Abundance and distribution of picoplankton in tropical, oligotrophic Lake Kivu, eastern Africa

1. We used flow cytometry to characterize freshwater photosynthetic picoplankton (PPP) and heterotrophic bacteria (HB) in Lake Kivu, one of the East‐African great lakes. Throughout three cruises run in different seasons, covering the four major basins, phycoerythrin‐rich cells dominated the PPP. Heterotrophic bacteria and PPP cell numbers were always high and spatial variations were modest. This represents an important difference from temperate and high latitude lakes that show high fluctuations in cell abundance over an annual cycle. 2. Three populations of picocyanobacteria were identified: one corresponded to single‐cells (identified as Synechococcus by epifluorescence microscopy, molecular methods and pigment content), and the two other that most probably correspond to two and four celled colonies of the same taxon. The proportion of these two subpopulations was greater under stratified conditions, with stronger nutrient limitation. 3. High PPP concentrations (c. 10⁵ cell mL^?1) relative to HB (c. 10⁶ cell mL^?1) were always found. Lake Kivu supports relatively less bacteria than phytoplankton biomass than temperate systems, probably as a consequence of factors such as temperature, oligotrophy, nutrient limitation and trophic structure. 4. A review of PPP concentration across aquatic systems suggests that the abundance of Synechococcus‐like cyanobacteria in large, oligotrophic, tropical lakes is very high. 5. Photosynthetic picoplankton cell abundances in the oligotrophic tropical lakes Kivu and Tanganyika are comparable to those of eutrophic temperate lakes. This apparently contradicts the view that PPP abundance increases with increasing eutrophy. More data on PPP in tropical lakes are needed to explore further this particular pattern.     

CHRYSOPHYTE POPULATION AND ENCYSTMENT PATTERNS IN TWO CANADIAN LAKES1

The dynamics of chrysophyte populations and the onset of encystment in relation to several physical, chemical, and biological variables were studied in two Canadian lakes, Lake Opinicon (LO) and Upper Rock Lake (URL). LO is shallow and polymictic during the ice-free season, whereas URL is deep and dimictic. Seasonal patterns of many limnological variables, as well as species composition and seasonal dynamics of chrysophyte assemblages, were broadly similar in both lakes. Three groups of chrysophytes were recognized: 1) taxa that were more abundant during ice cover and/or early spring and occasionally in late fall (Dinobryon spp. Ehrenberg and Synura spinosa Korsh), 2) taxa that seemed to prefer moderately warm water proglena americana Calkins and Ochromonas sp. Wyssotzki) and had maximum densities in late summer and occasionally in the fall, and 3) taxa that occurred in low numbers and did not exhibit any discernible seasonality in the two lakes (Mallomonas spp. Perty). Encystment rarely occurred during ice cover in both lakes. Stomatocyst concentrations were highest shortly after ice breakup (late April or early May), declined to trace levels by June, and remained at those levels until the end of the sampling season (late November). Encystment rates (cyst concentrations) were higher in dimictic URL. Encystment in both lakes coincided with concurrent changes in physical and chemical variables and chrysophyte vegetative cell densities, but it was not possible to pinpoint the exact conditions responsible for triggering encystment. Only a small proportion of chrysophytes encysted. Cyst production within a lake may vary from year to year.     

Field measurements of ingestion and assimilation for the Australian brine shrimp Parartemia zietziana Sayce (Crustacea: Anostraca)

Ingestion and assimilation of the brine shrimp Parartemia zietziana were measured in the field. Populations in Pink Lake and Lake Cundare were used; both lakes are shallow (< 1 m), saline (> 60°/00), and located in western Victoria; they have tow primary production. P. zietziana relies largely on organic matter in lake sediment for food. Ingestion rate was measured in situ in Pink by following the uptake of ¹⁴C from labelled sediment. The rate of faecal pellet production was measured (in both lakes) by allowing animals to defaecate for 24 h in a net suspended in the lake. Ingestion rate varied from 0.35 to 13 01 × 10^–1 mg dry wt h^-1 individual^-1 for a range in dry weight of shrimp from 0.2 to 2.3 mg individual^-1; for a similar range in weight, defaecation varied from 0.08 to 2.03 × 10^-1mg dry wt h^-1 individual^-1. These rates were converted to joules and assimilation efficiencies of 30–60% calculated. Regressions showed that change in dry weight of shrimp accounted for > 90% of the variance in both rates.     

Phytoplankton nutrient deficiency in lakes of the McMurdo dry valleys, Antarctica

1. The influence of inorganic nitrogen and phosphorus enrichment on phytoplankton photosynthesis was investigated in Lakes Bonney (east and west lobes), Hoare, Fryxell and Vanda, which lie in the ablation valleys adjacent to McMurdo Sound, Antarctica. Bioassay experiments were conducted during the austral summer on phytoplankton populations just beneath the permanent ice cover in all lakes and on populations forming deep-chlorophyll maxima in the east and west lobes of Lake Bonney. 2. Phytoplankton photosynthesis in surface and mid-depth (13 m) samples from both lobes of Lake Bonney were stimulated significantly (P < 0.01) by phosphorus enrichment (2 μM) with further stimulation by simultaneous phosphorus plus NH₄⁺ (20 μM) enrichment. Similar trends were observed in deeper waters (18 m) from the east lobe of Lake Bonney, although they were not statistically significant at P < 0.05. Photosynthesis in this lake was never enhanced by the addition of 20 μM NH₄⁺ alone. Simultaneous addition of phosphorus plus nitrogen stimulated photosynthesis significantly (P < 0.01) in both Lake Hoare and Lake Fryxell. No nutrient response occurred in Lake Vanda, where activity in nutrient-enriched samples was below unamended controls; results from Lake Vanda are suspect owing to excessively long sample storage in the field resulting from logistic constraints. 3. Ambient dissolved inorganic nitrogen (DIN) (NH4⁺+ NO₂^?+ NO₃^?): soluble reactive phosphorus (SRP) ratios partially support results from bioassay experiments indicating strong phosphorus deficiency in Lake Bonney and nitrogen deficiency in Lakes Hoare and Fryxell. DIN : SRP ratios also imply phosphorus deficiency in Lake Vanda, although not as strong as in Lake Bonney. Particulate carbon (PC): particulate nitrogen (PN) ratios all exceed published ratios for balanced phytoplankton growth, indicative of nitrogen deficiency. 4. Vertical nutrient profiles in concert with low advective flux, indicate that new (sensu Dugdale & Goering, 1967) phytoplankton production in these lakes is supported by upward diffusion of nutrients from deep nutrient pools. This contention was tested by computing upward DIN : SRP flux ratios across horizontal planes located immediately beneath each chlorophyll maximum and about 2 m beneath the ice (to examine flux to the phytoplankton immediately below the ice cover). These flux ratios further corroborated nutrient bioassay results and bulk DIN : SRP ratios indicating phosphorus deficiency in Lakes Bonney and Vanda and potential nitrogen deficiency in Lakes Hoare and Fryxell. 5. Neither biochemical reactions nor physical processes appear to be responsible for differences in nutrient deficiency among the study lakes. The differences may instead be related to conditions which existed before or during the evolution of the lakes.     

The construction of a diatom-based chlorophyll a transfer function and its application at three lakes on Signy Island (maritime Antarctic) subject to differing degrees of nutrient enrichment

1. Canonical correspondence analysis of a diatom and water chemistry dataset from fifty-nine maritime Antarctic lakes situated on Signy and Livingston Islands showed that nutrients and functions of nutrients (NH₄⁺, chlorophyll a) accounted for a significant fraction of the variance in the diatom data. 2. Weighted averaging regression was used to construct a diatom-based transfer function for inferring chlorophyll a concentrations from sediment core diatom assemblages. 3. The transfer function was applied to ²¹⁰Pb-dated sediment cores from three lakes (Moss, Sombre and Heywood) receiving different levels of nutrient input from fur seal populations, i.e. low, medium and high, respectively. 4. Moss Lake showed relatively stable reconstructed chlorophyll a values, and no evidence of recent eutrophication, agreeing with measured chlorophyll a concentrations at the site. 5. Changes in diatom assemblages and results of chlorophyll a reconstructions at Sombre Lake suggested that nutrient enrichment had occurred, which could be clearly linked to fluctuations in the measured water chemistry over the last 10–14 years. 6. Despite recorded increases in recent nutrient inputs there was no apparent diatom response at Heywood Lake.     

Primary productivity and trophic status of Lakes Sorell and Crescent,Tasmania

The primary productivity of two turbid, shallow lakes on the Tasmanian Central Plateau was determined by the C¹⁴ technique from half-light day incubations in situ. Graphical integration of depth-rate curves gave estimates of areal day rates of production and of annual rates.The 2 lakes are closely adjacent and very similar physically and chemically, but have very different phytoplankton populations. Lake Crescent has ten times the standing crop biomass of Lake Sorell but its greater turbidity restricts light penetration, and production per unit of surface per day and per year is only 2.6 times that of Sorell.With day rates of 25-(44)-93 mgCm^–2 and annual production of 16.9 gCm^–2 Lake Sorell could be regarded as oligotrophic. Consideration of standing crop biomass and morphometry however indicates oligo-mesotrophy. Lake Crescent with day rates of 35-(115)-250 mgCm^–2 and annual production of 45 gCm^–2 is moderately eutrophic.Incubations in constant light demonstrated considerable variation in production rates in different parts of Lake Crescent.     

Salt Lakes in Australia: Present Problems and Prognosis for the Future

Australia is a land of salt lakes and despite low human population density, many lakes are adversely impacted by a range of factors. Secondary salinisation is the most pernicious force degrading lakes, especially in south-west Western Australia where up to 30% of the landscape is predicted to be affected. Mining also impinges on many salt lakes in this state, mainly through the dewatering of saline groundwater. Exploitation of groundwater for irrigation caused some lakes in Victoria, Australia, to dry, especially the significant Red Rock Complex. Global climate change will result in new water balances in endorheic lakes, with most having less water, particularly the seasonal lakes of southern Australia. This has already happened in Lake Corangamite, Victoria, but the prime reason is diversion of inflowing floodwater. Consequently, the lake has retreated and become salinised compromising its status as a Ramsar site. Various other lakes suffer from enhanced sedimentation, have introduced biota or their catchments are being disturbed to their detriment. Enlightened management should be able to maintain some important lakes in an acceptable condition, but, for most others, the future is bleak.     

Bacterioplankton production in freshwater Antarctic lakes

1. Bacterioplankton production was measured in the water columns of two ultra‐oligotrophic, freshwater Antarctic lakes (Crooked Lake and Lake Druzhby) during an annual cycle. In both lakes bacterial production, measured by the incorporation of [³H] thymidine, continued in winter and showed a cycle over the year. The range of production was between 0 and 479 ng C L^?1 h^?1 in Crooked Lake and 0–354 ng L^?1 h^?1 in Lake Druzhby. 2. Abundance and mean cell volume both varied, producing marked changes in biomass during the year, with highest biomass occurring in the winter and early spring. Biomass showed similar seasonal trends in both lakes. 3. For most of the year inorganic forms of nitrogen and phosphorus were detectable in the water columns of the lakes and were unlikely to have limited bacterial production. Dissolved organic carbon (DOC) was below 3000 μg L^?1. Dissolved amino acids and carbohydrates contributed 5–25% of the DOC pool in Crooked Lake and 5–64% in Lake Druzhby. Dissolved carbohydrates were consistently low, suggesting that this may have been the preferred carbon substrate for bacterioplankton. 4. Aggregate associated bacteria had higher mean cell volume, abundances and production than freely suspended bacteria in Lake Druzhby, while in Crooked Lake aggregate associated bacteria consistently had higher mean cell volumes than free bacteria, but abundance and production were on occasion higher in free bacteria compared with aggregate associated communities. 5. The data indicated that production is limited by continuous low temperatures and the limited availability of suitable DOC substrate. However, the bacterioplankton functions year round, responding to factors other than temperature.     

Decomposition of dominant submerged macrophytes: implications for nutrient release in Myall Lake, NSW, Australia

Breakdown and nutrient dynamics of submerged macrophytes were studied in Myall Lake, Australia. Mass loss of Myriophyllum sulsagineum was the lowest (64.90%) among the studied macrophytes during the 322 days followed by charophytes (60.79%), whereas Najas marina and Vallisneria gigantea lost 91.15 and 86.02% of their respective initial mass during that time. The overall exponential breakdown rates of Najas marina and Vallisneria gigantea were similar, with k-values of 0.24 and 0.23 day⁻¹, respectively. These rates were significantly higher than the break down rates of charophytes (0.007 day⁻¹) and M. sulsagineum (0.008 day⁻¹). During growth phase, water column depicted lower nutrient concentrations while during decay period, significant increase in water column nutrients resulted. Release of nutrients from decomposing macrophytes and incorporation of these nutrients into sedimentary phase as well as uptake of nutrients by the growing macrophytes, can present a considerable cycling pathway of nutrients in Myall lake system. The results of this study suggest that different submerged macrophytes may differ appreciably in quality and may exhibit different decomposition rates, patterns and nutrient dynamics in aquatic ecosystems in general, and Myall lakes in particular.     

Photosynthesis,mixotrophy and microbial plankton dynamics in two high Arctic lakes during summer

Photosynthesis and microbial plankton dynamics of two lakes in the Kongsfjorden catchment of Spitzbergen (Svalbard Archipelago, 78°N) were investigated during the summer of 2000. One of the lakes (Tvillingvatnet) served as the water supply for the village of Ny Ålesund. The other was a deeper, larger unnamed lake in the Ossian Sarsfjella reserve—named Lake OS in this study. Both lakes can be classified as oligotrophic on the basis of chlorophyll a and inorganic nutrient concentrations. Chlorophyll a concentrations ranged between 0.33 and 1.65 µg l ^-1 in Tvillingvatnet and 0.2 and 0.3 µg l ^-1 in Lake OS. The phytoplankton was dominated by chrysophytes and cryptophytes, with the diatom Rhizosolenia and a number of dinoflagellate species. Rates of photosynthesis were typically low, between 24.5 µg and 1.0 mg l ^-1 day ^-1 in Tvillingvatnet (photosynthetic efficiency 0.004–0.26), and between 3.1 and 29.5 µg l ^-1 day ^–1 in Lake OS (photosynthetic efficiency 0.0019–0.0085). Among the phytoflagellates (PNAN) there were a number of mixotrophs which reached their peaks of abundance before other PNAN. Mixotrophy appeared to provide a competitive advantage. Grazing rates for the mixotroph Dinobryon ranged between 0.063 and 1.12 pg C cell ^-1 day ^-1 in Lake OS and between 0.22 and 1.11 pg cell ^-1 day ^-1 in Tvillingvatnet, with rates increasing between July and August. However, Dinobryon removed less than 1% of bacterial biomass day ^-1, while the heterotrophic nanoflagellates (HNAN) removed up to 28% of bacterial biomass day ^-1. Bacterial concentrations were low, with a maximum of 28.8×10 ⁸ l ^-1 in Tvillingvatnet and 23.6×10 ⁸ l ^-1 in Lake OS. Ciliated protozoan and rotifer diversity in the plankton was low.     

Effects of warming on annual production and nutrient‐use efficiency of aquatic mosses in a high Arctic lake

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Abundance and Diversity of Sulfur-Oxidizing Bacteria along a Salinity Gradient in Four Qinghai-Tibetan Lakes,China

Sulfur-oxidizing bacteria (SOB) play important roles in the sulfur cycle and are widespread in a number of environments, but their occurrence and relationship to geochemical conditions in (hyper)saline lakes are still poorly understood. In this study, the abundance and diversity of SOB populations were investigated in four Qinghai-Tibetan lakes (Erhai Lake, Gahai Lake 1, Gahai Lake 2 and Xiaochaidan Lake) by using quantitative polymerase chain reaction (qPCR) and soxB gene- (encoding sulfate thiohydrolase) based phylogenectic analyses. qPCR analyses showed that in the studied lakes, the total bacterial 16S rRNA and soxB gene abundances in the sediments were distinctly higher than in the overlying waters. The 16S rRNA gene abundance in the waters ranged 5.27 × 10⁶–6.09 × 10⁸ copies per mL and 7.39 × 10¹⁰–2.9 × 10¹¹ copies per gram sediment. The soxB gene abundance in the waters ranged from 1.88 × 10⁴ to 5.21 × 10⁵ per mL and 4.73 × 10⁶–2.65 × 10⁷ copies per gram sediment. The soxB gene in the waters of the two hypersaline lakes (Gahai Lake 2 and Xiaochaidan Lake) was more abundant (2.97 × 10⁵ and 5.21 × 10⁵ copies per mL) than that in the two low-salinity lakes (1.88 × 10⁴ and 3.36 × 10⁴ copies per mL). Phylogenetic analysis showed that Alpha- and Betaproteobacteria were dominant SOB in the investigated lakes, and the composition of proteobacterial subgroups varied with salinity: in freshwater Erhai Lake and low-salinity Gahai Lake 1, the SOB populations were dominated by the Betaproteobacteria, whereas in hypersaline Lake Gahai 2 and Xiaochaidan Lake, the SOB populations were dominated by Alphaproteobacteria. Overall, salinity played a key role in controlling the diversity and distribution of SOB populations in the investigated Qinghai-Tibetan lakes.     

Chemical limnology of soft water lakes in the Upper Midwest

Water samples from 36 lakes in northern Minnesota, Wisconsin, and Michigan were collected and analyzed during 1983–1984. All study lakes were dilute and had total alkalinities of less than 150 eq · L^–1. Minnesota lakes have hydrologic inputs from the watershed and inputs of base cations derived from the watershed. Study lakes in Minnesota had higher total alkalinities, dissolved organic carbon, and noncarbonate alkalinity as a result of watershed inputs. Lakes in Michigan and Wisconsin were precipitation-dominated seepage lakes that have lower concentrations of base cations than lakes in Minnesota. All of the study lakes have lower sulfate concentrations than expected, based on atmospheric wet deposition and evapotranspiration.Pore water samples collected from one of the study lakes—Little Rock Lake—in Wisconsin were used to calculate diffusive fluxes between the sediment and water column. According to these calculations, the sediments were a source of total alkalinity and Ca²⁺ and a sink for SO₄ ^2–. The sediment-water exchange of total alkalinity, Ca²⁺, and SO₄ ^2– appears to be important in the whole-lake budgets of these ions for Little Rock Lake.     

Primary and Bacterial Production in Two Dimictic Indiana Lakes

The relationship between primary and bacterial production in two dimictic Indiana lakes with different primary productivities was examined during the summer stratification period in 1982. Primary production rates were calculated from rates of H¹⁴CO₃⁻ incorporation by natural samples, and bacterial production was calculated from rates of [³H-methyl]thymidine incorporation by natural samples. Both vertical and seasonal distributions of bacterial production in the more productive lake (Little Crooked Lake) were strongly influenced by primary production. A lag of about 2 weeks between a burst in primary production and the subsequent response in bacterial production was observed. The vertical distribution of bacterial production in the water column of the less productive lake (Crooked Lake) was determined by the vertical distribution of primary production, but no clear relationship between seasonal maxima of primary and bacterial production in this lake was observed. High rates of bacterial production in Crooked Lake during May indicate the importance of allochthonous carbon washed in by spring rains. Bacterial production accounted for 30.6 and 31.8% of total (primary plus bacterial) production in Crooked Lake and Little Crooked Lake, respectively, from April through October. High rates of bacterial production during late September and October were observed in both lakes. Calculation of the fraction of bacterial production supported by phytoplankton excretion implies an important role for other mechanisms of supplying carbon, such as phytoplankton autolysis. Several factors affecting the calculation of bacterial production from the thymidine incorporation rates in these lakes were examined.     

Viral dynamics and patterns of lysogeny in saline Antarctic lakes

Antarctic lakes are extreme ecosystems with microbially dominated food webs, in which viruses may be important in controlling community dynamics. A year long investigation of two Antarctic saline lakes (Ace and Pendant Lakes) revealed high concentrations of virus like particles (VLP) (0.20–1.26 × 10⁸ ml⁻¹), high VLP: bacteria ratios (maximum 70.6) and a seasonal pattern of lysogeny differing from that seen at lower latitudes. Highest rates of lysogeny (up to 32% in Pendant Lake and 71% in Ace Lake) occurred in winter and spring, with low or no lysogeny in summer. Rates of virus production (range 0.176–0.823 × 10⁶ viruses ml⁻¹ h⁻¹) were comparable to lower latitude freshwater lakes. In Ace Lake VLP did not correlate with bacterial cell concentration or bacterial production but correlated positively with primary production, while in Pendant Lake VLP abundance correlated positively with both bacterial cell numbers and bacterial production but not with primary production. In terms of virus and bacterial dynamics the two saline Antarctic lakes studied appear distinct from other aquatic ecosystems investigated so far, in having very high viral to bacterial ratios (VBR) and a very high occurrence of lysogeny in winter.     

Patterns of ecological specialization among microbial populations in the Red Sea and diverse oligotrophic marine environments

Large swaths of the nutrient‐poor surface ocean are dominated numerically by cyanobacteria (Prochlorococcus), cyanobacterial viruses (cyanophage), and alphaproteobacteria (SAR11). How these groups thrive in the diverse physicochemical environments of different oceanic regions remains poorly understood. Comparative metagenomics can reveal adaptive responses linked to ecosystem‐specific selective pressures. The Red Sea is well‐suited for studying adaptation of pelagic‐microbes, with salinities, temperatures, and light levels at the extreme end for the surface ocean, and low nutrient concentrations, yet no metagenomic studies have been done there. The Red Sea (high salinity, high light, low N and P) compares favorably with the Mediterranean Sea (high salinity, low P), Sargasso Sea (low P), and North Pacific Subtropical Gyre (high light, low N). We quantified the relative abundance of genetic functions among Prochlorococcus, cyanophage, and SAR11 from these four regions. Gene frequencies indicate selection for phosphorus acquisition (Mediterranean/Sargasso), DNA repair and high‐light responses (Red Sea/Pacific Prochlorococcus), and osmolyte C1 oxidation (Red Sea/Mediterranean SAR11). The unexpected connection between salinity‐dependent osmolyte production and SAR11 C1 metabolism represents a potentially major coevolutionary adaptation and biogeochemical flux. Among Prochlorococcus and cyanophage, genes enriched in specific environments had ecotype distributions similar to nonenriched genes, suggesting that inter‐ecotype gene transfer is not a major source of environment‐specific adaptation. Clustering of metagenomes using gene frequencies shows similarities in populations (Red Sea with Pacific, Mediterranean with Sargasso) that belie their geographic distances. Taken together, the genetic functions enriched in specific environments indicate competitive strategies for maintaining carrying capacity in the face of physical stressors and low nutrient availability.     

安徽某铁矿不同矿山废水库中微生物群落结构特征

【目的】研究安徽某铁矿不同矿山废水库中微生物群落结构特征及其影响因素。【方法】对比分析了该铁矿3个大型废水库的地球化学特征,并用高通量测序技术研究了水体中微生物群落组成,进而用统计学方法解析了环境因子对微生物群落结构的影响。【结果】3个废水库中有2个为酸性,1个为中性,理化性质有明显的差异。近年形成的塌方采场废水库(TF) pH仅为2.55±0.01,Fe浓度高达154.95±0.78mg/L,SO_4~(2–)浓度为3374.86±3.81mg/L;形成于20世纪70年代的排土场废水库(PT)酸性略弱(pH 2.9±0.02),Fe浓度(34.57±4.00 mg/L)与TF相比明显降低,SO_4~(2–)浓度则高达10398.98±626.70 mg/L;东沙采场废水库(DS)则为中性(pH7.55),但SO_4~(2–)仍高达4162.99mg/L,主要的金属离子为Mg(594.90 mg/L)、Ca (650.10 mg/L)。3个废水库的原核生物多样性随pH的升高而升高。两个酸性废水库的原核生物组成较为接近,但TF的化能自养菌含量较高(69.54%±2.89%),PT的化能异养菌含量较高(64.45%±13.81%)。自养铁氧化菌Ferrovum在TF中的比例高达(64.17±1.84)%,在PT中则下降为(35.39±13.74)%。但PT中含有丰富的化能异养嗜酸菌如Acidicapsa(15.75%±3.99%)、Acidiphilium(10.65%±2.05%)、Acidisphaera (6.34%±1.02%)等。DS中虽然也含有较高的金属离子和SO_4~(2–),但其中的原核生物组成与TF和PT截然不同,主要为Limnohabitans (18.47%)、Rhodobacter (8.42%)等。3个废水库的真核生物群落主要由藻类组成,酸水库TF和PT中主要为棕鞭藻属(Ochromonas)和胶球藻属(Coccomyxa),棕鞭藻属在TF中(53.65%±2.02%)占优势,胶球藻属在PT中(68.84±10.4%)占优势,中性废水库DS中则主要是小环藻属(Cyclotella)(49.85%)。经统计学分析,pH是影响矿山废水微生物多样性和群落组成的主要环境因素。     

Patterns of Fish Growth along a Residential Development Gradient in North Temperate Lakes

Residential development of lakeshores is expected to change a variety of key lake features that include increased nutrient loading, increased invasion rate of nonnative species, increased exploitation rates of fishes by anglers, and alteration of littoral habitats. All of these factors may alter the capacity of lakes to support productive native fish populations. Fourteen north temperate lakes were surveyed to examine how growth rates of two common fish species (bluegill sunfish, Lepomis macrochirus; largemouth bass, Micropterus salmoides) varied along a residential development gradient. Size-specific growth rates for both species were negatively correlated with the degree of lakeshore residential development, although this trend was not statistically significant for largemouth bass. On average, annual growth rates for bluegill sunfish were 2.6 times lower in heavily developed lakes than in undeveloped lakes. This effect of lakeshore development on fish growth was not size specific for bluegills between 60 and 140 mm in total length. An index of population production rate that accounted for both the size-specific growth rate and the size distribution of fishes showed that bluegill populations were approximately 2.3 times less productive in highly developed lakes than in undeveloped lakes. Our results suggest that extensive residential development of lakeshores may reduce the fish production capacity of aquatic ecosystems. Received 29 April 1999; Accepted 26 October 1999.     

The role of environmental parameters in the structure of phytoplankton assemblages and cyanobacteria toxins in two hypereutrophic lakes

We evaluated the variability of cyanotoxins, water chemistry, and cyanobacteria communities in two hypereutrophic drowned river mouth lakes (Spring Lake and Mona Lake; summer 2006) in west Michigan, USA. Even with considerable geographical and watershed similarity, local variations in nutrient concentrations and environmental factors were found to influence the differences observed in cyanobacteria assemblages and cyanotoxins levels between the two lakes. Limnothrix sp. dominated the phytoplankton community in Spring Lake (82% of biovolume) and was negatively correlated with total phosphorus (TP) concentrations. Although Spring Lake was treated with alum during the previous year, Limnothrix sp. was able to bloom in the lower P environment. In contrast, the N₂-fixing cyanobacterium, Anabaena flos-aquae, dominated the phytoplankton in Mona Lake (64% of biovolume). N₂-fixing cyanobacteria dominance in Mona Lake was correlated with higher TP lower dissolved nitrogen levels. Cylindrospermopsis raciborskii was found in both systems; however, the toxin-producing polyketide synthetase gene was not present in either population. The higher TP in Mona Lake appeared to account for the 3-fold increase in cyanobacteria biovolume. Restoration plans for both lakes should include assessments of internal loading and continued phytoplankton monitoring to track the temporal distribution of cyanobacteria species and cyanotoxin concentrations.     

Nutrient growth conditions for Oscillatoria spp. and diatoms in two Norwegian lakes

The growth conditions and development of Asterionella formosa Hass., Tabellaria fenestrata (Lyngb.) Kütz., Fragilaria crotonensis Kitt., and Oscillatoria spp. have been studied in Lake Vansjø and Lake Mjøsa for many years. Some batch culture experiments have been performed with dilute phytoplankton populations in filtered water samples to support the field data. The results indicate that the concentrations of phosphate, nitrate, silicate or chelators/Fe were important factors regulating the spatial and temporal distribution of the populations. Based on the assumption that the growth rates were strongly dependent on the external nutrient concentration, S, the growth conditions were divided into three types. The nutrient concentration may be too low to support growth, i.e. the growth rate k ≤ 0, or the population is nutrient growth limited (0 < k < km) or not growth limited, i.e. the population has maximum growth rate (km). The nutrient concentration may be population density dependent, i.e. resource competition may occur, or density independent separately or in combination. The growth strategies of the populations in the two lakes were partly explained by studying the nutrient growth conditions of the populations.