Extracellular phosphatases in a Mediterranean reservoir: seasonal, spatial and kinetic heterogeneity

1. Epilimnetic alkaline phosphatase activity (APA) was measured in longitudinal profiles of the canyon‐shaped, eutrophic Sau Reservoir (Catalonia, Spain) during the autumn, winter and spring periods of 1997–2000. 2. The spatial pattern of APA depended on lake circulation. During periods of stable stratification, when the ratio of mixed to euphotic depth (z_mix/z_eu < 1.7) was low and the Chlorophyll a (Chl a) concentration high, APA was also comparatively high (0.5–3.4 μmol L^?1 h^?1) and located mostly in the >2‐μm size‐fraction. APA increased towards the dam at the same time as the concentration of soluble reactive phosphorus (SRP) decreased. In periods of unstable stratification, deep mixing (z_mix/z_eu > 2.4) and low Chl a concentration, APA was low (<0.1 μmol L^?1 h^?1) and without longitudinal changes, consistent with a high and stable SRP concentration. 3. A high input of mostly dissolved (in the <0.2‐μm size‐fraction) phosphatases from the river Ter was found in 1997–98. At the river inflow, independently of season and despite a continuously high SRP concentration, APA was approximately 0.7 μmol L^?1 h^?1 and decreased towards the dam within the inflowing, canyon‐like part of the reservoir. 4. Analysis of saturation kinetics revealed the kinetic heterogeneity of APA. Low‐affinity APA was localised in the >2‐μm (algal) size‐fraction while, in the <2‐μm (picoplankton, mainly bacteria + dissolved) size‐fraction, high‐affinity APA, or a mixture of both, was found. The presence of two kinetic components, whose substrate affinities (i.e. in Michaelis constants, K_m) differed, was confirmed statistically in 13 of 18 cases analysed. The range of K_mH values of the high‐affinity component was 0.15–2.4 μmol L^?1, K_mL values of the low‐affinity component ranged from 18 to 275 μmol L^?1.     

ALKALINE PHOSPHATASE IN FRESHWATER CLADOPHORA‐EPIPHYTE ASSEMBLAGES: REGULATION IN RESPONSE TO PHOSPHORUS SUPPLY AND LOCALIZATION1

Extracellular alkaline phosphatase enzyme activity (APA) is important for algal phosphorus (P) acquisition in P‐limited freshwater ecosystems and is often used as an indicator of P deficiency. APA allows access to organic P (monophosphate esters), but the regulation of APA in response to availability of both PO₄³⁻ and organic P is poorly characterized. This study aimed to examine the regulation of APA in freshwater Cladophora‐epiphyte assemblages in response to PO₄³⁻ and a hydrolyzable organic P source, and for the first time to apply enzyme linked fluorescence (ELF) to localize APA within freshwater macroalgal‐epiphyte assemblages. In response to elevated PO₄³⁻ concentrations, a component of net APA was suppressed, but there was also a constitutive APA, which was maintained even after prolonged exposure to nearly 1,000 μM PO₄³⁻ and saturation of internal P pools. When supplied with organic glycerol P as the sole P source, the algae maintained APA in excess of needs for supplying PO₄³⁻ for uptake, resulting in PO₄³⁻ release into the medium. Constitutive APA may be adaptive to growth under chronic P limitation in oligotrophic freshwater habitats. Excess APA and release of PO₄³⁻ could benefit different algal and bacterial partners within assemblages. APA in both Cladophora sp. and epiphytic algae was localized with ELF only when ethanol fixation was omitted. In algal subsamples exposed to different P treatments, there was no correlation between bulk APA (using 4‐methylumbelliferyl phosphate [MUP] substrate) and % cell labeling with ELF, suggesting that ELF labeling of APA was at best semiquantitative in the algal assemblages.     

Kinetics of alkaline phosphatase activity and phosphorus availability for phytoplankton and bacterioplankton in lake plu\see (North German Eutrophic Lake)

Annual studies of kinetics of alkaline phosphatase (APA) activity and phosphorus availability for microplankton in the photic zone of an eutrophic lake are reported. The total APA activity of microplankton varied strongly. V_max was highest during summer P depletion, and in autumn and winter total APA activity was low. The total APA specific activity of the microplankton was also highest (average 3.55 pmole PO₄ ^3– ng ATP^–1 min^–1) when ambient orthophosphate concentrations were very low. Both V_max and specific APA activity were not dependent on the biomass of microplankton; they were strongly affected by P available for microplankton. A differential filtration technique was used for separation of microplankton into two size classes, i.e., algal, larger than 3m, and bacterial fraction with size 0.2–3.0m. The algal size fraction had lower specific APA activity (average 1.224 pmole PO₄ ^3– ng ATP^–1 min^–1) and higher K_M values (38.8mole × liter^–1) than microorganisms which were smaller than 3m (2.011 pmole PO₄ ^3– ng ATP^–1 min^–1 and 25.4mole liter^–1, respectively). The K_M values of free, dissolved APA (36.8mole liter^–1) indicated that free APA was probably released by algae. Phytoplankton were major APA activity producers in the photic zone of the lake from March to November, and their activity constituted, on the average, 48.6% of the total APA activity in the water. Bacteria were the dominant APA activity producers in winter (41.3–44.9%); however, during other periods they contributed significantly (average 21.7%) to total APA activity. When surplus constituted less than 10% of particulate P in seston, phytoplankton produced high specific APA activity, and when surplus P was higher than 15%, the specific APA activity of phytoplankton size fraction rapidly decreased. APA of the bacterial size fraction of the seston was not affected by P concentrations. Orthophosphate was a competitive inhibitor of APA produced by microorganisms of the size fraction larger than 3.0m, and increasing concentrations of inorganic phosphate caused an increase in K_M values. The hypothetical metabolic-coupling between phytoplankton and bacterioplankton in the phosphorus cycle in conjunction with carbon metabolism in the lake is discussed.     

The collapse of benthic macroalgal blooms in response to self‐shading

1. Our goal was to use physiological indicators [photosynthesis–irradiance (P–I) response, nutrient status], population level feedbacks (self‐shading) and ambient environmental conditions (dissolved nutrients, light, temperature) to improve our understanding of the seasonal and spatial population dynamics of Cladophora. 2. Cladophora grew in three distinct phases, rapid growth early in the season (May–July), a mid‐season population collapse (July–August) and autumn re‐growth. Across all sites and dates, mean net maximal photosynthesis [P_M (NET)] was 6.9 ± 3.9 mg O₂ g DM^?1 h^?1, and α was 0.055 ± 0.025 mg O₂ g DM^?1 μm photons^?1 m^?2. Mean values for critical irradiance (I_CR) and the half‐saturation light intensity (I_K), were 42.9 ± 32.1 and 189.3 ± 123.8 μm photons^?1 m^?2 s^?1 respectively. 3. At most sites growth was phosphorus‐limited. Values of α were significantly higher at a site influenced by a nutrient enriched river plume, where algal growth was phosphorus‐sufficient. 4. Photoinhibition was not apparent in any of our P–I experiments. Even if photoinhibition had been apparent during in vitro P–I experiments, population level photosynthetic rates in the field would be little affected because intense self‐shading restricts inhibiting irradiances to the upper few mm–cm of the algal canopy. 5. Our physiological (P–I response) experiments contradicted previous assertions that high ambient temperatures, or nutrient deficiency, were primary causes of mid‐summer sloughing. In our study, sloughing occurred simultaneously at nutrient enriched and nutrient deficient sites, at temperatures well below critical values found during in vitro experiments, and our indicator of physiological condition (P–I response) remained unchanged leading up to, or immediately after, the sloughing event. 6. Self‐shading can reduce the convexity of the P–I response within in vitro incubations, even when the amount of algal material is low. Our experiments used 0.08 g DM of algal material that formed clumps c. 1 cm thick. Under these conditions, we estimated negligible (<1%) effects on P_M, a 12% reduction in apparent values of α, and 14% and 17% increases in values of the α‐dependent terms I_CR and I_K, respectively. 7. Our results are consistent with the hypothesis that a population‐level negative feedback (self‐shading) is responsible for sloughing in dense macroalgal beds. Sloughing was probably inevitable once macroalgal bed density and thickness surpassed a critical threshold. Cells towards the base of the bed received insufficient light to maintain metabolic balance, began to decay and weaken, and became increasingly susceptible to physical detachment from shear stress.     

Phytoplankton and bacterial alkaline phosphatase activities in relation to phosphate and DOP availability within the Gironde plume waters (Bay of Biscay)

Previous studies conducted on the continental shelf in the Southeast Bay of Biscay influenced by Gironde waters (one of the two largest rivers on the French Atlantic coast) showed the occurrence of late winter phytoplankton blooms and phosphorus limitation of algal growth thereafter. In this context, the importance of dissolved organic phosphorus (DOP) for both algae and bacteria was investigated in 1998 and 1999 in terms of stocks and fluxes. Within the mixed layer, although phosphate decreased until exhaustion from winter to spring, DOP remained high and phosphate monoesters made up between 11 to 65% of this pool. Total alkaline phosphatase activity (APA, V_max) rose gradually from winter (2-8 nM h⁻¹) to late spring (100-400 nM h⁻¹), which was mainly due to an increase in specific phytoplankton (from 0.02 to 3.0 nmol μgC⁻¹ h⁻¹) and bacterial APA (from 0.04 to 4.0 nmol μgC⁻¹ h⁻¹), a strategy to compensate for the lack of phosphate. At each season, both communities had equal competitive abilities to exploit DOP but, taking into account biomass, the phytoplankton community activity always dominated (57-63% of total APA) that of bacterial community (9-11%). The dissolved APA represented a significant contribution. In situ regulation of phytoplanktonic APA by phosphate (induction or inversely repression of enzyme synthesis) was confirmed by simultaneously conducted phosphate-enrichment bioassays. Such changes recorded at a time scale of a few days could partly explain the seasonal response of phytoplankton communities to phosphate depletion.     

The effects of reduced phosphorus and nitrogen loading on phytoplankton in Mondsee,Austria

Following restoration measures (ring canalization, treatment plant with phosphorus precipitation), phosphorus loading declined step-wise from 26.21 t year^–1 to 9.18 t year^–1 during the period 1979–1984 while P-retention increased from 48% to 78%. Phosphorus loading was poorely correlated with precipitation. Inorganic nitrogen load, largely NO₃– N, did not decline but was significantly correlated with precipitation (r = 0.95) throughout the investigation period (1978–1989).Total phosphorus loading reached acceptable levels in 1984 when compared to critical loading calculated according to Vollenweider (1976). Phosphorus input to the lake has remained at these levels in recent years.Average annual chlorophyll-a concentrations and biovolume of phytoplankton in the top 20 m layer of the lake decreased, in correspondence with the respective phosphorus concentrations, from eutrophic to mesotrophic levels. The decline was accompanied by a drastic reduction of blue-green algal populations, and especially of Oscillatoria rubescens D.C..     

Heterotrophic bacterial activity and primary production in a hypertrophic African lake

The relationship between heterotrophic bacteria and phytoplankton in the epilimnion (0–10 m) of hypertrophic Hartbeespoort Dam, South Africa, was examined by statistically analyzing three years of parallel measurements of heterotrophic bacterial activity (glucose uptake) and phytoplankton particulate and dissolved organic carbon production. Algal biomass ranged between 4.0 and 921.1 mg Chl a m^-3 at the surface. Primary production varied between 69.5 and 3010.0 mg C m^-2h^-1 while algal production of dissolved organic carbon (EDOC) ranged from 2.5 to 219.2 mg C m^-2h^-1. Bacterial numbers reached a summer peak of 44.23 × 10⁶ cells ml^-1 in the first year and showed no depth variation. The maximum rate of glucose uptake, V_max, reached a peak of 5.52 g C l^-1h^-1. V_max, maximum glucose concentration (K_t + S_n) and glucose turnover time (T_t) were usually highest at the surface and decreased with depth concomitant with algal production. At the surface, V_max was correlated to EDOC (r = 0.59, n = 67, p < 0.001) and primary production (r = 0.71, n = 70, p < 0.001). At 5 and 10 m, V_max was correlated to integral euphotic zone (~ 4 m) algal production and bacterial numbers. Glucose turnover time was inversely related to integral algal production (r = -0.72, n = 70, p < 0.001) and less strongly to bacterial numbers. The data indicated that although bacterial numbers and biomass were low relative to algal biomass in this hypertrophic lake, the heterotrophic bacteria attained high rates of metabolic activity as a result of enhanced algal production of available organic carbon.     

The role of Gloeotrichia echinulata in the transfer of phosphorus from sediments to water in Lake Erken

The abundance of Gloeotrichia echinulata colonies in the sediments of Lake Erken and their phosphorus content were investigated to determine the contribution of Gloeotrichia colonies to total sediment phosphorus. Moreover, the potential size of the algal inoculum and the migration to the water during summer were estimated.The surplus phosphorus content of the resting colonies in the sediment was about 45% of total phosphorus, which maximized at 8.5 µg P (mg dw)^–1 or 81 ng P colony^–1. The C:P ratio (by weight) in the early colonies appearing in the lake water was 50:1, while the ratio stabilized at 150 during the major migration period. The internal supply of surplus phosphorus was used during the pelagic growth of the colonies.The internal phosphorus loading to the epilimnion of Lake Erken due to Gloeotrichia migration could, from the measurements of the increase in particulate epilimnetic phosphorus, be estimated at 40 mg P m ^–2 or 2.5 mg P m^–2 d^–1 in late July and early August. Determination of the number of colonies in the sediment before and during the migration verified this value to be a conservative estimate of the internal phosphorus loading due to Gloeotrichia migration to the epilimnion in Lake Erken.The sediment P content calculated from the P concentration in early epilimnion colonies resulted in a value of 35 µg P (g dw)^–1 as a maximum. This corresponds to only 3% of the total phosphorus content in Lake Erken sediment.     

氮磷比对有机磷环境中微藻砷代谢的生态风险效应北大核心

【目的】探究了以单酯磷D-葡萄糖-6-磷酸二钠(D-glucose-6-disodium phosphate,GP)为唯一磷源时含砷(As5+)水体中不同氮磷质量比对铜绿微囊藻(Microcystis aeruginosa)生长及砷代谢和微囊藻毒素(microcystins,MCs)释出的影响。【方法】将氮磷饥饿状态的藻细胞于含砷水体中不同氮磷比条件下进行实验,通过测定藻细胞密度(OD680)、叶绿素a(chlorophyll a,Chla)、实际光合产率(Yield)、超氧化物歧化酶(superoxide dismutase,SOD)、砷的存在形态以及微囊藻毒素含量,分析该藻在砷胁迫下的生理响应以及砷代谢机制。【结果】氮磷饥饿状态藻细胞对较高GP水平(0.1 mg/L)下的低氮磷比有更好的适应性,较低GP水平(0.02 mg/L)下的高氮磷比能显著促进培养初期藻细胞的OD680、Chla和Yield;SOD在培养初期与末期受氮磷比影响显著。GP环境下铜绿微囊藻经8 d培养后氮磷比为10:0.1介质中的砷表现为以亚砷酸盐(As^(3+))为主,占水体总砷(total arsenic,TAs)含量的78.8%,其余氮磷比环境中仍以As^(5+)为主,藻体砷形态则均以As^(5+)为主,氮磷比为1:0.1时有机砷占藻体TAs比例最高。藻细胞砷代谢受GP水平影响显著,较高GP环境(0.1 mg/L)下砷的代谢总量也更高,氮磷比为10:0.1时砷代谢以As^(5+)的还原和As^(3+)释出为主,低GP环境下(0.02 mg/L)砷代谢的甲基化水平提高。介质中MCs的含量与GP水平有关,较高GP的低氮磷比水体中MCs含量最低。【结论】研究结果对全面了解有机磷源含砷水体中藻华暴发及砷生态风险的科学管控具有重要意义。     

Effect of algal growth on the availability of phosphorus,iron and manganese in rice soils

Summary Laboratory experiments were conducted to study the effect of algal growth on the change of (I) pH, (II) available phosphorus and (III) solubility of iron and manganese content in five waterlogged alluvial rice soils of West Bengal, India. The results showed that the algal growth initially caused an increase in the soil pH, which later declined to the original value in some of the soils. The available phosphorus content decreased upto 90 days of their growth and began to increase towards the later period of incubation. The drastic fall of water soluble plus exchaneable manganese content of the soils due to algal growth was accompanied by similar increase in reducible manganese content. No appreciable change in water soluble plus exchangeable ferrous iron content was encountered but theN-NH₄OAC(pH 3) extractable iron due to algal growth progressively decreased with the progress of the incubation period.     

Alteration of phytoplankton phosphorus status during enrichment experiments: implications for interpreting nutrient enrichment bioassay results

We compared the results of phosphorus-enrichment bioassay experiments with alkaline phosphatase activity (APA) assays as indicators of phosphorus (P) limitation of in situ phytoplankton growth. In 4-d experiments, phytoplankton APA decreased or remained unchanged in P-enriched samples, but increased in unenriched samples, indicating a rapid alteration of the P status of the unenriched algae during the experimental incubations. In direct comparisons of enrichment bioassays and APA assays of reservoir phytoplankton samples, the results of the two methods corresponded in general, although contradictory results were not uncommon. Our data support the conclusion that enrichment experiments can indicate the potential for nutrient limitation of algal growth in the absence of other limiting factors, but do not necessarily demonstrate the occurrence of in situ nutrient limitation of phytoplankton production.     

Phosphorus transformations in the epilimnion of humic lakes: biological uptake of phosphate

SUMMARY. 1. The hypothesis that dissolved humic material (DHM) stimulates bacterial involvement in phosphorus transformations and may thus lead to decreased accessibility of phosphorus to algae was investigated by studying three small forest lakes in southern Finland representing a wide range of concentrations of DHM. 2. Other chemical differences between the three lakes were slight, although the most humic lake exhibited higher concentrations of total phosphorus and of molybdate-reactive phosphorus. Bacterial biomass did not differ significantly between the lakes, but algal biomass was significantly lower at higher DHM concentrations. Consequently the ratio of algal biomass to bacterial biomass was significantly lower in the most humic lake. 3. Uptake of phosphorus from added ³³PO₄ was partitioned between algal and bacterial size fractions by differential filtration. No significant variation between lakes was found in the proportion of particulate ³³P recovered from the algal fraction. 4. Turnover times for phosphate were significantly longer in the most humic lake and also showed lower variability. In general turnover times were long in comparison with values reported from many other lakes. Only briefly in mid summmer did turnover times in two of the lakes shorten to values which would indicate that demand for phosphate was outstripping supply. 5. Short-term storage of samples from the most humic lake stimulated biological incorporation of ³³P, but additions of nitrogen and iron had little effect on phosphate uptake. 6. In these small forest lakes it is probable that no single nutrient consistently limits plankton development. Since no evidence was found that DHM shifts the balance of plankton phosphate uptake away from algae towards bacteria, the influence of DHM on phosphorus transformations may rather be through chemical regulation of free phosphate availability.     

An experimental demonstration of trophic interactions affecting water quality of Rice Lake, Ontario (Canada)

Eight cylindrical enclosures (3 m diameter, 2.7 m long, V = 20m³) were installed in eutrophic Rice Lake (Ontario, Canada) in late spring of 1987. Fish (yearling yellow perch (Perca flavescens) and macrophytes (Potamogeton crispus) presence and absence were set at the beginning of the experiment to yield four combinations of duplicate treatments. The purpose of the experiment was to determine if the phytoplankton, zooplankton, macrophytes and fish species resident in the lake interact to influence water quality (major ions, phosphorus, algal densities and water clarity).The presence of fish was associated with: (1) decreased biomass of total zooplankton, (2) decreased number of species in the zooplankton, (3) decreased average size of several zooplankton taxa, (4) higher total phosphorus concentrations, (5) higher phytoplankton and chlorophyll a concentrations, (6) lower water clarity, (7) lower potassium levels during macrophyte die-back, (8) lower pH and higher conductivity in the presence of macrophytes. Biomass of large Daphnia species (but not total zooplankton) was highly correlated with the algal response (r ² = 0.995) and was associated with reduced biomass of several algal taxa including some large forms (Mougeotia, Oedogonium) and several colonial blue-green algae. However, no significant control of late summer growth of the bloom-forming blue-green alga Anabaena planctonica Brun. was achieved by the Daphnia presence-fish absence treatment. Release of phosphorus to the water column during the die-back of P. crispus was not an important phenomenon.     

Comparison of phosphorus deficiency indices during a spring phytoplankton bloom in a eutrophic reservoir

1. Phosphorus limitation was studied along the eutrophic, canyon-type ?ímov reservoir (Czech Republic) during a spring phytoplankton bloom. Concentration of soluble reactive phosphorus (SRP), C:P molar ratio in seston, extracellular alkaline phosphatase activity (APA), and P limitation (bioassay) were used as indices for phosphorus deficiency in the phytoplankton. 2. SRP, C:P, APA, and P limitation indicated a moderate P deficiency in the downstream, but not upper, part of the reservoir. 3. Significant correlations between these parameters were found in the downstream part. Chlorophyll a concentration correlated with APA and P limitation in the upper part. 4. APA was significantly enhanced in the phosphorus-deficient phytoplankton. However, APA was apparently not related to total biomass or species composition of the phytoplankton. 5. Generally, APA was closely correlated with pH in the reservoir. However, extracellular alkaline phosphatases, with a pH optimum above 9.0, were induced and active only during the phytoplankton bloom, whereas low background activity of extracellular phosphatases was found at low chlorophyll a concentrations (winter, clear-water phase).     

Observations on carbon dioxide fixation in the Nant-y-moch reservoir,Ceredigion, Wales

High rates of carbon dioxide fixation, both in the light and the dark have been observed in Nant-y-Moch Mountain reservoir; these are surprising because of the sparse algal population. The values of CO₂-carbon fixed in the light per milligramme of Chlorophyll a per hour, ranged from 0.1167 mgC/ mg Chl a/hr. to infinity and are much higher than any recorded previously. The uptake of ¹⁴C-sucrose and ¹⁴C glucose was also determined. Factors which might possibly account for these observations, either alone or in combination are discussed. Possible CO₂ fixing agencies include detritus, flagellates and other organisms capable of obtaining energy by dark reactions (chemotrophs).     

Undesirable side-effects of water hyacinth control in a shallow tropical reservoir

1. Based on a comprehensive data set collected monthly during 8 years (1997–2004), we evaluated the effects of mechanical removal of Eichhornia crassipes on the limnological characteristics and algal biomass of a polymictic shallow tropical reservoir. 2. Interrupted time series analyses indicated that the limnological responses to macrophyte removal can be classified as an ‘abrupt permanent impact’ implying that the overall mean of the time‐series shifted promptly after intervention. These analyses indicated a significant increase for pH, total phosphorus, total phytoplankton and cyanobacterial biomass, and a decrease in water transparency and CO₂ concentrations in the surface water; also, the increase in water stability, increase of bottom soluble reactive phosphorus (SRP) and decrease in bottom oxygen levels. 3. Cyclic anoxic periods previously observed during springs and summers were replaced by a persistent period of anoxic conditions in the sediment overlying water. Anoxic conditions were suitable for SRP release from sediments. Heavy cyanobacterial blooms became more persistent, maximum biomass (4229 mm³ L⁻¹) was 30 times larger, the blooms frequently reached 2 m and sometimes the bottom of the reservoir, contrasting to the preremoval period in which it reached at most 1 m deep. 4. The long‐term P dynamics in the system, initially driven by allochthonous nutrient loadings were replaced by internal ecological processes. Water hyacinth removal markedly accelerated the process of eutrophication due to internal feedback mechanisms, leading to a switch to a more turbid state. Biological feedback mechanisms were driven by cyanobacterial blooms by enhancing water stability, oxygen anoxia at the bottom and by increasing suitable conditions for P internal loading. These data support the hypothesis of the role of cyanobacterial blooms as an important factor impairing water quality and driving the ecosystem towards a stable degraded state. 5. These findings have important implications for the restoration of shallow stratifying eutrophic lakes, as the alternative degraded state is most likely to occur when compared with their non‐stratifying counterparts. Moreover, feedback mechanisms in tropical and subtropical shallow lakes seem to be stronger than in temperate ones, as stratification events are more likely to occur over the year, intensifying system resilience to restorative strategies.     

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.     

Temperature shift-induced changes in the antioxidant enzyme system of cyanobacteriumSynechocystis PCC 6803

The 24 h effect of low (20°C) and high (43°C) temperature on the antioxidant enzyme activities and lipid peroxidation was investigated in intact cells of the cyanobacteriumSynechocystis PCC 6803 grown at 36°C. At low temperature treated cells, the superoxide dismutase, catalase and glutathione peroxidase activities were significantly higher and the protein content lower than in high temperature treated cells. The increase of hydroxyl free radical level and malonyldialdehyde formation, when algal cells were exposed to low temperature, were due to the stimulated production of superoxide radicals O₂ ⁻ and hydrogen peroxide (H₂O₂).     

Short-term variation in extracellular phosphatase activity: possible limitations for diagnosis of nutrient status in particular algal populations

Species-specific extracellular phosphatase activity (PA) was studied in natural phytoplankton with an emphasis on short-term variation in PA and its consequences for the diagnosis of inorganic phosphorus (P) deficiency. Extracellular PA was measured over 3 days at 4-h intervals to compare variations in algal PA in two distinct aquatic environments: in the Římov reservoir and acidified Plešné Lake. PA was directly detected in an epifluorescence microscope using fluorescently labelled enzyme activity; the fluorescence signal was quantified using image analysis. High short-term variation was found both in the total PA of the samples and specific PA of Ankyra ancora (G. M. Smith) Fott, Monoraphidium dybowski (Wolosz.) Hind. and Kom.-Legn., and Chlorogonium fusiforme Matwienko. The specific PA of these species varied between zero and 157 fmol cell⁻¹ h⁻¹. The PA of Chlorogonium fusiforme showed a significant relationship with the time of day. During the study period, populations of the certain species were present that differed in physiological status and activity due to diurnal or random changes caused by mixing in the reservoir or lake, or both. Therefore, species-specific extracellular PA should be interpreted carefully, as an indicator of P deficiency in freshwater phytoplankton if based on limited number of samplings.     

Controls on chlorophyll-a in nutrient-rich agricultural streams in Illinois, USA

Nitrogen and phosphorus are the primary nutrients that affect water quality in streams in the midwestern USA and high concentrations of these nutrients tend to increase algal biomass. However, how nutrients interact with physical controls in regulating algal biomass is not well known in agricultural streams. Eighteen streams in east-central Illinois (USA) were sampled during June and September 2003 to analyze factors possibly regulating algal biomass. Additionally, two shaded and two non-shaded sites in the Embarras River in east-central Illinois were sampled intensively from June to December 2003. Both sestonic and periphytic chlorophyll-a (chl-a) were analyzed, and periphytic chl-a was assessed on natural substrata and unglazed ceramic tiles. Although high concentrations of nutrients were found in these streams (mean total P = 0.09–0.122 mg l⁻¹ and mean NO₃-N=4.4–8.4 mg l⁻¹), concentrations of sestonic chl-a were low among all sites and both sampling periods (<18 mg m⁻³, median values of 5 and 3 in June and September, respectively). Filamentous algae were an important component of the algal communities in streams with stable substrata. Periphytic chl-a was generally not related to the concentration of N or P in the water column, and in non-shaded streams periphyton appeared at times to be light-limited due to turbid water. Turbidity was found to be an important factor controlling chl-a on ceramic tiles across the 18 sites and for the Embarras River sites; chl-a decreased exponentially in concentration (132–0 mg m⁻²) as turbidity increased from 4 to 39 NTU (r ² = 0.80). In general, the interaction between hydrology and light (turbidity) likely controlled algal biomass in these nutrient-rich, agricultural streams.