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1.
A bloom of the cyanobacteria Microcystis aeruginosa was sampled over the summer and fall in order to determine if the spatial and temporal patterns in cell density, chlorophyll a (chl a) concentration, total microcystins concentration, and percent microcystins composition varied with environmental conditions in San Francisco Estuary. It was hypothesized that the seasonal variation in Microcystis cell density and microcystin concentration was ecologically important because it could influence the transfer of toxic microcystins into the aquatic food web. Sampling for Microcystis cell density, chl a concentration, total microcystins concentration and a suite of environmental conditions was conducted biweekly at nine stations throughout the freshwater tidal and brackish water regions of the estuary between July and November 2004. Total microcystins in zooplankton and clam tissue was also sampled in August and October. Microcystis cell density, chl a concentration and total microcystins concentration varied by an order of magnitude and peaked during August and September when and αB were high. Low streamflow and high water temperature were strongly correlated with the seasonal variation of Microcystis cell density, total microcystins concentration (cell)−1 and total microcystins concentration (chl a)−1 in canonical correlation analyses. Nutrient concentrations and ratios were of secondary importance in the analysis and may be of lesser importance to seasonal variation of the bloom in this nutrient rich estuary. The seasonal variation of Microcystis density and biomass was potentially important for the structure and function of the estuarine aquatic food web, because total microcystins concentration was high at the base of the food web in mesozooplankton, amphipod, clam, and worm tissue during the peak of the bloom. Handling editor: D. Hamilton  相似文献   

2.
Seasonal changes in abundance of the testate amoeba Penardochlamys sp. and its food vacuole contents were investigated in relation to blooms of the cyanobacteria Microcystis spp. in a hypertrophic pond from April 1999 to March 2000. The behavior of the amoeba feeding on M. aeruginosa and M. wesenbergii was also observed in the laboratory. The amoeba was detectable from late May to November 1999 during the blooms of Microcystis spp. Cell densities of the amoeba fluctuated between 1.4 and 350 cells ml–1 with some sporadic peaks, which did not coincide with rapid decreases in the abundance of Microcystis spp. Food vacuoles contained only Microcystis cells; other prey items were not found, suggesting that this amoeba utilized only the cyanobacteria as food. The amoeba was frequently found attached to Microcystis colonies, but was not associated with other suspended particles. Observation of the amoeba feeding revealed the feeding mechanism and that the amoeba was able to graze on both species of Microcystis. These results suggest that the trophic coupling of these organisms is substantial, although grazing by the amoeba is not sufficient to regulate the dynamics of Microcystis populations in this hypertrophic pond.  相似文献   

3.
In order to assess the effects of the introduced bivalve Limnoperna fortunei on water-column properties of Salto Grande reservoir, experiments were conducted using six 400 L mesocosms: 2 with 100 mussels, 2 with 300 mussels, and 2 controls (without mussels). At 0, 1, 2, 3, 7, 14, 21, 28, and 35 days we measured nutrient and chlorophyll a concentrations, counted and identified the phytoplankton, and estimated the density, size, and number of cells of the colonies of Microcystis spp. Cumulative periphyton growth and total accumulated sediments were assessed in all enclosures at the end of the experiment. Throughout the experiment, in the controls ammonia and phosphates dropped to near zero, whereas in the mesocosms with L. fortunei they increased two- to tenfold. Nitrates decreased in all mesocosms. In the presence of the mussel, chlorophyll a and algal cells dropped until day 3 increasing thereafter, whereas in the controls they increased from day 0. Periphyton growth and sediment accumulation were significantly higher in the mesocosms with mussels that in the controls. Cell density, proportion of colonial cells and colony size of Microcystis spp. increased in all enclosures, but these increases were dramatically (and very significantly) higher in enclosures with 100 and, especially, with 300 mussels, than in the controls. Our results indicate that L. fortunei modifies nutrient concentrations and proportions, and promotes aggregation of solitary Microcystis spp. cells into colonies; both these effects can favor blooms of this often noxious cyanobacteria.  相似文献   

4.
Cyanobacteria produce a wide spectrum of biologically active compounds such as microcystins, whose effects on photoautotrophic organisms and role in the aquatic ecosystems have been most intensively discussed and studied. In our study, we examined effects of semipurified Microcystis extract containing microcystins (0.2–20 nM corresponding to 0.2–20 μg L−1) on age-induced cell differentiation of filamentous cyanobacterium Trichormus variabilis. The heterocyst and akinete formation was significantly decreased after exposure to extract containing 2 or 20 nM of microcystins within 10 days of exposure. To our knowledge, this is the first information about effects of metabolites produced by planktonic cyanobacteria on cell differentiation in filamentous cyanobacteria. Since the effects were induced by environmentally relevant concentrations of microcystins, our observations may indicate not only that microcystins or other bioactive peptides could affect conservation, overwintering, and nitrogen-fixation in filamentous cyanobacteria under environmental conditions but also contribute to the understanding of possible signaling function of cyanobacterial metabolites.  相似文献   

5.
Bacteria of the genus Vibrio are an important component of marine ecosystems worldwide. The genus harbors several human pathogens, for instance the species Vibrio parahaemolyticus, a main cause for foodborne gastroenteritis in Asia and the USA. Pathogenic V. parahaemolyticus strains emerged also in Europe, but little is known about the abundance, pathogenicity and ecology of V. parahaemolyticus especially in Northern European waters. This study focuses on V. parahaemolyticus and its close relative Vibrio alginolyticus in the North Sea (Helgoland Roads, Germany). Free-living, plankton-attached and shellfish-associated Vibrio spp. were quantified between May 2008 and January 2010. CFUs up to 4.3 × 103 N l−1 and MPNs up to 240 N g−1 were determined. Phylogenetic classification based on rpoB gene sequencing revealed V. alginolyticus as the dominant Vibrio species at Helgoland Roads, followed by V. parahaemolyticus. We investigated the intraspecific diversity of V. parahaemolyticus and V. alginolyticus using ERIC-PCR. The fingerprinting disclosed three distinct groups at Helgoland Roads, representing V. parahaemolyticus, V. alginolyticus and one group in between. The species V. parahaemolyticus occurred mainly in summer months. None of the strains carried the virulence-associated genes tdh or trh. We further analyzed the influence of nutrients, secchi depth, temperature, salinity, chlorophyll a and phytoplankton on the abundance of Vibrio spp. and the population structure of V. parahaemolyticus. Spearman Rank analysis revealed that particularly temperature correlated significantly with Vibrio spp. numbers. Based on multivariate statistical analyses we report that the V. parahaemolyticus population was structured by a complex combination of environmental parameters. To further investigate these influences is the key to understanding the dynamics of Vibrio spp. in temperate European waters, where this microbial group and especially the pathogenic species, are likely to gain in importance.  相似文献   

6.
Habitat choice in relation to environmental factors of two coexisting calanoid copepod species, Eudiaptomus gracilis and E. graciloides, was studied in a mesotrophic lake and in large indoor mesocosms. Both species and sexes showed pronounced diel vertical migration (DVM) in the field. In 12 m deep mesocosms with free ranging fish DVM was observed and species increased day depth over time. No changes were observed in copepod day depth over time in experiments with fish kairomone. It is hypothesized that fish kairomone acts as an early warning system to copepods which respond by moving deeper, but only as far as the thermocline. For full DVM, a nearby mechanical stimulus is necessary. Thus, as fish go deeper to feed, copepods retreat. The response of copepods to fish predation, in the presence of low and high numbers of Daphnia,shows that copepods effectively use Daphniaas living shields to avoid predation. The two species adopt different vertical migration strategies depending on whether there are high or low numbers of Daphniapresent. A dominant feature of mesocosm experiments was the night time aggregating (lekking) of E. gracilis males at the surface. When the spring and autumn percentages of risk takers in the epilimnion were compared, E. gracilis, particularly males, suffered the greatest cost.  相似文献   

7.
The zooplankton biomass of Lake Boeckella (Hope Bay, Antarctic Peninsula) is strongly dominated by the calanoid copepod Boeckella poppei Mrázek. This work analyses the grazing impact of this copepod on the two dominant fractions of phytoplankton, pico- and nanoplankton, and on the bacterioplankton. By means of in-situ experiments using microcosms, the following hypotheses were tested: (a) the early stages of the copepod mainly graze on phytoplankton; (b) the pre-adult and adult stages graze on phytoplankton and benthic algae. Copepods were separated into two groups of maturity: early stages, and pre-adult and adult stages. The following treatments were performed: (1) only nano- and picoplankton, (2) nano- and picoplankton+periphyton, and (3) only periphyton, for each one of the two copepod maturity groups, and (4) control (without copepods). The variation in nano-phytoplankton density was analysed after 2 and 4 days. The results determined a significant grazing on the nano-phytoplankton fraction in all microcosms containing copepods of both maturity groups (P<0.01). The effect on the nano-phytoplankton was greater when the copepods did not have another source of food (P<0.01). No significant differences between the maturity groups were observed (P>0.05). We also conclude that the copepods use the periphyton as an alternative source of food, which was corroborated by the analysis of gut content. In general, no significant differences among treatments were recorded for the pico-sized fraction (pico-phytoplankton and bacterioplankton), which would suggest that no direct grazing on this fraction exists.  相似文献   

8.
Larvae and juveniles of the roughskin sculpin Trachidermus fasciatus were collected during four cruises in March and April 2001 to study the distribution and diet of the fish in the Chikugo estuary in the northern Ariake Bay, Japan. Sampling was conducted at seven stations covering an area approximately 30 km in length along the estuary, with salinity ranging from nearly 0 to about 30 PSU. Gut contents were analyzed by separating, identifying, and counting the prey organisms. Plankton samples were collected during each cruise to study the numerical composition and abundance of copepods in ambient water. A total of 1790 larvae and juveniles of T. fasciatus were collected; they were distributed in the five uppermost stations, covering approximately 20 km and ranging in salinity from 0.4 to 27.4 PSU. The fish fed almost exclusively on a single calanoid copepod species, Sinocalanus sinensis, which was numerically the most abundant in the ambient water at the upper part of the Chikugo River. At the lower part of the river, the fish positively selected S. sinensis while negatively selecting other species. T. fasciatus larvae experienced a high proportion of empty guts and low feeding intensity at around 9.0 mm body length, which appears to be associated with commencement of exogenous feeding. The proportion of empty guts reduced sharply at subsequent stages and was accompanied by an increase in feeding intensity. The oligohaline and mesohaline areas are important nursery grounds, and S. sinensis is an important prey copepod species for the larval and juvenile T. fasciatus in the Chikugo estuary.  相似文献   

9.
A Streptomyces strain, NT0401, was isolated from soil that selectively inhibited Microcystis strains but did not affect other microorganisms. Based on its morphology, physiology and 16S rDNA sequence, it was identified as Streptomyces grisovariabilis. The active substance produced by NT0401 was a water-soluble compound with a Mr <1 kDa that was stable over a broad pH range and at 100°C for 20 min. This organism should be a potential environment-friendly strain for control of Microcystis blooms.  相似文献   

10.
Toxic cyanobacterial blooms constitute a threat to human safety because Microcystis sp. releases microcystins during growth, and particularly during cell death. Therefore, analysis of toxic and nontoxic Microcystis in natural communities is required in order to assess and predict bloom dynamics and toxin production by these organisms. In this study, an analysis combining fluorescence in situ hybridization (FISH) with flow cytometry (FCM) was used to discriminate between toxic and nontoxic Microcystis and also to quantify the percentage of toxic Microcystis present in blooms. The results demonstrate that the combination of FISH and flow cytometry is a useful approach for studying the ecology of Microcystis toxin production and for providing an early warning for toxic Microcystis blooms.  相似文献   

11.
A survey was conducted for the presence of cyanobacteria toxins in Lake Kotokel due to a few cases of Haff disease registered in 2008–2009 caused by consumption of fish from Lake Kotokel, and wildlife mortality including large fish kill. The aims of this study were to determine what cyanotoxins (if any) were present in the lake, to describe phytoplankton composition including morphology, density, and species diversity of cyanobacteria, as well as to evaluate the trophic state of the lake. Samples were collected from both nearshore and central sites in August of 2009. Aphanocapsa holsatica dominated the phytoplankton. The presence of toxigenic genotypes of Microcystis spp. and Anabaena lemmermannii was detected by sequencing of PCR-amplified aminotransferase domain of microcystin synthetase gene. LR, RR, and YR microcystin (MC) variants were detected with liquid chromatography-UV mass spectrometry. The data do not shed light on the etiology of Haff disease in Lake Kotokel region, nevertheless taking into account the recreational importance of the lake and its direct connection to Lake Baikal, a necessity to monitor cyanobacteria in these water bodies is evident. This is the first report on simultaneous detection of MC-producing genotypes and MCs in the Lake Baikal region.  相似文献   

12.
Recombination has been suggested to be an important factor for the genetic variation of bacterial genes, but few studies have dealt with intragenic recombination between the same or closely related species of cyanobacteria. Here we provide strong evidence for recombination in the microcystin synthetase (mcy) gene cluster of the toxic cyanobacteria Microcystis spp. This gene cluster contains 10 genes (mcyA to J) that encode a mixed polyketide synthase (PKS)/nonribosomal peptide synthetase (NRPS) complex. mcy gene sequences were determined for four selected regions (within mcyA, D, G, and J) within the mcy gene cluster from 1 Canadian and 10 Asian toxic Microcystis and compared with previously published mcy sequences. Split decomposition analysis indicated a reticulate phylogeny of mcyA, and several potential recombination tracts of mcyA were identified by the RDP analysis and a runs test implemented in GENECONV. In contrast, no recombination was detected in the mcyD, G, and J sequences. However, discrepancies among the four mcy gene genealogies were evident from the results of independent split decomposition analyses, which were further supported by incongruence length difference (ILD) tests. Taken together, these findings suggest that both intragenic and intergenic recombination within the mcy gene cluster contributes to the genetic diversity of the mcy genes of Microcystis spp.This article contains online supplementary material.  相似文献   

13.
The re-emergence of Gymnodinum catenatum blooms after a 10 year hiatus of absence initiated the present investigation. This study aims to evaluate the exposure of small pelagic fishes to paralytic shellfish toxins (PST) during blooms of G. catenatum. Sardines (Sardina pilchardus) were selected as a representative fish species. In order to assess toxin availability to fish, both intracellular PSTs (toxin retained within the algal cells) and extracellular PSTs (toxin found in seawater outside algal cells) were quantified, as well as toxin levels within three fish tissue matrices (viscera, muscle and brain). During the study period, the highest cell densities of G. catenatum reached 2.5 × 104 cells l−1 and intracellular PST levels ranged from 3.4 to 398 ng STXeq l−1 as detected via an enzyme linked immunosorbent assay (ELISA). Measurable extracellular PSTs were also detected in seawater (0.2–1.1 μg STXeq l−1) for the first time in Atlantic waters. The PST profile in G. catenatum was determined via high performance liquid chromatography with fluorescence detection (HPLC-FLD) and consisted mostly of sulfocarbamoyl (C1+2, B1) and decarbamoyl (dcSTX, dcGTX2+3, dcNEO) toxins. The observed profile was similar to that reported previously in G. catenatum blooms in this region before the 10-year hiatus. Sardines, planktivorous fish that ingest a large number of phytoplankton cells, were found to contain PSTs in the viscera, reaching a maximum of 531 μg STXeq kg−1. PSTs were not detected in corresponding muscle or brain tissues. The PST profile characterized in sardine samples consisted of the same sulfocarbamoyl and decarbamoyl toxins found in the algal prey with minor differences in relative abundance of each toxin. Overall, the data suggest that significant biotransformation of PSTs does not occur in sardines. Therefore, planktivorous fish may be a good tracer for the occurrence of offshore G. catenatum blooms and the associated PSTs produced by these algae.  相似文献   

14.
Cyanobacteria blooms (especially Microcystis spp.) are thought to alter dominance of large-sized daphnids into small-sized metazoan zooplankton. However, several field investigations show different phenomena. Laboratory experiments were conducted based on the hypothesis that different Microcystis spp. concentrations would influence competitive outcomes using two algal combinations of different concentrations and four species of cladocerans. In the algal combination of 50 mg l−1 colonial Microcystis spp. + 1 mg l−1 Scenedesmus obliquus (fresh weight), Daphnia carinata was absent during the experiment in competition with other cladocerans. Decreasing colonial Microcystis spp. concentration (10 mg l−1) resulted in a shift from dominance by small-sized cladocerans to dominance by D. carinata. No significant effects of different concentrations of colonial Microcystis spp. on competitive outcomes were shown among three small-sized cladocerans. These results support the idea that cyanobacteria concentration affects the dominance status of large-bodied daphnid.  相似文献   

15.
In this study, the methanol extract of Arthrospira (Spirulina) platensis was examined for acute and subchronic toxicities. The extract did not produce any sign of toxicity within 7 days after feeding it at a single high dose of 6 g kg−1 body weight to female and male Swiss mice. For the subchronic toxicity test, the extract at doses of 6, 12, and 24 mg kg−1 body weight was orally administered to six male and six female Wistar rats daily for 12 weeks. Throughout the study period, we did not observe any abnormalities on behavior, food and water intakes, and health status among the treated animals. The hematology and clinical chemistry parameters of treated groups did not significantly differ from those of the controls in both sexes. Postmortem examination of the test groups also showed no abnormalities in both gross and histological findings. These results thus suggest that the methanol extract of A. platensis did not cause acute or subchronic toxicity in our experimental animals.  相似文献   

16.
The predatory copepod Acanthocyclops americanus is commonly found in shallow ponds and lakes in Mexico. We tested the survivorship and reproduction-related variables of this copepod, isolated from Lake Huetzalin (Xochimilco, Mexico City), on four mixed diets comprising two algal concentrations (0.8 × 106 and 1.6 × 106 cells ml−1 of Scenedesmus acutus) and two rotifer densities (1 and 8 ind. ml−1 of Brachionus havanaensis). Survivorship patterns of naupliar and adult stages of A. americanus were evaluated at 24 ± 1°C. The demography experiments were initiated with five females and five males (1:1 ratio, 5th copepodites) into each of the 16 test [= 4 diet combinations × 4 replicates (cohorts)] jars containing 50 ml medium and with one of the chosen algal-rotifer diets. Data on the survival of naupliar and copepodites (up to V) showed a high death rate (80%) in A. americanus by the time nauplii reached the adult stage. The duration of developmental time for nauplii and the copepodite V (C-V) was about 28 days by which time A. americanus became adults. Food combination had a significant effect on both survival and duration of the naupliar and C-V but not on the development time in C-I, C-II, C-III, and C-IV. Naupliar stages showed highest life expectancy at low food levels (rotifers or alga). Adult male and female copepods showed better survival under higher availability of rotifers in the medium, while higher algal density resulted in steep mortality during the first days. Average adult lifespan and life expectancy of A. americanus varied from 19 to 23 days, depending on the diet combination. Age-specific reproductive output (m x ) of female A. americanus showed low reproduction (<10 offspring per female on any given day) when raised on low rotifer density, regardless of the algal concentration. On the other hand, enhanced offspring production (average of about 50 nauplii per female) was recorded when the rotifer density was higher (i.e., 8 ind. ml−1). Gross and net reproductive rates varied from 48 to 438 and 27 to 318 offspring per female, respectively. In general, higher availability of rotifers in the medium resulted in higher (8–10 times as compared to those in low rotifer density) gross and net reproductive rates. There was no significant difference in the generation time (13–14 days) in relation to the diet combination. The rate of population increase (r) (range: 0.29–0.69 per day) increased with greater abundance of B. havanaensis in the medium. Thus, our study showed that inclusion of animal protein in the diet is necessary for high population growth rates of A. americanus.  相似文献   

17.
Chemical communication may inform about the location of prey, predators, co-specifics, and mate partners in zooplankton. In this study, we evaluated several life-history traits of the rotifer, Brachionus calyciflorus, exposed to conditioned media by a rotifer predator (Asplanchna brightwelli) and a cladocera competitor (Daphnia similis), quantifying population growth and life-table demography at two algal food levels (2.0 and 0.5 × 106 cells ml−1 of Chlorella pyrenoidosa). At both food levels, B. calyciflorus grown in predator-conditioned media had lower population abundance and slower population growth rate than controls. Conversely, the competitor-conditioned media treatments produced both higher rotifer population abundance and faster population growth rate than controls. Life-history parameters varied significantly depending on the presence of predator and competitor-conditioned media. The Asplanchna-conditioned media significantly decreased gross reproductive rate (GRR): 8–9 offsprings per female; net reproductive rate (R 0): 6–7 offsprings per female; population growth rate (r): 0.34–0.37 day−1; and increased generation time (T): 5.5–5.6 days. On the other hand, The Daphnia-conditioned media significantly increased the GRR (13–14 offsprings per female); net reproductive rate (8–9 offsprings per female); population growth rate (0.42–0.43 day−1); and decreased generation time (4.9–5.0 days). However, the effects of food level on the life-history characteristic were not significant in both treatments. Maximum values of the population abundance and the population growth rate are significantly influenced by the predator densities and pre-culture time. This study suggests that rotifers use variable life-history strategies (low reproduction and high survivorship versus high reproduction and low survivorship) based on the presence of predators and competitors.  相似文献   

18.
Ingestion rates and selectivity of the Arctic pelagic amphipod Themisto libellula were studied experimentally in Kongsfjorden (Svalbard, 78°N) during the summer period. Feeding incubations were conducted on naturally occurring copepod communities at different concentrations ranging from 25 to 250 preys L−1. The ingestion rates increased with food availability from 1.3 to 17.7 preys ind−1 day−1, which corresponded to 0.3–11% of body carbon day−1. Despite the high prey concentration used in the experiments the satiation level was not reached. We suggested that T. libellula is able to take the maximum benefit from dense patches of preys, which represent a good adaptation to the high variability in food supply characteristic of polar environment. Copepodids stage III of Calanus spp. appeared to be the preferred preys of T. libellula. Smaller copepods such as Oithona similis and Pseudocalanus spp., were also selected but only when their relative abundance exceeded 25% of the total prey available. The potential predation impact of T. libellula is discussed in relation to the mesozooplankton small-scale patchiness and predator abundance.  相似文献   

19.
A genetic transformation system has been developed for callus cells of Crataegus aronia using Agrobacterium tumefaciens. Callus culture was established from internodal stem segments incubated on Murashige and Skoog (MS) medium supplemented with 5 mg l−1 Indole-3-butyric acid (IBA) and 0.5 mg l−1 6-benzyladenine (BA). In order to optimize the callus culture system with respect to callus growth and coloration, different types and concentrations of plant growth regulators were tested. Results indicated that the best average fresh weight of red colored callus was obtained on MS medium supplemented with 2 mg l−1 2,4-dichlorophenoxyacetic acid (2,4-D) and 1.5 mg l−1 kinetin (Kin) (callus maintenance medium). Callus cells were co-cultivated with Agrobacterium harboring the binary plasmid pCAMBIA1302 carrying the mgfp5 and hygromycin phosphotransferase (hptII) genes conferring green fluorescent protein (GFP) activity and hygromycin resistance, respectively. Putative transgenic calli were obtained 4 weeks after incubation of the co-cultivated explants onto maintenance medium supplemented with 50 mg l−1 hygromycin. Molecular analysis confirmed the integration of the transgenes in transformed callus. To our knowledge, this is the first time to report an Agrobacterium-mediated transformation system in Crataegus aronia.  相似文献   

20.
This study has two main objectives, the first being the determination of net phytoplankton primary production to explain the phytoplankton’s function in a wetland carbon cycle, while the second objective is to relate this function with the phytoplankton assemblage composition. The annual variation in the phytoplankton production was monitored monthly for more than a year (2007–2008) in the semiarid eutrophic, hydrologically-perturbed “Tablas de Daimiel” National Park wetland. The phytoplankton fraction considered in this study comprised all organisms between the size 3 and 100 μm. The total biomass of phytoplankton was obtained by counting algae and calculating their volume, while net primary production and respiration were quantified by in situ incubations with the Winkler method. The respiration ranged from undetectable to 0.07 mgO2 l−1 h−1; net photosynthesis reached 0.20 mgO2 l−1 h−1. Net primary production was maximum at the end of the warm period (October 2007), and other peaks occurred at the start of summer (July 2007) or spring (March 2008). When maximum production took place, phytoplankton was mainly composed of small fast-growing chlorophytes (Tetraselmis cf. fontiana or Chlamydomonas sp.), in addition to some of the large, S-strategist algae (Peridinium umbonatum, Microcystis flos-aquae, Euglena sp.). The phytoplankton metabolism in “Tablas de Daimiel” was autotrophic as a whole due to changing contributions of algal groups. Only chlorophyte biomass was statistically related to net primary production. The conclusion reached is that this shallow eutrophic semiarid wetland possessed an annual net autotrophic production of phytoplankton fraction resulting from the small, fast-growing algae enhanced by hydrological perturbations that interrupted the autogenic course of S strategists.  相似文献   

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