Winter phytoplankton communities in different depths of three mesotrophic lakes (Łęczna-Włodawa Lakeland, Eastern Poland)

Phytoplankton samples were collected from three mesotrophic lakes: Piaseczno, Rogóźno and Krasne during winter seasons (from January to March). The samples were analyzed for species analysis and abundance of planktonic algae in relation to different depths of water column (0–7 m). Selected water physical-chemical parameters were also measured. Abundance of phytoplankton depended strongly on the thickness of snow and ice cover or mixing conditions. The maximal phytoplankton total number reached about 5 × 10⁶ ind. L⁻¹ beneath the clear ice in the Krasne Lake, minimal numbers were recorded under the thick snow and ice layers in the Piaseczno Lake (2 × 10³ ind. L⁻¹). The winter phytoplankton communities were dominated by flagellates principally cryptomonads (Cryptomonas spp. Rhodomonas minuta), euglenophytes (Trachelomonas volvocina, T. volvocinopsis), dinoflagellates (Peridinium bipes, Gymnodinium helveticum) and chrysophytes (Mallomonas elongata, M. akrokomos, Dinobryon sociale) or non-motile small species of blue-green algae (e.g. Rhabdoderma lineare, Limnothrix redekei), diatoms (Stephanodiscus spp., Asterionella formosa), and green algae (e.g. Scenedesmus spp., Monoraphidium spp.). Phytoplankton abundance and structure showed differentiation during the winter season and along the water column as well.     

Spring phytoplankton of 54 small lakes in southern Finland

The abundance and species composition of phytoplankton communities were studied rapidly following the spring ice-melt in 54 small Finnish lakes that form a unique mosaic of water bodies. Phytoplankton biomass and cell density varied among the study lakes with a factor 100 between the lowest and highest values. Highest biomass and densities of phytoplankton characterized small ( < 0.05 km²) lakes with moderate or high water colour (> 80 mg Pt l^–1). In contrast, biomass was low in clear-water lakes and lakes where water throughflow was strong. Typically one species dominated most phytoplankton communities, and usually comprised up to about 45% of the total phytoplankton biomass. Two-thirds of the 103 taxa observed were Chrysophyceans and Chlorophyceans. The most common taxa wereChlamydomonas spp. (Chlorophyceae) andCryptomonas ovata (Cryptophyceae).     

Larger zooplankton in Danish lakes after cold winters: are winter fish kills of importance?

Winter fish kills can be intense under ice in shallow lakes, and have cascading effects on the food web and ultimately on lake water clarity. In maritime Western Europe, winters are usually mild, but occasional colder periods may also have strong effects on lake fish communities. Global warming may have disproportionate effects by delaying freezing and shortening the period of ice coverage. We studied differences in zooplankton (cladocerans, copepods, and rotifers): phytoplankton biomass, zooplankton community structure, and individual body size among 37 Danish lakes of various depths, chemical characteristics, and trophy, by comparing four winters of different severity (mean winter temperatures ranging from −1.19°C in 1996 to +2.9°C in 1995). We found that crustacean mean body sizes were significantly larger in the summer following a severely cold winter. The zooplankton communities in the summer after a cold winter had a significantly larger proportion of larger-bodied species and taxa. Phytoplankton biomass, expressed as chlorophyll-a (chl-a), was lower and zooplankton herbivory (chl-a:TP index), higher, in the summer after the severely cold winter of 1995/1996. All these effects were stronger in shallow lakes than in deep lakes. Changes in zooplankton during summer 1996, compared with other years, were likely caused by fish kills under ice during the preceding severe winter of 1995–1996. Fish kills due to under ice oxygen depletion would be expected to occur earlier and be more complete in the shorter water columns of shallow lakes. With climate change, severe winters are predicted to become less frequent and the winters to be milder and shorter. In general, this is likely to lead to higher winter survival of fish, lower zooplankton grazing of phytoplankton the following summer and more turbid waters, particularly in shallow eutrophic lakes.     

Cladocera assemblages from the surface sediments of south-central Ontario (Canada) lakes and their relationships to measured environmental variables

Fossil cladoceran remains preserved in surface sediment samples from 44 oligotrophic lakes in south-central Ontario were examined to evaluate the relationships between species assemblages and measured environmental variables. Differences in cladoceran assemblages were related to physical and chemical variables using multivariate techniques. Redundancy Analysis (RDA) identified five environmental variables as significantly influencing assemblage composition: sulphate (SO₄²⁻), calcium (Ca²⁺), pH, maximum lake depth (Z _max) and dissolved organic carbon (DOC). There was a distinct separation of lakes and taxa along the ion gradient based on SO₄, Ca and pH. Additionally, cladoceran communities in coloured, shallow lakes had relatively higher abundances of littoral chydorid species and the pelagic taxa Holopedium spp., and the Daphnia pulex complex. Deep, clear lakes had relatively higher abundances of other pelagic taxa. Predation by fish (measured as presence–absence) and Chaoborus (measured as density) were less significant than some of the physico-chemical variables in influencing cladoceran assemblage structure. However, this could be due to the limited resolution of the predation data that was available at the time of this study. The distribution of cladocerans in the surface sediment, and their relation to these important environmental variables, suggests that there is considerable potential for the use of sedimentary cladoceran remains as environmental indicators in south-central Ontario lakes. Handling editor: J. Saros     

Taxon-specific variation in the stable isotopic signatures (δ13C and δ15N) of lake phytoplankton

1. The variability in the stable isotope signatures of carbon and nitrogen (δ¹³C and δ¹⁵N) in different phytoplankton taxa was studied in one mesotrophic and three eutrophic lakes in south‐west Finland. The lakes were sampled on nine to 16 occasions over 2–4 years and most of the time were dominated by cyanobacteria and diatoms. A total of 151 taxon‐specific subsamples covering 18 different phytoplankton taxa could be isolated by filtration through a series of sieves and by flotation/sedimentation, followed by microscopical identification and screening for purity. 2. Substantial and systematic differences between phytoplankton taxa, seasons and lakes were observed for both δ¹³C and δ¹⁵N. The values of δ¹³C ranged from ?34.4‰ to ?5.9‰ and were lowest in chrysophytes (?34.4‰ to ?31.3‰) and diatoms (?30.6‰ to ?26.6‰). Cyanobacteria were most variable (?32.4‰ to ?5.9‰), including particularly high values in the nostocalean cyanobacterium Gloeotrichia echinulata (?14.4‰ to ?5.9‰). For δ¹³C, the taxon‐specific amplitude of temporal changes within a lake was usually <1–8‰ (<1–4‰ for microalgae alone and <1–8‰ for cyanobacteria alone), whereas the amplitude among taxa within a water sample was up to 31‰. 3. The values of δ¹⁵N ranged from ?2.1‰ to 12.8‰ and were high in chrysophytes, dinophytes and diatoms, but low in the nitrogen‐fixing cyanobacteria Anabaena spp., Aphanizomenon spp. and G. echinulata (?2.1‰ to 1.6‰). Chroococcalean cyanobacteria ranged from ?1.4‰ to 8.9‰. For δ¹⁵N, the taxon‐specific amplitude of temporal changes within a lake was 2–6‰, (2–6‰ for microalgae alone and 2–4‰ for cyanobacteria alone) and the amplitude among taxa within a water sample was up to 11‰. 4. The isotopic signatures of phytoplankton changed systematically with their physical and chemical environment, most notably with the concentrations of nutrients, but correlations were non‐systematic and site‐specific. 5. The substantial variability in the isotopic signatures of phytoplankton among taxa, seasons and lakes complicates the interpretation of isotopic signatures in lacustrine food webs. However, taxon‐specific values and seasonal patterns showed some consistency among years and may eventually be predictable.     

Phytoplankton dynamics and structure: a comparative analysis in natural and man-made water bodies of different trophic state

Previous investigations on Sicilian man made lakes suggested that physical factors, along with the specific morphology and hydrology of the water body, are important in selecting phytoplankton species. In particular, the variations of the z _mix/z _eu ratio due to the operational procedure to which reservoirs are generally subject were recognised as a trigger allowing the assemblage shift. To investigate if these variations may be considered analogous to those occurring in natural lakes as trophic state and phytoplankton biomass increase, causing a transparency decrease and a contraction of the euphotic depth, phytoplankton were collected in two natural water bodies, one mesotrophic (Lake Biviere di Cesarò) the other eutrophic (Lake Soprano), and compared with those collected in two reservoirs with analogous trophic characteristics (Lake Rosamarina, mesotrophic and Lake Arancio, eutrophic). Particular attention was paid to the dynamics of two key groups: Cyanophytes and chlorophytes. In all four water bodies, transparency mainly depended on chlorophyll level. Annual average value of phytoplankton biomass in the mesotrophic environments was below 2.0 mg l^–1, whereas in the eutrophic systems it was well above 10 mg l^–1. All water bodies showed the presence of cyanophytes (e.g. Anabaena spp., Anabaenopsis spp., Microcystis spp., Planktothrix spp.) and chlorophytes (e.g. Chlamydomonas spp., Botryococcus spp., Oocystis spp., Scenedesmus spp., Pediastrum spp.), but their relative proportions and body size dimensions were different. In particular, small colonial chlorophytes and large-colony forming cyanophytes were most common in the most eutrophic water bodies, whereas larger colonies of green algae in those with a lower trophic state. The results showed that, under the same climatic conditions, autogenic (increase of biomass, decrease in light penetration and euphotic depth) and allogenic (use of the stored waters, anticipated breaking of the thermocline, increase of the mixing depth) processes may shift the structure of phytoplankton assemblage in the same direction even though the quantity of biomass remains linked to nutrient availability.     

Some aspects of the seasonal distribution of flagellates in mountain lakes

In a larger regional survey in Tyrol, phytoplankton species composition and biovolume of mid-altitude and high-mountain lakes was studied. Results from eight lakes showed that flagellates (mainly Chrysophyceae, Dinophyceae, and Cryptophyceae) are important components of the phytoplankton.In the mid-altitude lakes a spring and an autumn maximum of Chrysophyceae as well as a summer maximum of large dinoflagellates are observed, whereas Cryptophyceae and Dinophyceae show irregular distributions. In the high-mountain lakes the seasonal variations of phytoplankton, including flagellates, are limited by the long duration of the winter situation. However similar sequences of phytoplankton assemblages as in the midaltitude lakes can be observed. Flagellates in high-mountain lakes are important to sustain phytoplankton standing crop under the winter snow and ice cover.In order to show similarities and differences of high-mountain and mid-altitude lakes, vertical profiles of phytoplankton from three lakes and seasonal patterns of Gymnodinium uberrimum from two lakes are compared. In addition the patterns of cryptomonads differing in their ecological requirements (Cryptomonas spp. and Rhodomonas minuta) are shown for a meromictic mid-altitude lake.     

Winter severity determines functional trait composition of phytoplankton in seasonally ice‐covered lakes

How climate change will affect the community dynamics and functionality of lake ecosystems during winter is still little understood. This is also true for phytoplankton in seasonally ice‐covered temperate lakes which are particularly vulnerable to the presence or absence of ice. We examined changes in pelagic phytoplankton winter community structure in a north temperate lake (Müggelsee, Germany), covering 18 winters between 1995 and 2013. We tested how phytoplankton taxa composition varied along a winter‐severity gradient and to what extent winter severity shaped the functional trait composition of overwintering phytoplankton communities using multivariate statistical analyses and a functional trait‐based approach. We hypothesized that overwintering phytoplankton communities are dominated by taxa with trait combinations corresponding to the prevailing winter water column conditions, using ice thickness measurements as a winter‐severity indicator. Winter severity had little effect on univariate diversity indicators (taxon richness and evenness), but a strong relationship was found between the phytoplankton community structure and winter severity when taxon trait identity was taken into account. Species responses to winter severity were mediated by the key functional traits: motility, nutritional mode, and the ability to form resting stages. Accordingly, one or the other of two functional groups dominated the phytoplankton biomass during mild winters (i.e., thin or no ice cover; phototrophic taxa) or severe winters (i.e., thick ice cover; exclusively motile taxa). Based on predicted milder winters for temperate regions and a reduction in ice‐cover durations, phytoplankton communities during winter can be expected to comprise taxa that have a relative advantage when the water column is well mixed (i.e., need not be motile) and light is less limiting (i.e., need not be mixotrophic). A potential implication of this result is that winter severity promotes different communities at the vernal equinox, which may have different nutritional quality for the next trophic level and ecosystem‐scale effects.     

Macroinvertebrates as indicators of fish absence in naturally fishless lakes

1. Little is known about native communities in naturally fishless lakes in eastern North America, a region where fish stocking has led to a decline in these habitats. 2. Our study objectives were to: (i) characterise and compare macroinvertebrate communities in fishless lakes found in two biophysical regions of Maine (U.S.A.): kettle lakes in the eastern lowlands and foothills and headwater lakes in the central and western mountains; (ii) identify unique attributes of fishless lake macroinvertebrate communities compared to lakes with fish and (iii) develop a method to efficiently identify fishless lakes when thorough fish surveys are not possible. 3. We quantified macroinvertebrate community structure in the two physiographic fishless lake types (n = 8 kettle lakes; n = 8 headwater lakes) with submerged light traps and sweep nets. We also compared fishless lake macroinvertebrate communities to those in fish‐containing lakes (n = 18) of similar size, location and maximum depth. We used non‐metric multidimensional scaling to assess differences in community structure and t‐tests for taxon‐specific comparisons between lakes. 4. Few differences in macroinvertebrate communities between the two physiographic fishless lake types were apparent. Fishless and fish‐containing lakes had numerous differences in macroinvertebrate community structure, abundance, taxonomic composition and species richness. Fish presence or absence was a stronger determinant of community structure in our study than differences in physical conditions relating to lake origin and physiography. 5. Communities in fishless lakes were more speciose and abundant than in fish‐containing lakes, especially taxa that are large, active and free‐swimming. Families differing in abundance and taxonomic composition included Notonectidae, Corixidae, Gyrinidae, Dytiscidae, Aeshnidae, Libellulidae and Chaoboridae. 6. We identified six taxa unique to fishless lakes that are robust indicators of fish absence: Graphoderus liberus, Hesperocorixa spp., Dineutus spp., Chaoborus americanus, Notonecta insulata and Callicorixa spp. These taxa are collected most effectively with submerged light traps. 7. Naturally fishless lakes warrant conservation, because they provide habitat for a unique suite of organisms that thrive in the absence of fish predation.     

Calcium content of littoral Cladocera in three softwater lakes of the Canadian Shield

Aqueous calcium (Ca) concentrations are declining in softwater lakes of the Canadian Shield largely because of decades of acid deposition and afforestation following timber harvesting. Populations of pelagic cladoceran taxa with high Ca requirements, especially Daphnia spp., are declining in response to reduced aqueous Ca availability. However, the Ca content, and thus the requirements of littoral cladoceran taxa are unknown; therefore, the potential vulnerability of this major component of lake ecosystems to ongoing regional Ca decline remains uncertain. Here, we identify the body Ca concentration of nine littoral cladoceran taxa collected from three lakes in the Muskoka-Haliburton region of Ontario, Canada. Ca content differed among taxonomic groups, ranging from 3.6 mg g⁻¹ for Acroperus harpae to 23.2 mg g⁻¹ for Disparalona spp. Perhaps surprisingly, some littoral microcrustacean taxa have Ca burdens comparable to the Ca-rich daphniids. Therefore, there may be differential responses to ongoing lake water Ca declines among taxa within littoral communities as has been observed among open-water taxa, with potential ecological repercussions for near-shore food webs.     

Effects of resuspension and eutrophication level on summer phytoplankton dynamics in two hypertrophic areas of Lake Taihu,China

The composition of summer phytoplankton communities in two highly eutrophic areas of Lake Taihu, a large, shallow water body in China (area 2,338 km²; mean depth 2.0 m), was studied in relation to selected environmental variables. Both areas have multiple uses, including drinking water supply, fisheries, and recreation. The first area, Meiliang Bay, is much closer to central lake and subjected to wind-driven sediment resuspension, while the second, at Wuli Lake, is less affected by wave action but is considered more nutrient-rich due to its proximity to human inhabitation. By comparing two lake areas, we attempt to advance the understanding and management of different regions of shallow lakes, mainly differing in resuspension levels and eutrophication levels. A comparison of these two lake areas indicated a higher standing crop of phytoplankton in Wuli Lake than in Meiliang Bay. Canonic Correspondence Analysis indicated that nutrient status and sediment resuspension were the principal factors influencing phytoplankton composition in both areas. The dominant algal group in Meiliang Bay comprised immobile R-type strategists (Reynolds in Growth and reproductive strategies of freshwater phytoplankton, 1988), including filamentous diatoms and green algae (Aulacoseira granulate and Planctonema lauterbornii), and recruitment-beneficial RS-type strategists, mainly the colonial cyanobacteria (Microcystis spp.). The community of Wuli Lake was dominated by S-strategists, principally motile flagellates (Cryptomonas spp.) and C-strategists including small nonmotile centric diatoms and chlorococcal green algae.     

Effects of the halocline on water quality and phytoplankton composition in a shallow brackish lake (Lake Obuchi, Japan)

The relationships of the halocline to both water quality and phytoplankton composition in Lake Obuchi, a shallow brackish lake in northern Japan, were investigated from April 2001 to December 2004. The halocline in this lake became stronger in summer (July–September, mean maximum density gradient 4.3–5.8 ρ_tm⁻¹) but weaker in spring, fall, and winter (1.9–3.3 ρ_tm⁻¹). Although the difference in water quality between the upper and lower layers separated by the halocline was high in summer, nutrients (PO₄³⁻-P and NH₄⁺-N) were eluted from the bottom sediment as levels of dissolved oxygen decreased in the bottom layer because of the strong stratification caused by the halocline formed over the long term. Moreover, phytoplankton taxa composition also differed between the upper and lower layers in summer, but was similar in other seasons. The dominant phytoplankton taxa in the upper layer in summer were Skeletonema costatum and Cyclotella spp., whereas in the lower layer, Gymnodinium spp. (Dinophyceae) and Chlorophyceae, which prefer eutrophic and low dissolved oxygen conditions, dominated. This suggests that the halocline was related to differentiations in both water quality and ecosystem components between the upper and lower layers in the brackish lake water.     

Cell death in lake phytoplankton communities

1. The fraction of living and dead phytoplankton cells in seven Florida lakes was assessed by using the cell digestion assay, a non‐staining membrane permeability test. The cell digestion assay is an effective method to analyse cell viability in complex natural phytoplankton communities. 2. The lakes examined ranged widely in phytoplankton abundance and community composition. The variability in the percentage of living cells (% LC) was high among the taxonomic groups forming the different phytoplankton communities, ranging from 19.7% to 98% LC. 3. All cells within single cyanobacteria filaments were determined to be either dead or alive, suggesting physiological integration of the cells within colonies. 4. Within each lake, the dominant taxa generally exhibited the highest proportion of living cells. A high proportion of living cells was found to be a characteristic of the different taxa forming the communities of eutrophic lakes. The average value for the % LC for all groups comprising the phytoplankton communities in each of the lakes ranged from 29.9 ± 7.2 to 80.4 ± 4.0 (mean ± SE) and varied strongly and positively with chlorophyll a concentration. 5. These results suggest phytoplankton cell death to be an important process structuring phytoplankton communities in lakes, particularly in oligotrophic ones.     

Ecological sensitivity of marl lakes to nutrient enrichment: evidence from Hawes Water,UK

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Autotrophic Picoplankton: Community Composition,Abundance and Distribution across a Gradient of Oligotrophic British Columbia and Yukon Territory Lakes

Autotrophic picoplankton communities were examined in eleven oligotrophic lakes from a broad geographic region of western Canada, representing a variety of physico-chemical and biological conditions. During our study, several of the lakes were treated with additions of inorganic nitrogen and phosphorus fertilizers. Picoplankton communities in most lakes were dominated (>70%) by unicellular or colonial coccoid cyanobacteria, provisionally identified by morphological and autofluorescence properties as Synechococcus. Also common in some lakes were red-fluorescing cyanobacteria and Chlorella-like eucaryotes. Autotrophic picoplankters contributed from 36-63% to total chlorophyll, from >2-26% to total phytoplankton carbon, and from 29–53% to total photosynthesis. Average populations ranged from >5-10,000 cells·ml⁻¹ in winter and early spring to 65-75,000 cells · ml⁻¹ in summer and fall. Peak densities in most lakes occurred in August-September and most populations were within the epilimnion or metalimnion/hypolimnion boundary. Subsurface peaks were prevalent only in untreated, strongly stratified lakes. Eucaryotic picoplankters became dominant in acidic (pH < 6.2), humic lakes. Colonial picoplankters were more common in more productive interior lakes in August, and though present, were uncommon in coastal systems. Picoplankton populations exhibited large increases under ice in a Yukon lake, and their abundance and seasonal distribution showed little relation to temperature or to light. Fertilization of lakes resulted in picoplankton population increases (>2x) and the elimination of subsurface peaks. Nutrients were considered to be one of the major factors controlling population abundance in these oligotrophic lakes with average pH < 6.5.     

Light climate and phytoplankton photosynthesis in maritime Antarctic lakes

The responses of phytoplankton populations to seasonal changes in radiation flux in two Antarctic lakes with extensive winter ice-cover are described. A phytoplankton capable of photosynthesis was found throughout the year in both systems. During winter, low incident radiation combined with thick layers of snow and ice prevented in situ photosynthesis becoming detectable. The beginning of spring was marked by a reduction in snow cover which resulted in a considerable increase in surface penetrating radiation. Planktonic algae rapidly adapted to utilise these increased levels efficiently, though they still showed characteristics of strong shade adaptation.Loss of ice cover at the start of the short open water period further increased the radiation levels and a summer population developed which was much less shade adapted. Saturation and photoinhibition effects were widespread during this period as the algae proved unable to utilise high radiation levels efficiently. They were however effective at the radiation fluxes prevalent in the lower part of the rapidly circulating water columns.     

An experimental investigation of phytoplankton nutrient limitation in two contrasting low arctic lakes

We investigated whether phytoplankton communities in two lakes in SW Greenland were phosphorus or nitrogen limited. The study lakes have contrasting water chemistry (mean conductivities differ ten fold) and are located near Kangerlussuaq, SW Greenland (~67°N, 51°W). A microcosm nutrient enrichment experiment was performed in June 2003 to determine whether nitrate or phosphate addition stimulated phytoplankton growth. Samples were analysed for species composition, biomass, and alkaline phosphatase activity (APA). Initially, both lakes had extremely low total phosphorus but high total nitrogen concentrations and high APA, suggesting that the phytoplankton were phosphorus limited prior to the start of the experiment. The phytoplankton composition and biomass (mainly Ochromonas spp.) responded to phosphate but not to nitrate addition. In both lakes, chlorophyll a increased significantly when phosphate was added. Furthermore, APA was significantly lower in the two lakes when phosphate was added compared to the control and the nitrogen addition treatment. The dominance of mixotrophic phytoplankton and high DOC values suggest that these lakes may be regulated by microbial loop processes.     

Benthic communities of exposed littoral sand-flats in eighteen Dutch lakes

Benthic communities of the exposed sandy littoral zone in eighteen Dutch lakes were sampled using a small, hand-held core (surface area 0.8 cm²). A community dominated by chironomid larvae, chydorid cladocerans, nematodes and copepods was found to be characteristic for this habitat, especially in large and/or deep lakes. Bray-Curtis similarities between sites were generally high (>70%). A hierarchical cluster analysis distinguished two clusters at a 75% similarity level only, varying in copepod and chydorid densities. The chironomidsCladotanytarsus andStictochironomus reached high densities in many lakes, whileRhynchotalona falcata, Monospilus dispar, Disparalona rostrata, Alona spp. andChydorus spp. (mainlyChydorus piger) were the most abundant chydorid taxa.     

珠江口南沙河涌浮游植物群落结构时空变化及其与环境因子的关系

随着城市生态健康理念的提出,城市河涌生态健康也受到了前所未有的关注。为更好的了解河涌的水环境和浮游植物现状,于2015年3月至2016年2月对珠江口南沙河涌8个站位水环境和浮游植物群落结构进行调查。结果显示:共发现浮游植物164种(属),隶属7门73属,其中以绿藻种类最多,达33属79种,占48.17%;硅藻次之,17属41种,占25%。优势种为梅尼小环藻(Cyclotella meneghiniana)、假鱼腥藻属(Pseudanabaena sp.)和小球藻(Chlorella vulgaris)。浮游植物细胞密度在0.19×10~6—101.34×10~6个/L内变动,呈现单峰型,在4月发生拟菱形弓形藻(Schroederia nitzschioides)水华,14涌密度高达87.38×10~6个/L,随后因强降雨细胞密度骤降。浮游植物群落的季节演替基本符合PEG(Plankton Ecology Group)模型,从冬季的硅藻,到春夏季的绿藻,再到秋季的蓝藻。One-way ANOVA分析显示,各月份浮游植物细胞密度差异显著(P0.01)。Pearson相关性分析表明绿藻细胞丰度变化主导着浮游植物总丰度的变化(r=0.454,P0.01)。运用Margalef物种丰富度指数、Shannon物种多样性指数、Pielou均匀度指数对水体进行评价表明,调查水体呈中度污染。相关加权营养状态指数表明,河涌全年处于富营养化状态。浮游植物聚类分析表明,时间异质性较高,总体相似性较低;空间上相似性较高,人为活动可能是导致空间差异的关键因子。冗余分析显示,叶绿素a、溶解氧、盐度、水温、总氮和p H与浮游植物群落结构关系最为密切。p H对硅藻门浮游植物影响较大,碱性条件适宜直链藻生长,春季水华形成的驱动因子是盐度、温度和总氮。     

Nutrient limitation of phytoplankton communities in Subarctic lakes and ponds in Wapusk National Park, Canada

We determined the limiting nutrient of phytoplankton in 21 lakes and ponds in Wapusk National Park, Canada, using nutrient enrichment bioassays to assess the response of natural phytoplankton communities to nitrogen and phosphorus additions. The goal was to determine whether these Subarctic lakes and ponds were nutrient (N or P) limited, and to improve the ability to predict future impacts of increased nutrient loading associated with climate change. We found that 38% of lakes were not limited by nitrogen or phosphorus, 26% were co-limited by N and P, 26% were P-limited and 13% were N-limited. TN/TP, DIN/TP and NO₃ ⁻/TP ratios from each lake were compared to the Redfield ratio to predict the limiting nutrient; however, these predictors only agreed with 29% of the bioassay results, suggesting that nutrient ratios do not provide a true measure of nutrient limitation within this region. The N-limited lakes had significantly different phytoplankton community composition with more chrysophytes and Anabaena sp. compared to all other lakes. N and P limitation of phytoplankton communities within Wapusk National Park lakes and ponds suggests that increased phytoplankton biomass may result in response to increased nutrient loading associated with environmental change.