The effects on the plankton community of filter-feeding Sacramento blackfish,Orthodon microlepidotus

Summary A series of mesocosm experiments was performed to assess the effects on the plankton community of filter-feeding Sacramento blackfish (Cyprinidae; Orthodon microlepidotus). Phytoplankton size-frequency distribution, zooplankton abundance, primary production, potential secondary production, and nutrient concentrations were measured. Blackfish reduce numbers of both evasive and nonevasive zooplankton and large phytoplankton while enhancing nanoplankton densities. Blackfish also significantly increase primary production and potential secondary community production. Levels of dissolved inorganic phosphorus and ammonia-nitrogen also increase. The effects of blackfish are generally similar to those reported for other filter-feeding fish.     

Influence of phytoplankton fractions on growth and reproduction of tropical cladocerans

The influence of two seston fractions, < 20 µm (nanoplankton) and 20 µm (microplankton), on growth and reproduction of cladoceran species with different sizes, from Lake Monte Alegre, was evaluated through individual growth and life table experiments. Size, shape and other features of the algae in the fractions were described. The procedure of filtering lake water through a 20 µm net for seston fractionation caused mutual contamination. The smallest cladoceran species, Ceriodaphnia cornuta Sars and Moina micrura Kurz, produced larger clutch sizes and exhibited higher intrinsic rates of population growth (r) in the nanoplankton, despite contamination of their food by inedible algae. The largest species, Simocephalus mixtus Sars, produced larger clutch sizes in the microplankton. There were no differences in juvenile biomass growth between treatments for M. micrura and Daphnia gessneri Herbst, but lower value of the exponential growth rate (g) in the microplankton was significant for M. micrura. Fecundity (eggs/total female) of M. micrura was significantly lower in the microplankton, while D. gessneri did not reproduce in this fraction, at the end of growth experiments. Spines, colonies, cenobium, filaments, hard cell wall, and gelatinous sheaths, represented constraints to cladoceran reproductive performance, besides algae size. Microplankton contamination by nanoplanktonic algae, in the experiments, probably minimized the negative effect of inedible algae. Nanoplankton was more suitable for the smallest species and microplankton for the largest one.     

THE RELATIVE PHOTOSYNTHETIC EFFICIENCY OF ASTERIONELLA FORMOSA HASS. (BACILLARIOPHYTA) IN NATURAL PLANKTON ASSEMBLAGES1

The relative photosynthetic efficiencies of net vs. nanoplankton and diatom vs. non-diatom plankton were estimated weekly in Woodcock Creek Reservoir, Pennsylvania, from April to November 1979, using carbon-14 in conjunction with size fractionation and germanium as a diatom inhibitor. The nanoplankton contributed a proportionally greater share of the community carbon assimilation than their share of the community biomass would indicate. Diatoms, specifically Asterionella formosa Hass. were photosynthetically inefficient and contributed significantly less than expected. The mechanism of periodic diatom dominance in lakes remains unexplained.     

Mixotrophic nanoplankton in oligotrophic surface waters of the Sargasso Sea may employ phagotrophy to obtain major nutrients

The vertical distribution and abundance of mixotrophic nanoplanktonwas examined during two cruises to the Sargasso Sea south ofBermuda. Fluorescently labeled bacteria and cyanobacteria wereused as tracers of ingestion in experiments designed to determineabundances of mixotrophic nanoplankton. Phagotrophic nanoplanktonic(2–20 µm) algae ranged from undetectable to >100ml^–1, and were more abundant near the surface (up to 140ml^–1) than in the deeper euphotic zone. On two occasions,50% of the phototrophic nanoplankton in surface waters wereobserved with ingested fluorescent tracers. The contributionof mixotrophic algae to the total phototrophic nanoplanktonassemblage in the deep chlorophyll maximum, however, did notexceed 0.5%. It is possible that mixotrophic algae were moreabundant in the deep chlorophyll maximum, but were not phagotrophicallyactive. Two 4 day experimental incubations were subsequentlycarried out to examine the adaptive significance of phagotrophicbehavior for algae in surface waters of the Sargasso Sea. Greatermixotrophic nanoplankton abundances were observed in treatmentsthat received no nutrient inputs and were limited by the availabilityof inorganic nutrients during the experiments. A decrease inthe abundance of mixotrophic algae or a decrease in their phagotrophicactivity occurred with nutrient enrichment. Based on the experimentalresults, we suggest that phagotrophy was a mechanism by whichthese algae supplemented nutrient acquisition during periodsof low dissolved nutrient concentrations. Higher abundancesof mixotrophic nanoplankton observed in the upper 50 m of theSargasso Sea may have been due to the generally low nutrientconcentrations in these waters.     

Effects of experimental design and nitrogen on cabbage butterfly oviposition

Summary To test the prediction that P. rapae egg densities increase with N fertilizer in large-scale systems as they do in model systems with potted plants, we used field experiments with Brassica oleracea var. acephala L. (collards and kale) planted in pots or large field plots, and treated with different levels of nitrogen fertilizer. In small-scale field experiments with potted kale and collards, egg densities were significantly higher on plants with high N than those with low N. But in larger scale experiments with field-grown collards, average seasonal P. rapae egg densities were not significantly correlated with leaf N content. These differences among experiments did not depend on the magnitude of the difference in foliage N levels.     

Design and analysis of biodiversity field experiments

The design and analysis of field experiments have figured prominently in the current debate about biodiversity and ecosystem function. These debates have identified important issues about species traits, functional groups, and community assembly, but a broader debate needs to include discussions about the scale (grain and extent) of experiments relative to natural spatial and temporal heterogeneity. In addition, alternative statistical analyses need to be explored that focus on comparison among several statistical models rather than simple hypothesis testing. Analyses of the first 2 years of data from a new biodiversity field experiment are used to illustrate these concepts. A traditional one-way ANOVA demonstrates the expected increase in aboveground biomass with higher levels of vascular plant diversity. Further analysis demonstrates that this relationship is absent when the community contains either Arrhenatherum elatius or Holcus lanatus, two dominant species of grass. Variance in biomass is also a function of diversity and both spatial and temporal heterogeneity are significant factors in the analysis despite precautions taken to minimize them. These examples illustrate the degree to which the analysis of a field experiment influences the interpretation of the observed results. Ultimately, results from field experiments must be validated through continued comparisons among field experiments, mathematical models, laboratory trials, and mesocosm experiments.     

Partitioning of the food ration of marine ciliates between pico- and nanoplankton

Food size-range for 13 species of Tintinnina and 18 species of Oligotrichina were studied using electronic particle counting and in situ observation of food vacuole contents. Tintinnids consume nanoplankton in the size range 2–20 μm. Oligotrichous naked ciliates consume particles in the size range 0.5–10 μm. Ciliates smaller than 30 μm take 72% picoplankton and 28% nanoplankton. For ciliates between 30 μm and 50 μm the proportions are reversed (30% pico- and 70% nanoplankton), while the larger ciliates (> 50 μm) take nanoplankton almost exclusively (95% nano- and 5% picoplankton). A seasonal study of total Oligotrichida grazing showed that natural particles were consumed at rates that varied from 1 to 20 μg C 1⁻¹ day⁻¹. This included between 1 and 38% of the bacterioplankton production and 9 to 52% of the nanoplankton production. In the N-W Mediterranean the total ciliate production varied from 0.4 to 8.2 μg C 1⁻¹ day⁻¹. Research supported by CNRS-PIROCEAN AIP-RTM-953146-GRECO P4 and by CNRS-UA 716 (FR), and by the University of Nice and by the CNRS-GRECO 88 (MLP)     

Effects of temperature and ultraviolet radiation on microbial foodweb structure: potential responses to global change

1. A series of growth experiments were conducted with natural plankton communities from a lake and river in northern Quebec, to evaluate the response of microbial foodweb structure to changes in ambient temperature and solar ultraviolet radiation (UVR). 2. Bioassays were incubated for 6 days at two temperatures (10 and 20 °C) and three near-surface irradiance conditions [photosynthetically active radiation (PAR) + UVA + UVB, PAR + UVA, and PAR only). 3. The concentration of total bacteria showed no net response to temperature, but the percentage of actively respiring bacteria, as measured with the cellular redox probe 5-cyano-2,3-ditolyl tetrazolium chloride (CTC), was up to 57% higher at 20 °C relative to 10 °C. Chlorophyll a concentrations in the < 2 μm size fraction also reacted strongly to temperature, with a net increase of up to 61% over the temperature range of 10–20 °C. 4. The UVR effects were small or undetectable for most of the measured variables; however, the percentage of actively respiring bacteria was significantly inhibited in the presence of UVR at 20 °C, decreasing by 29–48% on day 6 in the lake experiments and by 59% on day 2 in one river experiment. 5. The results show differential sensitivity to temperature among organisms of microbial communities in subarctic freshwaters, and a resilience by the majority of micro-organisms to their present UVR conditions. Microbial foodwebs in northern freshwaters appear to be relatively unresponsive to short-term (days) changes in UVR. However, the observed responses to temperature suggest that climate change could influence community structure, with warmer temperatures favouring picoplanktonic phototrophs and heterotrophs, and a shift in nanoplankton species composition and size structure.     

Abiotic stress QTL in lettuce crop–wild hybrids: comparing greenhouse and field experiments

The development of stress‐tolerant crops is an increasingly important goal of current crop breeding. A higher abiotic stress tolerance could increase the probability of introgression of genes from crops to wild relatives. This is particularly relevant to the discussion on the risks of new GM crops that may be engineered to increase abiotic stress resistance. We investigated abiotic stress QTL in greenhouse and field experiments in which we subjected recombinant inbred lines from a cross between cultivated Lactuca sativa cv. Salinas and its wild relative L. serriola to drought, low nutrients, salt stress, and aboveground competition. Aboveground biomass at the end of the rosette stage was used as a proxy for the performance of plants under a particular stress. We detected a mosaic of abiotic stress QTL over the entire genome with little overlap between QTL from different stresses. The two QTL clusters that were identified reflected general growth rather than specific stress responses and colocated with clusters found in earlier studies for leaf shape and flowering time. Genetic correlations across treatments were often higher among different stress treatments within the same experiment (greenhouse or field), than among the same type of stress applied in different experiments. Moreover, the effects of the field stress treatments were more correlated with those of the greenhouse competition treatments than to those of the other greenhouse stress experiments, suggesting that competition rather than abiotic stress is a major factor in the field. In conclusion, the introgression risk of stress tolerance (trans‐)genes under field conditions cannot easily be predicted based on genomic background selection patterns from controlled QTL experiments in greenhouses, especially field data will be needed to assess potential (negative) ecological effects of introgression of these transgenes into wild relatives.     

Photosynthetic Activity of Dominant Algal Species in Eutrophic Shallow Lake (Großer Müggelsee,Berlin) Investigated by Microautoradiography

The photosynthetic activity of dominant phytoplankton in eutrophic shallow lake was investigated by the autoradiographic method in 1979 and 1980. It was shown by light and dark field microscopy that all species of Cyanophyta (Oscillatoria redekei, Oscillatoria agardhii, Aphanizomenon flos-aquae) were characterized by a continuously high uptake of NaH¹⁴CO₃. Similarly high photosynthetic activity was observed during the occurrence of Cryptomonas sp. and nanoplankton. Contrary to these observations, diatoms showed remarkably high portions of photosynthetically inactive biomass when their development was abundant. The reasons for this discrepancy between high biomass of diatoms and relatively low primary production (measured by the ¹⁴C-method and autoradiography) are discussed.     

Mixotrophic algae in three ice-covered lakes of the Pocono Mountains, U.S.A.

1. The occurrence and grazing activity of mixotrophic (phagotrophic) algae in three icecovered freshwater lakes of different trophic status were examined (oligotrophic Lake Giles, mesotrophic Lake Lacawac, eutrophic Lake Waynewood), Microbial population densities were low (4.1–7.2 × 10⁵ bacteria ml^?1 and 1.2–2.4 × 10³ nanoplanktonic protists ml^?1). All three nanoplankton communities were dominated by chloroplast-bearing forms (60–96%). 2. Mixotrophs formed up to 48% of the phototrophic nanoplankton in Lake Lacawac and were responsible for up to ～90% of the observed uptake of bacteria-sized particles. The abundance of mixotrophic algae in Lakes Giles and Waynewood were extremely low (3 and 2% of the phototrophic algae, respectively), and heterotrophs dominated nanoplankton bacterivory. 3. The overall impact of nanoplankton feeding activity on the bacterial assemblage was low under the ice in Lakes Giles and Waynewood. Removal rates of bacteria based on our particle uptake experiments were 1.0 and 4.0% of the bacterial standing stock day^?1 in these lakes, respectively. Removal rates were higher in Lake Lacawac and ranged from 4.9 to 11% of the bacterial standing stock day^?1 on 2 successive sampling days.     

Evaluation of the effectiveness of a consortium of three plant-growth promoting rhizobacteria for biocontrol of cotton Verticillium wilt

We investigated the biocontrol efficacy of a consortium of three rhizobacteria (hereafter BBS) against cotton Verticillium wilt, along with the effect on plant growth, in a series of greenhouse and field experiments. BBS treatment inhibited germination of Verticillium dahliae spores by 88.4%. Under greenhouse conditions, BBS reduced Verticillium wilt by 86.1% compared to the untreated control, and promoted plant growth by 49.9%. In field experiments, nine L/acre of BBS suspension reduced Verticillium wilt by 76.0% and increased cotton yields by 13.7%. Soil inoculation with BBS also improved the breaking tenacity and uniformity index of harvested cotton. Soil properties improved with BBS treatment in field experiments, including an increase in organic matter and the availability of nitrogen (N), phosphorus (P) and potassium (K). There was no significant difference in the biocontrol efficacy of BBS among eight tested cotton cultivars.     

The influence of macrophytes on a phytoplankton community in experimental conditions

The impact of submerged macrophytes or their extracts on planktonic algae was studied under experimental conditions. Live Ceratophyllum demersum L., its extract, and extracts of four other plant species induced modifications in the phytoplankton dominance structure. These modifications were: a decline in the number of Oscillatoria limnetica Lemm., which was the most numerous cyanobacterian species, and a decline in biomass and percentage contribution of all cyanobacteria to total algal biomass. This was accompanied by an increase in biomass and percentage contribution of green algae, especially Chlorella sp. and Chlamydomonas sp. Also, there was an increase in biomass and percentage contribution of nanoplankton (under 50 µm) to total phytoplankton biomass.The isolation of planktonic algae from direct influence of C. demersum by means of dialysis membranes caused an increase in number, biomass and percentage contribution of cyanobacteria. Release of organic compounds of over 3000 daltons by macrophytes apparently contributed to a decline of cyanobacteria by changing the phytoplankton dominance structure.     

Small nanoplankton and bacteria in the Western Ross Sea during sea-ice retreat (spring 1994)

Spatial changes of small nanoplankton (2–10 μm) were investigated in relation to sea-ice conditions, hydrography and receding ice processes in the Ross Sea (Antarctica) during spring 1994. Abundance and biomass of heterotrophic and autotrophic nanoplankton, as well as bacterioplankton, were determined along a south-north transect from the open waters polynya towards the pack ice. Autotrophic and heterotrophic nanoplankton biomass ranged from 758 to 4570 mgC m⁻² and from 3 to 387 mgC m⁻², respectively. Heterotrophic nanoplankton accounted, on average, for about 9% of the total (i.e. autotrophic plus heterotrophic) nanoplankton biomass. The size structure of both auto- and heterotrophic nanoplankton in the Ross Sea continental shelf receding ice edge was different from that of nanoplankton associated with the shelf break and open Antarctic ice-edge area. Generally, the highest heterotrophic biomass was found in the pack-ice zone on the continental shelf, while the highest heterotrophic contribution to the total nanoplankton biomass (up to 25%) was encountered at the shelf break where phytoplankton was largely dominated by 2- to 3-μm-size cells. Accepted: 2 May 1999     

Nutrient and Grazer-Mediated Effects on Picoplankton and Size Structure in Phytoplankton Communities

This study examines the factors which contribute to the abundance of algal picoplankton in lakes. A three-year field study of a meso-eutrophic lake was compared with observations from oligotrophic and highly eutrophic lakes in the region. Trophic state alone (oligotrophic vs. eutrophic) was not a good predictor of the importance of picoplankton; smaller cells were relatively abundant when phosphorus was limiting other phytoplankters, but also when nitrogen was in surplus. Subsequent field experiments found that picoplankton growth was stimulated by N, but not by P additions. This relationship was strongly affected by light and grazer levels. Grazers apparently mediate the effects of nutrient deficiency, and favor the growth of larger algal size classes, especially nanoplankton. The flux of P within experimental enclosures was controlled by picoplankton abundance under low nutrient conditions, but was a function of total phytoplankton biomass under P surplus.     

COMBINING NEW TECHNOLOGIES FOR DETERMINATION OF PHYTOPLANKTON COMMUNITY STRUCTURE IN THE NORTHERN GULF OF MEXICO 1

In situ analysis of phytoplankton community structure was determined at five stations along the Texas Gulf coast using two instruments, the Fluoroprobe and FlowCAM. Results were compared with traditional methods to determine community structure (pigment analysis and microscopy). Diatoms and small nanoplankton (most likely haptophytes) dominated the phytoplankton community at all stations. Estimated chl concentrations for diatoms+dinoflagellates obtained via the Fluoroprobe were not significantly different for three of the five stations sampled when compared with HPLC‐chemical taxonomy analysis, whereas the concentrations of green algal and cryptophyte chl were overestimated. The FlowCAM estimates of overall nanoplankton and microplankton cell abundance were not significantly different when compared with epifluorescence microscopy, and recorded images of phytoplankton cells provided a representative population of the phytoplankton community at each station. The Fluoroprobe and FlowCAM, when used in tandem, are potentially capable of determining the general characteristics of phytoplankton community structure in situ and could be an important addition to biological observing systems in the coastal ocean.     

Diurnal changes in zooplankton respiration rates and the phytoplankton activity in two Chilean lakes

Phytoplankton spatial distribution patterns in the Abra of Bilbao (a semienclosed coastal body of water) and adjacent shelf waters have been studied during June–July 1983 and May–June 1984. Small naked dinoflagellates, cryptophyceans and an unidentified nanoplankton component, were a common feature in all surveys. In July 1983 a dense bloom of nanoplankton developed inside the Abra which, in contrast to the community in the adhacent waters, contained high densities of small diatoms, naked dinoflagellates, cryptophyceans and the Haptophyta Phaeocystis pouchetii. Microplankton was mainly composed of dinoflagellates in July 1983, and of diatoms in June 1983 and May–June 1984. Microplankton abundance was highest in May–June 1984 and decreased from the shelf to the Abra. A principal component analysis performed separately on each cruise revealed the differences in the structure of the phytoplankton community between the Abra of Bilbao and the adjacent shelf waters.     

Photosynthetic efficiency and seasonality of autotrophic picoplankton in Lago Maggiore after its recovery

1. Lago Maggiore is a deep subalpine lake that has regained its previous oligotrophic state during the last decade after going through a process of cultural eutrophication and subsequent restoration. Autotrophic picoplankton (APP) have been studied in Lago Maggiore since 1992, with the primary production of the fraction being measured in 1994 and 1995. In 1998, we began to study the population structure and photosynthetic characteristics of APP after the restoration of the lake. We also compared the seasonal dynamics of APP and nanoplankton biomass and production. In this paper, we show the trend of annual and interannual dynamics of APP from 1993 to 1998. 2. Since 1993 and 1994, APP have almost tripled in numbers (mean values: 1993=44 × 10³ cell mL^–1 1998=123 × 10³ cell mL^–1) and in production. On average, APP produced 16 gC m^–2 year^–1 in 1994 and 41.2 gC m^–2 year^–1 in 1998, accounting for 10 and 20%, respectively, of total phototrophic production. 3. Although nanoplankton dominated in terms of biomass and production in the first 6 months of the year, APP were able to compete successfully with them in periods of P limitation. In September 1998, APP carbon fixation actually surpassed that of nanoplankton, reaching 13 mgC m^–3 h^–1 at 3 m. In a comparison of the daily chlorophyll‐specific photosynthesis rates of nanoplankton and APP, the latter proved to be more efficient in fixing carbon, particularly in summer and autumn. 4. While the spatial distribution of APP abundance and production followed the isotherms of 18 and 20 °C, and was strictly related to water stratification and thermocline appearance, the APP efficiency [mgC (mg chl)^–1Em^–2] had its maximum at the bottom of the euphotic zone, with irradiance in the range 0.5–18 μE m^–2 s^–1. 5. The existence of a significant regression between APP abundance and carbon fixation (P < 0.001, r²=0.92, d.f.=17; APP_prod=10.6 × 10³ cell mL^–1 + 15) enabled us to infer APP production from cell numbers, and supports the view that picocyanobacteria production is closely dependent on its standing crop. 6. Lago Maggiore is a good example of how APP in a stabilised oligotrophic system can, under favourable conditions, increase in numbers and effectively exploit the few available resources. We can conclude that the APP population reacts to the reduction in P‐levels, but only after several years of stable low nutrient conditions.     

Microbial introductions to apple leaves: Influences of altered immigration on fungal community dynamics

Fungal immigration to apple leaves in the field was altered by the introduction of populations ofChaetomium globosum orAureobasidium pullulans to surface-disinfested leaves either immediately following, or 6 days after, disinfestation. Total numbers of fungal individuals and numbers of filamentous fungal and yeast individuals were estimated and compared over time for 4–7 weeks on control leaves (leaves disinfested but no populations applied), onAureobasidium-treated, and onChaetomium-treated leaves. Fungal communities developing on leaves during three experiments in two different time frames (experiment 1: July 9–August 27; experiments 2 and 3: July 29–August 27), and thus under different immigration regimes, were also compared. Survival of introduced populations was not related to the presence of prior fungal immigrants. Rates of increase in total numbers of fungi and numbers of filamentous fungi and yeasts per leaf varied among experiments, apparently in relation to differences in immigration and environmental history. Differences among leaves in immigration had a short-term (days) influence on community size. However, no long-term effects of altered immigration on phylloplane fungal community size were evident.     

Experimental evidence of the grazing impact of <Emphasis Type="Italic">Boeckella poppei</Emphasis> on phytoplankton in a maritime Antarctic lake

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.