisms in urban areas may be greater than that in more rural landscapes. Moreover, biological succession and the dynamics of zooplankton can differ in urban ponds where we may observe a specific combination of environmental factors (e.g., an increase of eutrophication and pollution) compared to other water ecosystems. Therefore, ten urban artificial ponds were examined and the type of direct catchment area was established as the most important factor in the determination of zooplankton distribution. Different environmental factors structured zooplankton distribution between forest and meadow ponds. Low concentrations of oxygen as well as lack of fish, which was an effect of high concentrations of ammonium nitrogen, were responsible for the occurrence of littoral species and large crustacean species (e.g., Daphnia hyalina and Megacyclops viridis) in the case of forest ponds. Fish predation on large crustaceans and favourable food conditions (high concentration of chlorophyll a) created suitable conditions for the occurrence of pelagic species (e.g., Keratella cochlearis and K. quadrata) in the case of meadow ponds. Moreover, soon after the ice cover melted and before new macrophytes developed, previous-year dry rush stems created valuable refuge conditions for zooplankton in this type of pond. Despite anthropogenic pollution resulting from the close vicinity of the agglomeration of Poznań and unfavourable conditions attributed to the spring season a diverse zooplankton community occurred, reaching the level of 119 species in total.

Ecological requirements of ostracods in two limnocrene springs (Usta and Çetin Bey) were investigated between January 2000and July 2002 in Bolu, Turkey. Ten taxa (Candona neglecta, Darwinula stevensoni, Eucypris virens, Eucypris sp., Heterocyprisincongruens, Herpetocypris sp., Ilyocypris bradyi, Potamocypris sp., Scottia pseudobrowniana, Tonnacypris lutaria) werefound. Canonical correspondence analysis (CCA) showed that species with wide cosmopolitan distribution could tolerate highlevels of changes in environmental variables. CCA explained about 80 % and 77 % of the relationships between species andenvironmental variables in Usta and Çetin Bey springs, respectively. Electrical conductivity and dissolved oxygen were the twomost influential factors on species occurrence in each spring. According to Spearman correlation analysis, there were no significantrelationships between the numbers of species and environmental variables in both springs. When dissolved oxygen showedsignificant negative correlations to electrical conductivity, water and air temperature in Usta spring, such correlation wasonly negatively significant (P <0.05, r = -0.46) between electrical conductivity and dissolved oxygen in Çetin Bey spring.Clustering analysis (UPGMA) of presence/absence data divided species into four main groups. Results may suggest that anthropogenicactivities were responsible for decreasing water quality of these springs and reducing species richness. Although increasingincentives for conservation of natural springs were proposed, Çetin Bey spring and especially Usta spring are now highlydegraded.

La diapause des Cyclopides pélagiques d'un lac tempéré peu profond (Créteil, France) a été étudiée sur un cycle annuel. Une dormance estivale affecte les copépodites IV de Cyclops vicinus (avril-mai à février) et de Diacyclops bicuspidatus (mars à novembre) ; une diapause hivernale est observée chez les copépodites IV de Thermocyclops crassus et chez les subadultes de Thermocyclops oithonoides (septembre à avril).Quelques subadultes de Th. oithonoides ont un arrêt de développement sans pénétration dans le sédiment (diapause active).La biomasse moyenne de la fraction diapausante égale 36 % (15 % à 80 %) de la biomasse totale des Cyclopides pendant la période d'étude. Les effectifs benthiques, compris entre 27 000 ind. m-2 et 350 000 ind. m-2, représentent un potentiel de recolonisation du domaine pélagique de 6 ind. l-1 à 78 ind. l-1.Alors que la diapause hivernale de Th. oithonoides et de Th. crassus est probablement une adaptation à des conditions climatiques défavorables, la diapause estivale de C. vicinus serait une stratégie adaptative d'évitement de la prédation.

Freshwater systems on volcanic oceanic islands have very particular characteristics as a result of their geological origins, relatively small size, distances from source areas for colonizers, and distinct catchment morphology. These factors result in freshwater communities that are clearly distinct from continental systems. Chironomid spatial and temporal distribution was investigated in chironomid assemblages at 21 sites across the Azores Archipelago. Results using PERMANOVA, a permutational multivariate analysis of variance, indicated significant longitudinal differences in assemblages but none between islands or over time.

Links between hydromorphological and physicochemical variables and the community assemblage were assessed using DISTLM, a linear model for distance-based multivariate analysis. The percentage of variation explained by hydromorphological factors (31%) was slightly higher than that explained by physicochemical (28%) factors. Descriptors of land use (forest area, scrub area, natural area of catchment) and stream slope were found to be the best environmental predictors of chironomid assemblages in Azores. Physicochemical variables such as temperature, pH, nitrite, iron and conductivity were the principal drivers of change in chironomid composition in stream locations. Headwater sites, characterized by lower temperature, acid to neutral pH values, low conductivity, nutrient and metal concentrations were dominated by Rheocricotopus atripes. Mid-section sites, located in agricultural areas were dominated by Thienemanniella clavicornis. Urbanized lower reaches were characterized by higher temperature, pH, conductivity and nutrient levels and were dominated by Cricotopus sp. These results give essential information that allows us to predict the response of different chironomid species to hydromorphological and physicochemical gradients across the archipelago's streams contributing providing the basis for the development of tools for the implementation of the Water Frame Directive.

We studied the influence of lotic and lentic stretches on zooseston flux in the karstic barrage Plitvice Lakes (Croatia). Threesampling stretches were selected: 1) a channel with low inclination; 2) water flow through a deep lake (retention), and 3) a channelwith cascades and sharp inclination. At all sampling points of the three stretches, zooseston reached maximum abundance inSeptember (up to 147.39 ind/m3). Rotifers dominated in both zooseston abundance (58 to 80%) and diversity (66 taxa), followedby crustaceans (23 taxa), predominantly Cladocera and Copepoda. Kruskal-Wallis test suggest statistically significant differencesbetween the lentic and both lotic stretches in the biomass net flux of euplanktonic organisms. The net flux of benthicorganism biomass and changes in the biodiversity index were significantly different between the lentic stretch and lotic stretchwith high inclination. Lotic stretches had a greater influence on the increase of abundance and biomass of benthic and semiplanktonicorganisms than on the decrease of euplanktonic organisms. Principal Components Analysis of the species environmentrelationship explained 66% of the variance by the two first axes, where axis 1 accounted for 39% and axis 2 accounted for27% of the variance. Oligochaets, insect larvae, nematodes and total zooseston were associated with component 1, showing thattheir biomass was negatively correlated with discharge and dissolved oxygen concentration and positively correlated with temperature.Other taxa showed a similar correlation with component 1. Biomass of Cladocera, Copepoda, T. birostris, Polyarthraspp., K. cochlearis and rotifers was associated with component 2, and positively correlated with food resources such as chl-a,POM and DOM. The inference is that zooseston flux through Plitvice Lakes is a function of the hydrological and physiographicalfeatures of the stretches, as well as food resources from the lakes.

La consommation d'oxygène de 8 souches de Rotifères planctoniques a été mesurée à la même température (20°) à l'aide de la microméthode de Winkler. Les auteurs ont essayé d'utiliser la teneur en protéines comme base pour la comparaison de la respiration de Rotifères planctoniques de tailles différentes. Le rapport du logarithme du taux de respiration au logarithme de la teneur en protéine indique une corrélation linéaire de pente b = 0,72 en eau de source volcanique et b ~ 1 en eau d'étang calcaire. La plupart des Rotifères étudiés montrent un métabolisme respiratoire plus élevé en eau d'étang.

Ni la durée des expériences (dans la limite de 6 heures) et par conséquent la tension du milieu en oxygène (au-dessus de 50 % de la saturation), ni la densité en individus, ni la lumière, ni le type de régime alimentaire ne paraissent influencer le taux de consommation d'oxygène des Rotifères.

Phytoplankton dynamics and morphological plasticity were studied in an oligo-mesotrophic lake from late spring to earlyautumn in 2004 on a weekly to bi-weekly basis. This study aimed to evaluate the effect of environmental constraints on morphologicalplasticity and size structure of dominant species through the stratified period of the lake. While centric diatoms developedmainly in the epilimnion, Fragilaria crotonensis, Synedra sp. and Synedra acus were very well distributed through thewhole water column, as was the cryptophyte Plagioselmis nannoplanctica. A filamentous cyanobacterium, Planktothrix rubescens,was stratified in the metalimnion throughout the sampling period, while Mougeotia sp. was entrained through the wholewater column during the early autumn. Surface area / volume (S/V) ratios were calculated for the dominant species on each samplingdate. The minimum S/V ratio was found with centric diatoms, while maximal values measured were for Synedra sp. andAsterionella formosa. The S/V ratio of the only dominant cryptophyte, Plagioselmis nannoplanctica was also high; variationswere negatively correlated with NO3-N and PO4-P (-0.62 and -0.61, respectively) and positively correlated with euphotic depth(r=0.57) and temperature (r=0.82). On the other hand, the S/V ratios of Fragilaria crotonensis and Cyclotella ocellata each showedstrong negative correlation with silica concentrations (-0.55 and -0.62, respectively). It is concluded that allometric relationshipsshowed significant differences among species according to depth and season.

Life history patterns of the three most abundant chironomids were analysed in Lake Balaton by monitoring the populationdynamics of larvae and the presence of pupal exuviae. Procladius choreus had two generations per year with at least twoparallel cohorts during 1996-1999. Tanypus punctipennis had a trivoltine life cycle with occasional parallel cohorts, overlappinggenerations and long, interrupted emergence periods between 1997 and 1999. At Tihany, Chironomus balatonicus had only onedetectable generation between 1995 and 2002. Meanwhile at Keszthely, the abundance of C. balatonicus larvae peaked fromautumn to the beginning of summer in 1996/1997, 1998/1999 and 2000/2001, and it was continuously high from August 2001to October 2002. However, between these abundance peaks there were long periods when larvae could not be found at all. Theextreme population dynamics of C. balatonicus suggests that living conditions may fluctuate around the limit of this species inLake Balaton. Although C. balatonicus larvae practically were not found at both sampling sites from July 1997 to August 1998,and from July 1999 to August 2000, sporadic occurrence of exuviae during these periods proves that refuge areas should existwithin the lake. Possible background of the uncommon population dynamics of C. balatonicus is discussed.

Investigating forces driving the structure of aquatic communities has long been an important issue in ecology. In the present study, we focused on the effects of changed water supply for aquaculture ponds on phytoplankton, zooplankton and macrozoobenthos communities during two seasons of rearing common carp. We compared these communities between two types of inflow water: surface sources of water – a reservoir pond, two open wells and a small stream and groundwater – deep tube well. Significant changes were observed in environmental variables after the introduction of the groundwater source: oxygen concentration and water hardness decreased, while conductivity, phosphorus and un-ionized ammonia increased. Results revealed that all investigated groups, except Mollusca (macrozoobenthos), decreased in species richness, abundance and biomass due to changed water chemistry, but differed in the level of susceptibility. Rotifera and Cladocera were the most affected showing a sharp decline in density and number of species since 66% of species disappeared from the ponds. The abundance of Copepoda was relatively high although significantly lower under new conditions, with adults being more tolerant to changed inflow water than nauplii larvae. Phytoplankton had the highest potential to replace previous species with newcomers more adapted to changed water chemistry, providing 36 immigrant species, whereas 49 became extinct. Although mainly influenced by fish predation, Chironomidae (macrozoobenthos) were undoubtedly affected by changed water chemistry. These results suggest profound changes in three key ecological groups produced by significant changes of important environmental variables and water quality after the shift from surface to groundwater supply.

Eutrophication of freshwater lakes has led to blooms formed by cyanobacteria often associated with toxins harmful to livestock and humans. Environmental conditions that favor toxin production during cyanobacterial blooms are, however, not well understood. Moreover, the ability to use cyanobacteria quantity to assess the level of threat associated with toxin production is a topic of discussion. The purpose for this study was to examine Planktothrix agardhii dynamics in a shallow, temperate hypertrophic lake and to determine the factors that affect microcystin production. In addition, the relationship between P. agardhii morphology and microcystin production was examined. The study spanned 2 years, and we documented a perennial P. agardhii bloom that contributed up to 99% of the total biomass. Intracellular microcystins were primarily detected throughout the study, with the highest concentration in October. Microcystin concentrations ranged from 3.4 to 71.2 μg.L−1, and they had a strong, positive correlation with P. agardhii biomass. In contrast, the levels of weight-specific microcystin were relatively stable throughout the entire study, ranging from 0.23 to 1.18 μg.mg−1. We also found that environmental factors, such as water temperature, phosphate level, ammonium nitrogen and transparency, were the most related to microcystin production. Furthermore, a significant relationship between filament morphology and toxin concentration suggested that there were different morphotypes within the toxic and non-toxic populations of P. agardhii. Our study showed that P. agardhii biomass and filament morphology may be useful characteristics for the identification of threats associated with cyanotoxins.

A straightforward experimental approach was carried out to study the adaptation responses of a typical floating-leaved aquatic plant Nymphoides peltata to changes in water availability. N. peltata grown in terrestrial habitat was approximately 88.77% lower in total biomass, 62.75% higher in root biomass allocation, 80.9% higher in root-shoot ratio, and 54.5% longer in leaf longevity compared with N. peltata grown in aquatic habitats. Anatomical analyses suggest that aquatic-grown N. peltata exhibits a well-developed lacunal system in leaf, petiole, and coarse root. Moreover, aquatic-grown N. peltata had approximately a higher in lacunal system in leaf, petiole, and coarse root by 28.57%, 56.41% and 82.35%, respectively, than those of terrestrial-grown N. peltata. These results indicated that N. peltata was well adapted to the terrestrial habitat because of its biomass allocation, morphological, and anatomical strategies that depended on the increase in root biomass allocation and leaf longevity, as well as the decrease in the lacunal system volume in leaf, petiole, and coarse root. This indicates that N. peltata can develop multiple morphological and anatomical strategies, an integrated approach to enhance survival in dynamic and unpredictable environments.

Two-day life cycle tests with the rotifer Brachionus calyciflorus were run to study the nutritional quality effects to rotifers ofScenedesmus pectinatus grown under non-limiting, nitrogen limiting and phosphorus limiting conditions and the feedback of therotifers on the food algae. Under nutrient-limited conditions of its algal food Brachionus production was depressed, animals producedfewer eggs and were smaller sized. Clearance rates of Brachionus offered non-nutrient-limited and nutrient-limited foodwere similar. The number of cells per colony was similar for S. pectinatus under nitrogen limited and phosphorus limited conditionsboth in the absence and presence of Brachionus. Cell volumes of phosphorus limited S. pectinatus were larger than thoseof nitrogen limited cells. The most dramatic response of the food alga S. pectinatus was observed in non-nutrient-limited conditions:a strong size enlargement occurred only in the presence of Brachionus. This was caused by a higher share of eight-celledcolonies and larger individual cell volumes in the presence of rotifers than in their absence. S. pectinatus might gain an advantageof becoming larger in moving out of the feeding window of its enemy, but nutrient limited conditions might undermine theeffectiveness of such reaction.