Can physicochemical factors predict lipid content in phytoplankton?

1. We hypothesized that a large collection of reasonably standardized data for natural algal assemblages would reveal the influence of environmental factors on the fraction of recently produced photosynthate allocated to lipid.
2. Our analysis of photosynthate allocation in fresh- and saltwater systems showed that allocation of carbon to lipid was not well correlated with any one environmental factor.
3. Allocation to lipid increases with temperature up to 12 °C (lipid allocation = 11.7 + 1.70*temperature (°C), n = 48, r ² = 0.50, P < 0.001) but not above.
4. The relationship between temperature and lipid allocation appears to be caused by a unique convergence of nutrient limitation (nitrate or silicate) in the presence of facultative lipid-producing algae (diatoms or other chrysophytes) which occurs at or below 12 °C during stratification of the water column.     

Effects of enhanced ultraviolet-B radiation on the production of lipid, polysaccharide and protein in three freshwater algal species

1. The effects of ultraviolet-B (UVB-enhanced) radiation on the production of photosynthates (lipid, protein, polysaccharide and low molecular weight compounds) was examined for three species of algae. Cryptomonas sp., Nitzschia palea and Synechococcus elongatus were selected as representatives of the Cryptophyceae, Bacilliarophyceae and Cyanobacteria, respectively.
2. Laboratory experiments were performed at several UVB_weighted dose rates ranging from 0.018 to 0.391 mW cm^–2. These dose rates span the range of dose rates used in other studies.
3. Effects on the overall photosynthetic rate were observed, even at relatively low UVB_weighted dose rates (0.047 mW cm^–2).
4. The effect of UVB radiation on the fixation of carbon into the main macromolecular pools differed, depending not only on the dosage but on the species examined. However, the observed inhibitory effects were generally non-stochastic. In addition, within each species there were differences in the apparent sensitivity of the various fractions to inhibition by UVB radiation.
5. These results suggest that exposure to UVB radiation has the potential to alter the relative allocation of recently fixed carbon to lipid, protein, polysaccharide and low molecular weight compounds in algae in a species-specific manner.     

Resting eggs of lake- Daphnia I. Distribution, abundance and hatching of eggs collected from various depths in lake sediments

SUMMARY. 1. Resting eggs (ephippia) of Daphnia galeata, D. hyalina, D. cucullata and D. galeata X D. hyalina hybrids were collected from the upper 8 cm of sediments at various water depths from two glacial lakes in the Northern Federal Republic of Germany.
2. The horizontal distribution of 'intact' ephippia (i.e. those containing two apparently healthy eggs) was extremely patchy, with mean densities in the upper 8 cm ranging from 1325 to 113,907 m⁻² (coefficient of variation, C=65.5%; Schöhsee, Holstein) and 25,343–159,143 m⁻² (C=58.5%; Kel-lersee. Holstein). Ephippial densities were greatest in sediments from deep waters. The highest densities of intact ephippia were in the upper 4 cm of sediment, with progressively fewer with increasing depth.
3. A proportion of eggs obtained from sediments during autumn (September-November 1985) and spring (March-May 1986) were exposed to continuous light (approximately 18 W m⁻²) at three temperatures (6, 12, 20°C), and the incidence of hatches recorded.
4. Hatching success was consistently low (maximum 14.4%), with most hatching occurring at 12°C. There was no significant difference between the hatching success of ephippia collected during autumn and spring. Decapsulation of ephippial eggs inhibited hatching. Hatching was highly synchronous within each treatment, and the pattern did not differ significantly with sediment depth, though hatching success was lower in deep sediments. A huge reservoir of ephippia is present in lake sediments, of which a proportion may remain viable for many years. Periodic recruitment of hatchlings to the pelagic population may provide an important mechanism for the maintenance of genetic diversity in Daphnia populations, as well as affecting rates of evolutionary change.     

Effects of long-term exposure to elevated CO2 and increased nutrient supply on bracken (Pteridium aquilinum)

1. Bracken ( Pteridium aquilinum ) is an important fern with a global distribution. Little is known of the response of this species to elevated CO₂. We investigated the effects of high CO₂ (570 compared with 370 μmol mol^–1) with and without an increased nutrient supply (a combined N, P, K application) on the growth and physiology of bracken, growing in containers in controlled-environment glasshouses, over two full growing seasons. Results of growth and physiology determinations are reported for the second season.
2. Elevated CO₂ had little impact on the growth or allocation of dry mass in bracken. No significant changes were detected in dry mass of the total plant or any of the organs: rhizomes, roots and fronds. In contrast to the small effects of high CO₂, the high nutrient treatment caused a three-fold stimulation of total plant dry mass and an increase in the allocation of dry mass to above ground when compared with low nutrient controls.
3. Net photosynthetic rates in saturating light were increased by both high CO₂ and nutrient treatments, particularly in spring months (May and June). Growth in elevated CO₂ did not cause a down-regulation in light-saturated rates of photosynthesis. The increased carbon gain in the high CO₂ treatments was accompanied, in the low-nutrient plants, by higher concentrations of carbohydrates. However, in high-nutrient plants the CO₂ treatment did not cause an accumulation of carbohydrates. The absence of a growth response to elevated CO₂ in bracken despite significant increases in photosynthesis requires further investigation.     

Variation in the C:P ratio of suspended and settling seston and its significance for P uptake calculations

SUMMARY. 1. In 1981–84 limnocorral (LC) experiments were performed in Lake Lucerne. Switzerland, to manipulate the planktonic community by varying P fertilization and by removing large zooplankton (with a 95 μm screen).
2. The C:P ratios in both suspended and entrapped seston exceeded the 'ideal' C:P ratio of 106 proposed by Redfield, Ketchum & Richards (1963) when P was limiting algal growth.
3. P fertilization could decrease the sestonic C:P ratio to 106 only when P did not limit algal growth; P additions far exceeding the P loading of eutrophic lakes were necessary to obtain this situation.
4. Changes in epilimnetic C:P ratios were usually related to short- term changes in primary production, caused by variable in situ light conditions and turbulence, and subsequent variation in POC concentrations.
5. Entrapped seston in the 95 μm-filtered LCs showed C:P ratios slightly higher than those of suspended seston, indicating fast P release and slower C mineralization in the epilimnetic nutrient cycle.
6. Removing large crustacean zooplankton enhanced epilimnetic P mineralization, and C:P ratios of entrapped seston in the 95 μm-filtered LCs were increased.
7 Detritus formed a relatively high proportion of the seston and amounted to more than two-thirds of the measured POC concentration.
8. Calculations of algal P uptake using information on sestonic C:P ratios and ¹⁴C uptake rates are questionable, as long as detritus cannot be separated from algae and net carbon uptake cannot be accurately measured.     

Carbon source for reproduction in a spring ephemeral herb, Corydalis ambigua (Papaveraceae)

1.  The carbon source for reproduction in plants may differ between flowering and fruiting stages. To clarify how spring ephemerals use current photosynthetic products for reproduction, the allocation patterns of photosynthate at flowering and fruiting and the effects of resource limitation on reproductive performance in Corydalis ambigua were assessed.
2.  A ¹³C tracing experiment revealed that about 20% of the current photosynthetic carbon was used for reproduction at both flowering and fruiting. The proportion of ¹³C allocated to fruits was constant irrespective of the light level. In contrast, ¹³C translocation to tubers increased at fruiting, and this trend was accelerated when plants were shaded.
3.  Defoliation treatment significantly reduced nectar production and tuber mass, while seed production was not affected. Therefore, when carbon assimilation was limited, carbon was preferentially allocated to current reproduction (seeds) rather than to pollinator attraction (nectar) or storage (tuber).
4.  If seed production is partly supported by carbohydrate reserved in the old tissue of tubers, nectar and seed production may not compete strongly for carbon sources. In contrast to the ability of high seed production, the susceptibility of nectar production to current photosynthesis indicates that seed production of this species is basically limited by pollen capture.
5.  Therefore, temporal separation of resource pool for reproduction may mitigate the joint limitation of seed production between pollinator attraction and resource availability. Temporal variation of the sink–source balance of storage organ is crucial to understand the cost of reproduction in perennial plants.     

The balance between photosynthesis and grazing in Antarctic mixotrophic cryptophytes during summer

SUMMARY 1. Grazing and photosynthetic contributions to the carbon balance of planktonic, mixotrophic cryptophytes in Lakes Fryxell and Hoare in the Taylor Valley, Antarctica were measured during November and December 2000.
2. The cryptophytes never became entirely photosynthetic, although carbon derived from grazing decreased in December. Individual grazing rates ranged between 5.28 and 10.08 bacteria cell⁻¹ day⁻¹ in Lake Fryxell and 0.36–11.76 bacteria cell⁻¹ day⁻¹ in Lake Hoare. Grazing rates varied temporally and with depth in the water column. In Lake Fryxell, which is a meromictic lake, highest grazing occurred just above the chemocline. Individual photosynthetic rates ranged from 0.23 to 1.35 pg C cell⁻¹ h⁻¹ in Lake Fryxell and 0.074 to 1.08 pg C cell⁻¹ h⁻¹ in Lake Hoare.
3. Carbon acquisition by the cryptophyte community gained through grazing ranged between 8 and 31% during November in Lake Fryxell, dropping to between 2 and 24% in December. In Lake Hoare grazing contributed 12–21% of the community carbon budget in November and 1–28% in December. Around 4% of the carbon acquired from grazing and photosynthesis was remineralised through respiration.
4. Mixotrophy is probably a major survival strategy for cryptophytes in the extreme lakes of the Dry Valleys, because perennial ice-cover severely limits light penetration to the water column, whereas these phytoflagellates are not normally mixotrophic in lower latitude lakes. The evidence suggests that mixotrophy may be a mechanism for supplementing the carbon budget, as well as a means of acquiring nutrients for growth.     

Vertical partitioning of the phytoplankton assemblage in ultraoligotrophic Crater Lake, Oregon, U.S.A.

SUMMARY 1. The summertime phytoplankton assemblage in abysmally deep (Z_max: 589 m) Crater Lake, Oregon, consists of over 100 species, which are variously distributed in the upper 200 m of the vertical water column. The depth distribution of the lake's three most prevalent species follows a predictabk pattern: Nitzschia gracilis in the 0–20 m stratum, Tribonema sp. at mid–depth (80–20 m), and Stephanodiscus hantzschii in the lowermost stratum (160–200 m). These major species, which account for approximately 80% or more of the lake's total phytoplankton biomass and primary production, exist under atypical temperature, light, and nutrient conditions.
2. The spatial distribution of phytoplankton in Crater Lake resembles a three-tier structure. Unlike most lakes, where the entire phytoplankton communities exist in less disparate environmental conditions, or are vertically mixed periodically by storm events and seasonal lake turnover. the Crater Lake community is partitioned into stratified environments.
5. The disparate and unusual characteristics of these environments, and the hydrological and limnological stability of the lake basin, are perhaps important factors regulating the diversity, dominance. and partitioning of the lake's phytoplankton populations.     

A simple model of the eco-hydrodynamics of the epilimnion of Lake Tanganyika

1. The ecosystem response of Lake Tanganyika was studied using a four-component, nutrient–phytoplankton–zooplankton–detritus, phosphorus-based ecosystem model coupled to a nonlinear, reduced-gravity, circulation model. The ecosystem model, an improved version of the earlier eco-hydrodynamics model developed for Lake Tanganyika, was used to estimate the annual primary production of Lake Tanganyika and its spatial and temporal variability. The simulations were driven with the National Centres for Environmental Protection (NCEP) records for winds and solar radiation forcing.
2. The simulated annual cycles of the four ecosystem variables and the daily net primary production were compared with the observations. The comparison showed that simulations reproduced realistically the general features of the annual cycles of epilimnial phosphate, net primary production and plankton dynamics.
3. The climatic simulations for the years 1970–2006 yielded a daily averaged integrated upper layer net production ranging from 0.11 to 1.78 g C m⁻² day⁻¹ and daily averaged chlorophyll- a (chl- a ) from 0.16 to 4.3 mg m⁻³. Although the nutrient concentrations in the epilimnion during the strong wind years were high, the net production was low, which is partly because of the greater vertical mixing, produced by strong winds, exposing the phytoplankton to low light conditions in deeper waters. The simulated annual net production and chl- a agreed quite well with observed production available in the literature.
4. We envisage using this model to predict the future scenarios of primary productivity in the lake.     

Photosynthetic characteristics of the submerged macrophyte Ceratophyllum demersum

The photosynthetic and growth characteristics of Ceratophyllum demersum L. were investigated under laboratory conditions which simulated those encountered in the plants' normal environment. The carbon fixation rate of C. demersum measured with ¹⁴C at light and carbon saturation at pH 8.0 was 4.48 mg C (g ash-free dry weight)⁻¹ h⁻¹. It was lower at pH 6.5 than at pH 8.0. The light use efficiencies in quiescent plants and actively growing plants were 6.3 and 8.7 × 10⁻⁹ kg CO₂ J⁻¹, respectively, with corresponding maximum photosynthetic rates of 2.67 and 4.36 mg C (g ash-free dry weight)⁻¹ h⁻¹. Photorespiration in actively growing plants consumed 24% of the carbon fixed. Incubation with DCMU demonstrated that about one-third was refixed. The optimum temperature for carbon fixation was 25°C. The C₃-photosynthetic pathway was the main operational route as indicated by the early photosynthetic products (largely C₃-acids) and the absence of Krantz anatomy and the chlorophyll a:b ratio (2.7). The maximum relative growth rates ranged from 0.025 to 0.041 g ash-free dry weight (g ash-free dry weight)⁻¹ day⁻¹ in the field (Lake Vechten, 1 to 3 m depth classes).     

Seasonal heterotrophic flagellate and bacterial plankton dynamics in a large oligotrophic lake– Loch Ness, Scotland

1. Bacterial production in the 0–30 m water column of Loch Ness was measured using a dual labelling procedure with [³H] thymidine and [¹⁴C] leucine between May 1993 and June 1994. In most cases the uptake of the two labels did not covary, suggesting unbalanced growth. Rates of bacterial production varied from undetectable to 46.2 μg C l^–1 day^–1. Highest production coincided with the period of highest primary production, but carbon derived from this source was insufficient to meet the bacterial carbon demand, which was met by allochthonous humic inputs to the system.
2. Heterotrophic flagellate (HNAN) grazing rates, measured using fluorescently labelled bacteria, ranged between 10.3 and 24.5 bacteria cell^–1 day^–1 at temperatures between 5 and 15 °C. They removed up to 27% of the bacterial production per day.
3. Heterotrophic flagellate specific growth rates ranged from 0.043 to 0.093 h^–1 between 5 and 15 °C, giving generation times of 7.4–16.1 h.
4. bacterial and HNAN abundances were not coupled, but the highest HNAN grazing impact related to a time of high bacterial productivity.     

Host-plant quality alters grass/forb consumption by a mixed-feeding insect herbivore, Melanoplus bivittatus (Orthoptera: Acrididae)

Abstract.  1. Factors affecting the nutritional ecology of mixed-feeding, polyphagous herbivores are poorly understood. Mixed-feeding herbivores do better when they consume both forb and grass species although they typically feed primarily on forbs, which are of relatively higher protein content than grasses.
2. In a field experiment, we examined the effects of nitrogen and phosphorus fertilization and associated changes in host-plant C:N:P on proportional grass consumption by a mixed-feeding insect herbivore, Melanoplus bivittatus , using natural abundance stable carbon isotope (¹²C^/13C) methods. We also examined a grass-feeding ( Phoetaliotes nebrascensis ) and forb-feeding ( Hesperotettix viridis ) species.
3. The C isotope signatures of M. bivittatus collected from plots fertilized with nitrogen (+N), phosphorus (+P), nitrogen and phosphorus (+N+P) and no fertilizer were compared with the C isotope signatures of plants in those plots to determine the proportion of assimilated C derived from C₄ grasses and C₃ forbs in each plot. We also examined the relationship between M. bivittatus diets and plant C:N:P stoichiometry.
4. The proportion of grass assimilated approximately doubled in N-fertilized treatments (39.1 ± 0.1%) compared with non-fertilized treatments (19 ± <0.1%), an increase associated with decreased C:N and increased N:P of grasses.
5. These results indicate that mixed-feeding M. bivittatus can selectively feed to balance C:N:P intake even when choosing between two structurally and chemically different groups of plants.
6. The strong relationship between diet selection and grass stoichiometry also suggests that plant nutrient composition may be more important than defensive chemistry in food choice.     

Population dynamics and production of Jesogammarus annandalei, an endemic amphipod, in Lake Biwa, Japan

SUMMARY 1. Population dynamics and production of Jesogammarus annandalei , an endemic amphipod in Lake Biwa, were examined from April 1997 to June 1998. The life cycle of this species was 1 year with the new generation beginning in early autumn. They preferred low temperature (<12 °C) and their spatial distribution varied seasonally and accordingly.
2. In deep water, the abundance of J. annandalei ranged from 200 to 63 000 m⁻² and decreased towards summer and the biomass (0.01∼3.6 g C m⁻²) was on average comparable that of zooplankton. The density was much higher than that recorded by a study conducted 35 years ago.
3. Individual growth rate of this amphipod was high in winter and spring but decreased in summer. Annual production of J. annandalei (6.2 g C m⁻² year⁻¹) was only 2% of primary production but was at the higher end of the range reported for amphipods in oligo- and mesotrophic lakes.
4. These results are consistent with the view that Lake Biwa is becoming more eutrophic, with a consequent decrease in the abundance of predatory fish in the profundal zone.     

Production potential of aquatic plants in systems mixing floating and submerged macrophytes

1. To investigate whether the areal production of aquatic plant systems can be increased by growing floating and submerged plants together, the floating fern Azolla filiculoides was grown at six densities and two nutrient availabilities in the presence and absence of the submerged macrophyte Elodea canadensis .
2. High nutrient availability did not affect the total areal biomass production, but raised the internal N concentration of Azolla and Elodea by 34% and 152%, respectively, and the internal P concentration by 50% and 378%.
3. High Azolla density reduced Elodea production, whereas Elodea did not affect the production of Azolla . A maximal total production of 4.0 g DW m⁻² day⁻¹ was obtained at an Azolla density at and above full surface cover, when Elodea contribute with less than 10% to the total. This contribution did not raise the combined production of Azolla and Elodea significantly above the production of Azolla grown alone. Thus, maximum production was not enhanced by growing floating and submerged macrophytes together.     

Environmental factors influencing chlorophyll v. nutrient relationships in lakes

SUMMARY. 1. A model relating log chlorophyll a concentration to log epilimnetic total phosphorus (TP) concentration was re-examined based on: (a) comparative and temporal studies of four stratifying Wisconsin and other highly eutrophic temperate lakes; (b) comparative summer lake surveys from Iowa and Alberta.
2. Although P-limited, deeper lakes with long hydraulic residence times and low external and internal nutrient loading in summer had summer chlorophyll a yields below model predictions based on spring and summer epilimnetic TP concentrations.
3. For lakes with summer epilimnetic TP between 30 and 80 mg m⁻³, chlorophyll a concentrations exceeded model predictions based on summer TP. This relationship held even for Lake Delavan, Wisconsin, where the ratio of available N to P was unfavourably low during spring turnover, and where the trans-thermocline N:P flux ratio was sub-optimal for algal needs in early summer.
4. With increasing summer TP concentrations and/or increasing epilimnetic circulation depth (>5m), chlorophyll a concentrations fell below model predictions—independent of the potential for N-limitation. This plateauing in chlorophyll a response occurred at lower epilimnetic TP content (−2) in lakes with elevated non-algal light extinction coefficients. Using Tailing's algorithm for the'column compensation point' (algal photosynthesis = algal respiration over diel cycle), light limitation best explains this fall-off in chlorophyll a yield.
5. The failure of the Dillon & Rigler (1974) spring TP v . summer chlorophyll a model for these Wisconsin lakes is unrelated to N-limitation. Instead, it reflects internal adjustment in take TP in response to stratification and seasonal external P loading.     

遮荫对撂荒地草本群落生物量分配和养分积累的影响

城市化进程导致农村出现大量的撂荒地,了解撂荒地不同利用方式下的植物群落动态可为撂荒地利用与管理提供重要的基础数据。撂荒地栽植与没有栽植林木是否影响林下草本群落的生物量分配与养分积累仍有待于研究。采用50%—95%遮荫网处理,模拟林下光环境对撂荒地草本群落生物量分配和养分积累特征的影响。结果表明:随着遮荫强度增加,群落总生物量著降低。遮荫处理显著降低了地上生物量及其分配比例,而对根部生物量的影响不显著,却显著提高了根部生物量的分配比例。光照强度与总生物量和地上生物量呈极显著正相关。遮荫处理显著降低了群落地上部分C含量,显著提高了P、K含量,对N含量影响不显著;遮荫处理也显著提高了根部C、N、P含量,但对K含量的影响不显著。随遮荫强度增加,地上部分C、N、P、K的分配比例显著降低,根部C、N、P、K的分配比例显著提高。相关分析表明,光照强度仅与地上部分N含量、根部C、N、P含量极显著相关。遮荫处理显著降低了地上部分C∶N、C∶P和地下部分的C∶N,但对地下部分N∶P、C∶P影响不显著。可见,遮荫将影响撂荒地草本植物群落地上部分生物量和养分积累,而根部对光照强度改变的响应不敏感。     

THE EFFECT OF IRRADIANCE ON THE GROWTH OF DIFFERENT FORMS OF FRESHWATER DIATOMS: IMPLICATIONS FOR SUCCESSION IN ATTACHED DIATOM COMMUNITIES

The growth rates of 12 strains of attached freshwater diatoms were examined as a function of irradiance. The results were used to determine physiological parameters in the growth–irradiance relationships. Values of G _max (maximum growth rate) and I  _k (irradiance at half of G _max) correlated with successional phase. The development of attached diatom communities appears to proceed from high irradiance species to low irradiance species that appear later in the successional sequence. Thus, efficiency of light use is linked to successional strategy, as has been documented for terrestrial plants.     

Phytoplankton production and growth rate in Lake Tanganyika: evidence of a decline in primary productivity in recent decades

1. This study focused on phytoplankton production in Lake Tanganyika. We provide new estimates of daily and annual primary production, as well as growth rates of phytoplankton, and we compare them with values published in former studies.
2. Chlorophyll- a (chl- a ) in the mixed layer ranged from 5 to 120 mg chl- a  m⁻² and varied significantly between rainy and dry seasons. Particulate organic carbon concentrations were significantly higher in the south basin (with 196 and 166 mg C m⁻³ in the dry and the rainy season, respectively) than in the north basin (112 and 109 mg C m⁻³, respectively).
3. Carbon : phosphorus (C : P) ratios varied according to season. Phosphorus limitation seemed to occur more frequently than nitrogen limitation, especially during the rainy season. Severe P deficiencies were rare.
4. Measured particulate daily primary production ranged from 110 to 1410 mg C m⁻² day⁻¹; seasonal contrasts were well marked in the north basin, but less in the south basin, where primary production peaks occurred also in the rainy season. Estimates of annual primary production, based on daily primary production calculated from chl- a and water transparency, gave values lower than those reported in previous studies. Picophytoplankton accounted on average for 56% of total particulate production in the south basin during the wet season of 2003.
5. Phytoplankton growth rates, calculated from primary production, ranged from 0.055 to 0.282 day⁻¹; these are lower than previously published values for Lake Tanganyika.     

The simultaneous use of 14CO2 and 15N2 labelling techniques to study the carbon and nitrogen economy of legumes grown under natural conditions

A method based on simultaneous short-term exposure to ¹⁴CO₂ and ¹⁵N₂ is described for studying nitrogen fixation and distribution in legumes relative to carbon assimilation and use. Equipment designed to accomodate experiments under natural conditions with very little disturbance of the N₂ fixing association is used. It permits continuous measurement and regulation of variables such as air temperature, humidity and CO₂ concentration as well as soil aeration. Measurements of distribution and use of assimilates, respiration of nodulated roots, quantitative N₂ fixation and the distribution and fate of fixed N as a function of time lead to a precise estimation of C and N budgets for each labelling period. When experiments are done at several phenological stages they give a new insight into the complex C and N interrelations in legume symbiosis.
A series of trials throughout the growth period of Glycine max (L.) Merr. cv. Hodgson demonstrated the sensitivity of the method. The development of the plants from vegetative to reproductive stages was accompanied by a complete change in the distribution patterns of current assimilates and products of nitrogen fixation. Maximum sink strength moved from the leaves to the pods and seeds which ended up receiving 70% of the incoming C and 35% of the fixed N. The fact that up to 85% of fixed N in the plants was in the reproductive organs at maturity can be accounted for by remobilisation from vegetative parts.
The respiration of nodulated roots utilized 33% of carbon translocated to below-ground plant parts before nitrogen fixation started, but as much as 50% during the period of optimal fixation. The advantages and limitations of the isotopic method described are critically discussed as a prelude to future investigations.