Impact of intensive cage fish farming on the phytoplankton and periphyton of a Scottish freshwater loch

Nutrients, phytoplankton and periphyton were monitored in a 71 ha shallow, unstratified lake used for intensive cage culture of rainbow trout. Inorganic nitrogen, ortho-phosphate and suspended solids were significantly higher near the cages and the bottom and, although declining during summer, nutrients did not reach levels which limit phytoplankton growth. Microcystis aeruginosa dominated the phytoplankton, with surface chlorophyll a reaching 189 µg l^–1 in August, but with no subsequent bloom collapse or deoxygenation. A sub-dominant community of vernal diatoms and Pediastrum spp. persisted. Periphyton was dominated by Melosira italica-subarctica. Algal species and water quality showed the lake to be highly eutrophic. Chlorophyll values predicted from a phosphorus-dependent eutrophication model agreed with observations but light limitation by self-shading and suspended farm wastes, aided by wind-induced turbulence, is believed to control algal growth rates and biomass. Implications for environmental management of intensive freshwater cage farms are discussed.     

Impact of Holoshestes heterodon Eigenmann (Pisces: Characidae) on the plankton community of a subtropical reservoir: the importance of predation by Chaoborus larvae

We analyzed the effects of planktivorous Holeshestes heterodon Eigenmann (Characidae) predation on the plankton community of a small subtropical reservoir, using four enclosures (volume about 17.5 m³), open to the sediment, established in the littoral zone. Two enclosures were stocked with fish (mean TL 5.7 cm), at a density of about 4–5 fish m^–3 (approx. 8 g m^–3), whereas two remained fishless. The experiment lasted a little longer than one month. In the fish enclosures, most Crustacea and Chaoborus larvae remained scarce, probably as a result of visually selective fish predation. In both fishless enclosures, Chaoborus larvae became abundant. However, in only one of these did large individuals become relatively numerous; this discrepancy in the demographic structure of the Chaoborus populations between the two fishless enclosures is unexplained. Only in the fishless enclosure without appreciable numbers of large Chaoborus did densities of Crustacea increase greatly. It is suggested that in the enclosure containing large Chaoborus individuals, crustacean populations were prevented from developing due to predation pressure, while the small Chaoborus larvae of the other enclosure could not readily consume these prey. Rotifers were low in abundance in the absence of fish, probably as a consequence of Chaoborus predation. Phytoplankton density increased in all four enclosures, due probably to the lack of water flow. Only in the fishless enclosure with high densities of crustaceans did phytoplankton abundance decrease markedly at the end of the experiment, perhaps because of grazing losses.     

Indirect effects of fish on macrophytes in Bays Mountain Lake: evidence for a littoral trophic cascade

Summary We began this experiment to test specific hypotheses regarding direct and indirect effects of fish predation on the littoral macroinvertebrate community of Bays Mountain Lake, Tennessee. We used 24 m² enclosures in which we manipulated the presence and absence of large redear sunfish (Lepomis microlophus>150 mm SL), and small sunfish (L. macrochirus and L. microlophus <50 mm SL) over a 16-mo period. Here we report on effects of fish predation on gastropod grazers that appear to cascade to periphyton and macrophytes.Both large redear sunfish and small sunfish maintained low snail biomass, but snails in fish-free controls increased significantly during the first 2-mo of the experiment. By late summer of the first year of the experiment, the difference in biomass between enclosures with and without fish had increased dramatically (>10×). Midway through the second summer of the experiment, we noted apparent differences in the abundance of periphyton between enclosures containing fish and those that did not. We also noted differences in the macrophyte distribution among enclosures. To document these responses, we estimated periphyton cover, biovolume and cell size frequencies as well as macrophyte distributions among enclosures at the end of the experiment. When fish were absent, periphyton percent cover was significantly reduced compared to when fish were present. Periphyton cell-size distributions in enclosures without fish were skewed toward small cells (only 12% were greater than 200 m³), which is consistent with intense snail grazing. The macrophyte Najas flexilis had more than 60 x higher biomass in the fish-free enclosures than in enclosures containing fish; Potamogeton diversifolius was found only in fish-free enclosures. These results suggest a chain of strong interactions (i.e. from fish to snails to periphyton to macrophytes) that may be important in lake littoral systems. This contrasts sharply with earlier predictions based on cascading trophic interactions that propose that fish predation on snails would enhance macrophyte biomass.     

Seasonal succession of phytoplankton in a large subarctic river

The factors influencing the abundance of phytoplankton in the Yellowknife River, in the Canadian subarctic, were determined from collections made for 42 consecutive months from June 1975 to November 1978. The spring bloom of plankton occured during April of each year in response to changing light conditions. WhileChlamydomonas lapponica was dominant during this period, it was replaced during the early part of the summer by a rapid succession ofDinobryon species in whichD. cylindricum was followed byD. sociale and in turn byD. bavaricum andD. divergens. Although low nutrient levels permitted the development ofDinobryon during the summer, the abundance of diatoms was greatly limited by the concentrations of SiO₂ (< 0.1 g/m³). Algal densities began to decline in August and reached low overwintering levels by November. The absence of a fall bloom in densities was due to a combination of low temperatures and nutrient levels.P.O. Box 2310, Yellowknife, Northwest Territories, X1A 2P7, Canada     

浅水湖泊浮水植物芡实收割对浮游植物群落的影响

浅水湖泊生态系统正遭受广泛而强烈的人为干扰,但是对收割水生植物干扰的研究甚少。于2019年8月对芡实过度生长的陈瑶湖进行通道式分区收割工程,分析了收割芡实（Euryale ferox）前后不同处理组浮游植物群落的变化。研究期间共鉴定出浮游植物6门47属72种,其中收割前63种,收割后71种。收割后浮游植物的细胞密度和生物量均高于收割前,分别增加了39.78%和5.09%。收割芡实导致陈瑶湖浮游植物群落为由蓝藻-绿藻-硅藻-隐藻群落转变为蓝藻-绿藻-硅藻群落。其中蓝藻细胞密度和生物量显著高于收割前（P<0.05）,归因于有害蓝藻（铜绿微囊藻Microcystis aeruginosa、水华束丝藻Aphanizomenon flos-aquae、小颤藻Oscillatoria tenuis、卷曲鱼腥藻Dolicospermum circinale、小席藻Phormidium tenu）的增加。收割还导致了硅藻群落由附生型向浮游型硅藻的转变,表现为尖针杆藻（Ulnaria acus）减少,而颗粒直链藻极狭变种（Aulacoseira granulata var.angustissima）、梅尼小环藻（Cyclotella meneghiniana）增加。在芡实收割过程中,未收割组和河道的浮游植物群落结构在时空分布上无显著性差异（P>0.05）,但收割组在收割后的不同阶段内差异较为明显,其细胞密度和生物量随着收割实验的结束逐渐降低。浮游植物与环境因子的相关性分析表明,水生植被覆盖度、总磷、总氮、溶解氧和叶绿素a浓度是影响浮游植物细胞密度和生物量变化的主要环境因子。综合陈瑶湖水质状态,本研究认为收割芡实并不能缓解浅水湖泊富营养化状况,研究结果为浅水湖泊水生植被的管理提供理论依据。     

Cascading effects of the flathead grey mullet Mugil cephalus in freshwater eutrophic microcosmos

Flathead grey mullet (Mugil cephalus L.) stocked in fish ponds were long considered to feed primarily on detritus. However, recent research has found that they obtain much of their food from plankton and that they have a detrimental effect on pond zooplankton and large phytoplankton, whilst enhancing small phytoplankton. It has been suggested that flathead grey mullet may also increase the internal phosphorus loading of the ecosystem, which would also increase phytoplankton density. To test whether zooplankton removal or nutrient release from the sediment is the better explanation for phytoplankton enhancement in the presence of flathead grey mullet, the ecosystems of fish-less tanks, tanks with a 60 m mesh filter and tanks stocked at a fish density of 243 g m^-3 were compared. In the presence of flathead grey mullets, cladocerans, ostracods and chironomid larvae became scarcer than in the control tanks, while there were more small phytoplankton and mud snails. The green algae Cladophora sp. did not occur at all. The presence of a mechanical filter also reduced cladoceran, ostracod and chironomid densities and increased phtyoplankton and mud snail density. However, the situation observed in filter tanks was intermediate between that observed in the fish tanks and the control tanks, due to the lower filtering efficiency of the mechanical filter. The organic matter content of the sediment decreased throughout the experiment in the control and filter tanks, but remained stable in fish tanks. Phosphorus and nitrogen concentrations were not affected by any treatment. These results showed that flathead grey mullet enhanced phytoplankton development due to the removal of large cladocerans and not as a consequence of nutrient release. Furthermore, the flathead grey mullet strongly modified the benthic community, probably due to direct predation.     

Intercalibration of classical and molecular techniques for identification of Alexandrium fundyense (Dinophyceae) and estimation of cell densities

A workshop with the aim to compare classical and molecular techniques for phytoplankton enumeration took place at Kristineberg Marine Research Station, Sweden, in August 2005. Seventeen different techniques – nine classical microscopic-based and eight molecular methods – were compared. Alexandrium fundyense was the target organism in four experiments. Experiment 1 was designed to determine the range of cell densities over which the methods were applicable. Experiment 2 tested the species specificity of the methods by adding Alexandrium ostenfeldii, to samples containing A. fundyense. Experiments 3 and 4 tested the ability of the methods to detect the target organism within a natural phytoplankton community. Most of the methods could detect cells at the lowest concentration tested, 100 cells L⁻¹, but the variance was high for methods using small volumes, such as counting chambers and slides. In general, the precision and reproducibility of the investigated methods increased with increased target cell concentration. Particularly molecular methods were exceptions in that their relative standard deviation did not vary with target cell concentration. Only two of the microscopic methods and three of the molecular methods had a significant linear relationship between their cell count estimates and the A. fundyense concentration in experiment 2, where the objective was to discriminate that species from a morphologically similar and genetically closely related species. None of the investigated methods were affected by the addition of a natural plankton community background matrix in experiment 3. The results of this study are discussed in the context of previous intercomparisons and the difficulties in defining the absolute, true target cell concentration.     

Nutrient ratios and phytoplankton community structure in the large, shallow, eutrophic, subtropical Lakes Okeechobee (Florida, USA) and Taihu (China)

Analysis of ten- and four-year datasets for the large, shallow, subtropical, and eutrophic Lakes Okeechobee (USA) and Taihu (China), respectively, suggest that resource-ratio explanations for cyanobacteria dominance may not apply to these two lakes. Datasets were examined to identify relationships between nutrient ratios [total nitrogen (TN):total phosphorus (TP) and ammonium (NH₄ ⁺):oxidized N (NO _x )] and phytoplankton community structure (as proportions of cyanobacteria and diatoms to total phytoplankton biomass). Datasets were pooled by sampling month, averaged lake-wide, and analyzed with linear regression. In Okeechobee, the cyanobacteria proportion increased and the diatom proportion decreased with increasing TN:TP. In Taihu, cyanobacteria decreased with increasing TN:TP, but the opposite trend observed for diatoms was marginally significant. Okeechobee cyanobacteria increased and diatoms decreased with increasing NH₄ ⁺:NO _x , but no significant relationships between phytoplankton and NH₄ ⁺:NO _x were observed in Taihu. Both lakes had significant relationships between phytoplankton community structure and total nutrients, but these relationships were the opposite of those expected. Relationships between phytoplankton community structure and water quality parameters from the previous month resulted in improved relationships, suggesting a predictive capability. Statistical analysis of the entire datasets (not pooled) supported these and additional relationships with other parameters, including temperature and water clarity.     

Mechanisms responsible for the development of periphyton community structure during seasonal succession: the role of interspecies competition and plankton sedimentation

The development of periphyton community structure by exchange of organisms between substratum and water column (noninteractive mechanism) and by interspecific competition for surface (interactive mechanism) was studied during seasonal succession in Akulovsky water supply channel (the Upper Volga basin). The influence of exchange was assumed by similarity between the species composition of plankton and periphyton. At early stages of succession when the diatoms dominated in periphyton the community was formed mainly by phytoplankton sedimentation, while the competition for substratum didn't result in decrease of species diversity because the poor competitors were partly displaced by new colonists from the water column. Later when the green filamentous algae abundantly developed in periphyton, their numbers were probably controlled by factors not related to exchange of propagules. At the same time, the species structure of secondary periphyton cover developing on the thallus of filamentous algae depended mainly on the plankton sedimentation. At the last stages of seasonal succession when periphyton was represented by colonies of cyanobacteria and diatoms closely covering the substratum, the exchange of organisms between substratum and water column was not so important as interspecific competition for surface. As one could suppose, increase in biomass in this period resulted in the decrease of specificity as it was predicted by hypothesis of interactive community. In such a way, both mechanisms (interactive and noninteractive ones) took part in development of periphyton structure. Their relative influence changed in the course of seasonal succession.     

Physiological characteristics of inorganic nitrogen uptake by spatially separate algal communities in a nitrogendeficient lake

1. The physiological characteristics of nitrogen uptake by sublittoral and eulittoral (splash zone) epilithic periphyton as well as epipelic periphyton in N-deficient Castle Lake, California were determined by evaluating the half-saturation constants (K_t) and initial slopes (V_max/K_t) of uptake kinetics curves. These results were compared to similar studies of phytoplankton nitrogen uptake in this lake. 2. The strategies of nitrogen uptake differed among the various communities and were largely determined by the proximity of each to pools of available dissolved inorganic N (DIN). 3. The sublittoral algae did not have a high biological affinity for either NH₄ or NO₃ and depended on nitrogen fixation for their N supply. The eulittoral community showed an increased capacity for DIN uptake at low substrate concentration, but not as high as measured for the phytoplankton community. Epipelic algae live immediately adjacent to a large pool of interstitial sediment NH₄ and showed no physiological adaptations for surviving in a N-deficient environment. 4. K_t values for all benthic communities were approximately two orders of magnitude greater than ambient substrate levels. In contrast, the half-saturation constants for NH₄+ NO₃ uptake by phytoplankton were very similar to in situ levels of these nutrients.     

A Comparison of Phytoplankton and Algae Growing on Plexiglas Slides in an Oligotrophic Lake

This study compares the composition and dynamics of phytoplankton communities and of periphyton communities growing on Plexiglas slides in a temperate oligotrophic lake. Ordination methods were used in the comparisons, which covered a three-month period during the summer. Plankton and periphyton communities differed considerably, and shared only six common taxa. An ephemeral Dinobryon pulse occurred simultaneously in both communities, and represented the largest source of variability in community composition. Recovery from the pulse was faster in the phytoplankton than in the periphyton. Attachment to substrata, through its effect on cell turnover times and nutrient cycling, is hypothesized to provide a temporal refuge for plankton species during periods of unfavorable environmental conditions.     

Spatial patterns of benthic diatoms,carbohydrates and mud on a tidal flat in the Ems-Dollard estuary

The chlorophyll a content and two operational fractions of carbohydrate (water extractable and EDTA extractable) were measured every three months during one year along transects on a tidal flat in the Ems-Dollard estuary (The Netherlands). Chlorophyll a was used as an indicator of microphytobenthos biomass, which was composed predominantly of epipelic diatoms. Both carbohydrate fractions correlated significantly with chlorophyll a. EDTA extractable carbohydrates were more resistant towards degradation than the water extractable fraction. During most of the year, concentrations of chlorophyll a and carbohydrates were low, but in June, high concentrations of up to 90 g chlorophyll a/g sediment were found in a narrow zone running parallel to the channel. Maximum concentrations of water extractable carbohydrates and EDTA extractable carbohydrates ranged between 800–1200 and 600–800 g/g sediment, respectively. The mud content was high (± 90%) at the margin of the tidal flat. This was not limited to the growth season of the diatoms, but was observed throughout the year. This indicated that the high mud content at the mudflat margin was mainly caused by hydrodynamic factors, and not by biostabilization. In June, exceptionally high diatom densities were found in sediment with a high mud content. There was only minor evidence that biostabilization by epipelic diatoms lead to a further increase in the mud content of the sediment.     

Plankton community of a polyhumic lake with and without Daphnia longispina (Cladocera)

The impact of Daphnia longispina (Cladocera) on the plankton food web was studied in a polyhumic lake where this species comprised almost all zooplankton biomass. Plastic enclosures (volume 7 m³) were inserted into the lake retaining the initial water stratification except that in one enclosure zooplankton was removed. After the removal of Daphniaa rotifer, Keratella cochlearis, ciliates and heterotrophic nanoflagellates increased markedly and the density and biomass of bacteria decreased. Edible algal species, Cryptomonas rostratiformisand three small chrysophytes,Ochromonas, Pedinella and Spinifermonas, took advantage of the removal of Daphnia, while more grazing-resistant species declined. In spite of the changes in the species composition of phytoplankton, the removal of Daphnia did not affect the biomass, primary production or respiration of plankton. The results implied that the density of heterotrophic flagellates and ciliates was controlled by Daphnia, but in its absence the former took its role as the bacterial grazers.     

Structure, biomass, production and depth distribution of periphyton on artificial substratum in shallow lakes with contrasting nutrient concentrations

1. To examine how the vertical distribution of periphytic biomass and primary production in the upper 0–1 m of the water column changes along an inter‐lake eutrophication gradient, artificial substrata (plastic strips) were introduced into the littoral zones of 13 lakes covering a total phosphorus (TP) summer mean range from 11 to 536 μg L^?1. Periphyton was measured in July (after 8 weeks) and September (after 15 weeks) at three water depths (0.1, 0.5 and 0.9 m). 2. Periphyton chlorophyll a concentration and dry weight generally increased with time and the communities became more heterotrophic. Mean periphytic biomass was unimodally related to TP, reaching a peak between 60 and 200 μg L^?1. 3. The proportion of diatoms in the periphyton decreased from July to September. A taxonomic shift occurred from dominance (by biovolume) of diatoms and cyanobacteria at low TP to dominance of chlorophytes at intermediate TP and of diatoms (Epithemia sp.) in the two most TP‐rich lakes. 4. The grazer community in most lakes was dominated by chironomid larvae and the total biomass of grazers increased with periphyton biomass. 5. Community respiration (R), maximum light‐saturated photosynthetic rate (P_max), primary production and the biomass of macrograzers associated with periphyton were more closely related to periphyton biomass than to TP. Biomass‐specific rates of R, P_max and production declined with increasing biomass. 6. Mean net periphyton production (24 h) was positive in most lakes in July and negative in all lakes in September. Net production was not related to the TP gradient in July, but decreased in September with increasing TP. 7. The results indicate that nutrient concentrations alone are poor predictors of the standing biomass and production of periphyton in shallow lakes. However, because periphyton biomass reaches a peak in the range of phosphorus concentration in which alternative states occur in shallow lakes, recolonisation by submerged macrophytes after nutrient reduction may potentially be suppressed by periphyton growth.     

Algal communities near Cape Maclear,southern Lake Malaŵi,Africa

Algal communities were compared among benthic and net plankton samples from Cape Maclear, Lake Malai. In the cool mixing season (from May to August), rocks were overgrown byCladophora orCalothrix, accompanied by the diatomsRhopalodia, Cymbella, andNavicula. These diatoms, together withEpithemia andCocconeis, were epiphytic onCladophora and macrophytes. In sandy areas, the common diatoms wereRhopalodia, Fragilaria, Epithemia, Navicula, Surirella, andMelosira.In all phytoplankton samples, taken with a 10 µm mesh net, cyanophyte cells were the most common (70 to 80%), especially those ofOscillatoria. Biomass, however, was dominated byPeridinium from November to April and byAnabaena andOscillatoria from September to April when the mixolimnion was stratified. Among the chlorophytes,Oedogonium was the most common, especially from May into December whenPleodorina became more common.Diatoms dominated the biomass in the mixing season (May to September):Stephanodiscus in May, followed byMelosira nyassensis and lanceolateNitzschia species from mid-June through August. For the rest of the year the epilimnion was stratified and theseNitzschia species were virtually the only diatoms present.Benthic and planktonic communities share few taxa: benthic taxa never made up more than 2% of cells in offshore tows. This conclusion contrasts with previous reports, especially regardingSurirella. Consequently, an abundance of benthic taxa in sediment cores may be interpreted as lower lake level if sediment redistribution can be excluded. The seasonality of the planktonic diatoms is compatible with current ecological hypotheses, and therefore increases their value as paleolimnological indicators.     

A 2-year experimental study on nutrient and predator influences on food web constituents in a shallow lake of north-west Spain

1. A 2‐year study was carried out on the roles of nutrients and fish in determining the plankton communities of a shallow lake in north‐west Spain. Outcomes were different each year depending on the initial conditions, especially of macrophyte biomass. In 1998 estimated initial ‘per cent water volume inhabited’ (PVI) by submerged macrophytes was about 35%. Phytoplankton biomass estimated as chlorophyll a was strongly controlled by fish, whereas effects of nutrient enrichment were not significant. In 1999 estimated PVI was 80%, no fish effect was observed on phytoplankton biomass, but nutrients had significant effects. Water temperatures were higher in 1998 than in 1999. 2. In the 1998 experiment, cladoceran populations were controlled by fish and cyanobacteria were the dominant phytoplankton group. There were no differences between effects of low (4 g fresh mass m^?2) and high (20 g fresh mass m^?2) fish density on total zooplankton biomass, but zooplankton biomass was higher in the absence of fish. With the high plant density in 1999, fish failed to control any group of the zooplankton community. 3. Total biovolume of phytoplankton strongly decreased with increased nutrient concentrations in 1998, although chlorophyll a concentrations did not significantly change. At higher nutrient concentrations, flagellate algae became more abundant with likely growth rates that could have overcompensated cladoceran feeding rates. This change in phytoplankton community composition may have been because of increases in the DIN : SRP ratio. Both chlorophyll a concentration and total phytoplankton biovolume increased significantly with nutrients in the 1999 experiment. 4. A strong decline of submerged macrophytes was observed in both years as nutrients increased, resulting in shading by periphyton. This shading effect could account for the plant decline despite lower water turbidity at the very high nutrient levels in 1998.     

Disturbance events affecting phytoplankton biomass,composition and species diversity in a shallow,eutrophic, temperate lake

The seasonal changes in phytoplankton biomass and species diversity in a shallow, eutrophic Danish lake are described and related to different disturbance events acting on the phytoplankton community.Both the spring diatom maximum and the summer bloom of the filamentous blue-green alga, Aphanizomenon flos-aquae (L.) Ralfs, coincided with low values of phytoplankton species diversity and equitability. Diatom collapse was mainly due to internal modifications as nutrient depletion (Si, P) caused by rapid growth of phytoplankton, and increased grazing activity from zooplankton. A large population of Daphnia longispina O.F. Müller in June effectively removed smaller algal competitors, thus favouring the development of a huge summer bloom (140 mm³ l^–1) of Aphanizomenon flos-aquae. Heavy rainfall and storms in late July increased the loss of Apahnizomenon by out-flow and disturbed the stratification of the lake. These events caused a marked decline in phytoplankton biomass but had no effect on species diversity. A second storm period in late August circulated the lake completely and was followed by a rapid increase in phytoplankton diversity, and a change in the phytoplankton community structure from dominance of large, slow-growing K-selected species (Aphanizomenon) to small, fast-growing r-selected species (cryptomonads).     

Periphytic food and predatory crayfish: relative roles in determining snail distribution

Summary In the laboratory and field, we examined how periphyton (food of snails) and predatory crayfish influenced snail distribution in Trout Lake, a permanent, northern Wisconsin lake. Laboratory experiments (with no crayfish) tested the importance of periphyton biomass in determining snail preference among rocks, and among rock, sand, and macrophyte substrates. Among rocks with four different amounts of periphyton, periphyton biomass and the number of Lymnaea emarginata, Physa spp., and Amnicola spp. were positively related. A similar, but non-significant, trend occurred for Helisoma anceps. A field experiment at a site in Trout Lake where predation risk was low confirmed the preference by snails for periphyton covered rocks; more snails colonized rocks with periphyton than rocks without. When given a choice of rock, sand, and macrophytes in the laboratory, L. emarginata preferred high periphyton biomass and rock. Laboratory and field results contrasted with the distribution of snails in Trout Lake; no snails occurred in areas with abundant periphyton-covered rocks, but snails were abundant nearby on scattered rocks with little periphyton. However, where snails were absent, crayfish were abundant (14.5 crayfish-trap^–1-day^–1), and where snails were abundant, crayfish were rare (3.2 crayfish-trap^–1-day^–1), suggesting that crayfish predation reduced snails. The hypothesis that the negative association between snail and periphyton biomass resulted from snail grazing was supported by the results of a field snail enclosure-exclosure experiment (1 m² cages; n=3). All experiments and observations therefore suggest that: 1) crayfish predation is more important than a preference for high periphyton biomass in determining snail distribution in Trout Lake; 2) periphyton biomass is negtively related to snail grazing; and 3) crayfish had a positive indirect effect on periphyton by preying on grazing snails.     

Spatial and temporal dynamics of phytoplankton communities along a regulated river system, the Nakdong River, Korea

The longitudinal distribution and seasonal fluctuation of phytoplankton communities was studied along the middle to lower part of a regulated river system (Nakdong River, Korea). Phytoplankton biomass decreased sharply in the middle part of the river (182 km upward the estuary dam), and then increased downstream reaching a maximum at the last sampling station (27 km upward the estuary dam). In contrast, there was little downstream fluctuation in species composition, irrespective of pronounced differences in nutrient concentrations (TN, TP, NO₃, NH₄, PO₄) as well as in algal biomass. In the main river channel, small centric diatoms (Stephanodiscus hantzschii, Cyclotella meneghiniana) and pennate diatoms (Synedra, Fragilaria, Nitzschia) were dominant from winter to early spring (November–April). A mixed community of cryptomonads, centric and pennate diatoms, and coenobial greens (Pediastrum, Scenedesmus) was dominant in late spring (May–June). Blue-green algae (Anabaena, Microcystis, Oscillatoria) were dominant in the summer (July–September). A mid-summer Microcystis bloom occurred at all study sites during the dry season, when discharge was low, though the nutrient concentration varied in each study site. Nutrients appeared everywhere to be in excess of algal requirement and apparently did not influence markedly the downstream and seasonal phytoplankton compositional differences in this river.