REACTIVE OXYGEN METABOLISM IN THE TROPICAL BROWN ALGA DICTYOTA DICHOTOMA

There is a growing understanding that phagotrophic ciliates are often important members of aquatic communities in terms of their trophic role and mobilization of small cell production to higher consumers. As formidable consumers of small phytoplankton species they are likely to be also important in determining the community composition of the pico- and nanophytoplankton assemblages. Dilution method experiments were conducted during the winter and summer in the South Slough, an arm of the Coos Bay on the southern Oregon coast, to assess the impact of ciliate grazing on two size fractions of chlorophyll (0.2 to 5 mm and> 5 mm) and on the growth and abundance of specific phytoplankton groups, particularly cryptophytes and Synechococcus sp. The premise of the dilution technique is that grazers are diluted with their food and the observed rate of change in chlorophyll or phytoplankton abundance is linearly related to the dilution factor. Results from previous studies using the dilution technique have been given in terms of the grazing impact of microzooplankton on total chlorophyll. The findings of the research presented using a more rigorous application of the dilution method suggest that ciliates are differential in their grazing of phytoplankton, targeting small phytoplankton biomass and preying selectively on components of the assemblage that constitute this biomass.     

Grazing impact of microzooplankton on a diatom bloom in a mesocosm as estimated by pigment-specific dilution technique

To investigate the impact of microzooplankton grazing on phytoplankton bloom in coastal waters, an enclosure experiment was conducted in Saanich Inlet, Canada during the summer of 1996. Daily changes in the microzooplankton grazing rate on each phytoplankton group were investigated with the growth rates of each phytoplankton group from the beginning toward the end of bloom using the dilution technique with high-performance liquid chromatography (HPLC). On Day 1 when nitrate and iron were artificially added, chlorophyll a concentration was relatively low (4.3 μg l⁻¹) and 19′-hexanoyloxyfucoxanthin-containing prymnesiophytes were predominant in the chlorophyll biomass. However, both the synthetic rates and concentrations of 19′-hexanoyloxyfucoxanthin declined before bloom, suggesting that 19′-hexanoyloxyfucoxanthin-containing prymnesiophytes weakened. Chlorophyll a concentration peaked at 23 μg l⁻¹ on Day 4 and the bloom consisted of the small chain-forming diatoms Chaetoceros spp. (4 μm in cell diameter). Diatoms were secondary constituents in the chlorophyll biomass at the beginning of the experiment, and the growth rates of diatoms (fucoxanthin) were consistently high (>0.5 d⁻¹) until Day 3. Microzooplankton grazing rates on each phytoplankton group remarkably increased except on alloxanthin-containing cryptophytes after the nutrient enrichments, and peaked with >0.6 d⁻¹ on Day 3, indicating that >45% of the standing stock of each phytoplankton group was removed per day. Both the growth and mortality rates of alloxanthin-containing cryptophytes were relatively high (>1 and >0.5 d⁻¹, respectively) until the bloom, suggesting that a homeostatic mechanism might exist between predators and their prey. Overall, microzooplankton grazing showed a rapid response to the increase in phytoplankton abundance after the nutrient enrichments, and affected the magnitude of the bloom significantly. High grazing activity of microzooplankton contributed to an increase in the abundance of heterotrophic dinoflagellates with 7-24 μm in cell size, the fraction of large-sized (>10 μm) chlorophyll a, and stimulated the growth of larger-sized ciliates after the bloom.     

Seasonal variations in upwelling and in the grazing impact of copepods on phytoplankton off A Coruña (Galicia, NW Spain)

The impact of grazing by copepods on phytoplankton was studied during a seasonal cycle on the Galician shelf off A Coruña (NW Spain). Grazing was estimated by measuring the chlorophyll gut content and the evacuation rates of copepods from three mesh-size classes: 200-500 (small), 500-1000 (medium), and 1000-2000 μm (large). Between February 1996 and June 1997, monthly measurements of water temperature, chlorophyll concentration, primary production rates, and copepod abundance, chlorophyll gut content, and evacuation rates were taken at an 80-m-deep, fixed shelf station. Additionally, the same measurements were collected daily during two bloom events in March and in July 1996. Small copepods were the most abundant through the seasonal cycle. The highest grazing impact, however, was due to the medium and large size classes. Grazing by small copepods exceeded grazing by medium and large copepods only during phytoplankton spring blooms. The impact of copepod grazing (considering all size fractions) was generally low. On average, 2% of the phytoplankton biomass and 6% of the primary production were removed daily by the copepod community. Maximum grazing impact values (9% of the phytoplankton biomass and 39% of the primary production) were found in mid-summer. These results suggest that most of the phytoplankton biomass would escape direct copepod grazing in this upwelling area.     

Microbial Community Structure and Dynamics in the Largest Natural French Lake (Lake Bourget)

We investigated the dynamics and diversity of heterotrophic bacteria, autotrophic and heterotrophic flagellates, and ciliates from March to July 2002 in the surface waters (0–50 m) of Lake Bourget. The heterotrophic bacteria consisted mainly of “small” cocci, but filaments (>2 μm), commonly considered to be grazing-resistant forms under increased nanoflagellate grazing, were also detected. These elongated cells mainly belonged to the Cytophaga-Flavobacterium (CF) cluster, and were most abundant during spring and early summer, when mixotrophic or heterotrophic flagellates were the main bacterial predators. The CF group strongly dominated fluorescent in situ hybridization–detected cells from March to June, whereas clear changes were observed in early summer when Beta-proteobacteria and Alpha-proteobacteria increased concomitantly with maximal protist grazing pressures. The analysis of protist community structure revealed that the flagellates consisted mainly of cryptomonad forms. The dynamics of Cryptomonas sp. and Dinobryon sp. suggested the potential importance of mixotrophs as consumers of bacteria. This point was verified by an experimental approach based on fluorescent microbeads to assess the potential grazing impact of all protist taxa in the epilimnion. From the results, three distinct periods in the functioning of the epilimnetic microbial loop were identified. In early spring, mixotrophic and heterotrophic flagellates constituted the main bacterivores, and were regulated by the availability of their resources mainly during April (phase 1). Once the “clear water phase” was established, the predation pressure of metazooplankton represented a strong top-down force on all microbial compartments. During this period only mixotrophic flagellates occasionally exerted a significant bacterivory pressure (phase 2). Finally, the early summer was characterized by the highest protozoan grazing impact and by a rapid shift in the carbon pathway transfer, with a fast change-over of the main predators contribution, i.e., mixotrophic, heterotrophic flagellates and ciliates in bacterial mortality. The high abundance of ciliates during this period was consistent with the high densities of resources (heterotrophic nanoflagellates, algae, bacteria) in deep layers containing the most chlorophyll. Bacteria, as ciliates, responded clearly to increasing phytoplankton abundance, and although bacterial grazing impact could vary largely, bacterial abundance seemed to be primarily bottom-up regulated (phase 3).     

Microbial processes in a high-latitude fjord (Kongsfjorden, Svalbard): II. Ciliates and dinoflagellates

The composition and ecological role of ciliates and dinoflagellates were investigated at one station in Kongsfjorden, Svalbard, during six consecutive field campaigns between March and December 2006. Total ciliate and dinoflagellate abundance mirrored the seasonal progression of phytoplankton, peaking with 5.8 × 10⁴ cells l⁻¹ in April at an average chlorophyll a concentration of 10 μg l⁻¹. Dinoflagellates were more abundant than ciliates, dominated by small athecates. Among ciliates, aloricate oligotrichs dominated the assemblage. A large fraction (>60%) of ciliates and dinoflagellates contained chloroplasts in spring and summer. The biomass of the purely heterotrophic fraction of the ciliate and dinoflagellate community (protozooplankton) was with 14 μg C l⁻¹ highest in conjunction with the phytoplankton spring bloom in April. Growth experiments revealed similar specific growth rates for heterotrophic ciliates and dinoflagellates (<0–0.8 d⁻¹). Food availability may have controlled the protozooplankton assemblage in winter, while copepods may have exerted a strong control during the post-bloom period. Calculations of the potential grazing rates of the protozooplankton indicated its ability to control or heavily impact the phytoplankton stocks at most times. The results show that ciliates and dinoflagellates were an important component of the pelagic food web in Kongsfjorden and need to be taken into account when discussing the fate of phytoplankton and biogeochemical cycling in Arctic marine ecosystems.     

Abundance, species composition of microzooplankton from the coastal waters of Port Blair, South Andaman Island

ABSTRACT: BACKGROUND: Microzooplankton consisting of protists and metazoa <200 mum. It displays unique feeding mechanisms and behaviours that allow them to graze cells up to five times their own volume. They can grow at rates which equal or exceed prey growth and can serve as a viable food source for metazoans. Moreover, they are individually inconspicuous, their recognition as significant consumers of oceanic primary production. The microzooplankton can be the dominant consumers of phytoplankton production in both oligo- and eutrophic regions of the ocean and are capable of consuming >100% of primary production. RESULTS: The microzooplankton of the South Andaman Sea were investigated during September 2011 to January 2012. A total of 44 species belong to 19 genera were recorded in this study. Tintinnids made larger contribution to the total abundance (34%) followed in order by dinoflagellates (24%), ciliates (20%) and copepod nauplii (18%). Foraminifera were numerically less (4%). Tintinnids were represented by 20 species belong to 13 genera, Heterotrophic dinoflagellates were represented by 17 species belong to 3 genera and Ciliates comprised 5 species belong to 3 genera. Eutintinus tineus, Tintinnopsis cylindrical, T. incertum, Protoperidinium divergens, Lomaniella oviformes, Strombidium minimum were the most prevalent microzooplankton. Standing stock of tintinnids ranged from 30--80 cells.L-1 and showed a reverse distribution with the distribution of chlorophyll a relatively higher species diversity and equitability was found in polluted harbour areas. CONCLUSIONS: The change of environmental variability affects the species composition and abundance of microzooplankton varied spatially and temporarily. The observations clearly demonstrated that the harbor area differed considerably from other area in terms of species present and phytoplankton biomass. Further, the phytoplankton abundance is showed to be strongly influenced by tintinnid with respect to the relationship of prey--predator. Consequently, further investigation on microzooplankton grazing would shed light on food web dynamics.     

Temporal stability of marine phytoplankton in a subtropical coastal environment

We investigated the temporal stability of phytoplankton at a subtropical coastal site for 9 months by conducting chlorophyll and flow cytometric measurements at relatively high frequency (roughly at 2–5 day interval). Phytoplankton cells were grouped based on their sizes obtained from flow cytometric signals. We also conducted dilution experiments to estimate the growth and grazing mortality rates of different phytoplankton groups to assess whether the temporal stability of phytoplankton abundances was related with phytoplankton growth/grazing rates. Based on size-fractionated chlorophyll measurements, there was some indication that smaller phytoplankton cells were more stable than larger ones. However, by cytometric counting, there was no evidence for greater stability in small cells. Synechococcus, which had the lowest stability and dominated the <1 μm size class, showed a strong seasonal cycle that was highly dependent on temperature whereas eukaryotes did not have evident seasonal cycles. In general, biomass of a phytoplankton group consisting of several size classes was more stable than that of its sub component, consistent with the hypothesis that higher diversity leads to higher stability, probably related with the effect of statistical averaging (portfolio effect). Stability of heterotrophic bacteria was much higher than that of phytoplankton, leading to the speculation that bacteria were more diverse than phytoplankton. Phytoplankton stability was not related with their growth or grazing mortality rates. Our study suggests that species diversity should be taken into account when considering the temporal stability of phytoplankton.     

三门湾浮游动物的季节变动及微型浮游动物摄食影响

2002年8月、11月、2003年2月和5月,在三门湾进行了4个航次生物、化学和水文等专业综合调查。根据采集的浮游动物样品的分析鉴定及海上现场实验结果,对浮游动物的群落组成、生物量、丰度、多样性指数的分布和季节变动及其浮游动物对浮游植物的摄食影响进行研究。结果表明,三门湾浮游动物有67属,89种,16类浮游幼体,主要可划分为4个生态类群：以近岸低盐类群为主,其优势种为中华哲水蚤Calanus sinicus、真刺唇角水蚤Labidocera etwhaeta、捷氏歪水蚤Tortanus derjugini、太平洋纺锤水蚤Acartiapacifica、中华假磷虾Pseudeuphausia sinica和百陶箭虫Sagitta bedoti等。半咸水河口类群、暖水性外海类群和广布种相对较少。浮游动物生物量和丰度的平面分布趋势除了夏季有所差异外,其它季节基本一致。2月份和5月份,浮游动物生物量和丰度,从湾顶向湾口呈逐渐增加趋势;8月份,湾口区生物量最高,而丰度高值区出现在湾顶部;11月份,生物量和丰度的平面分布相对均匀。浮游动物种类多样性指数有明显的季节变化,其动态变化与浮游动物种数和丰度的变化一致。微型浮游动物对浮游植物存在摄食压力,且有季节变化,摄食率的变化在0．18．0．68d^-1,微型浮游动物的摄食率低于相同季节的浮游植物生长率。微型浮游动物对浮游植物摄食压力的变化范围为16．1％-49．1％d^-1,对初级生产力摄食压力的变化在58．3％-83．6％d^-1。11月份,微型浮游动物对浮游植物和初级生产力的摄食压力均出现最高值。     

Pheopigment dynamics,zooplankton grazing rates and the autumnal ammonium peak in a Mediterranean lagoon

The dilution technique was used to estimate chlorophyll and pheopigment, net and gross production as well as zooplankton grazing over a 12-month period in a coastal lagoon in Southern France. Chlorophyll a (Cha) based gross growth rates of phytoplankton ranged from undetectable in February to 2.6 day⁻¹ in June, corresponding to 3.8 divisions per day. Cha-based grazing rates ranged from undetectable in February to 1.1 d⁻¹ in June. The seasonal growth pattern of picoplankton was similar to that of the whole community, with a peak in July, corresponding to four divisions per day. Grazing processes represented from 20 to 150% of the phytoplankton daily growth, and the grazing pressure was stronger on small phytoplankton cells than on larger cells. Gross growth rates of phytoplankton were related to zooplankton grazing rates, and both were related to water temperature. Mesozooplankton which escaped sampling or oysters had to be also invoked as additional sinks for the primary production. In the fall, pheopigment concentrations greater than chlorophyll concentrations coincided with high ammonium levels in the water column. Pheopigment a production rates were highly correlated to chlorophyll -based microzooplankton grazing rates. The pheopigment a to chlorophyll a ratio was correlated with ammonium concentrations and could be used an index of the balance between ammonium supply (degradation) and demand (uptake by phytoplankton). In addition, pheopigment degradation rates in absence of grazing could be related to irradiance, indicating photo-degradation of these compounds.     

Microzooplankton grazing and the control of phytoplankton biomass in the Suwannee River estuary,USA

Microzooplankton grazing can have significant impacts on the distribution and abundance of phytoplankton, thereby influencing the frequency and duration of algae blooms. Observations of high ciliate abundances in the Suwannee River estuary, Florida, suggest a significant potential for top-down pressure on the phytoplankton community by microzooplankton. We examined the composition of the microzooplankton and determined grazing mortality losses for phytoplankton within the Suwannee River estuary from 2001 to 2002. Our results indicated grazing mortality rates of 1.4 d⁻¹, equivalent to a loss of up to 76% of phytoplankton standing crop and up to 83% of total daily primary production. The microzooplankton community was primarily composed of ciliates, dinoflagellates, and copepod nauplii. The densities of ciliates in the estuary were comparable to densities reported in highly eutrophic ecosystems (9,400–72,800 ciliates l⁻¹). Grazing pressure on small phytoplankton may be further enhanced because ciliates and small dinoflagellates have growth rates similar to those of phytoplankton, and therefore can keep up with surges in abundance. Handling editor: Judit Padisak     

Protozooplankton in the Weddell Sea,Antarctica: Abundance and distribution in the ice-edge zone

Summary Protozooplankton were sampled in the iceedge zone of the Weddell Sea during the austral spring of 1983 and the austral autumn of 1986. Protozooplankton biomass was dominated by flagellates and ciliates. Other protozoa and micrometazoa contributed a relatively small fraction to the heterotrophic biomass. During both cruises protozoan biomass, chlorophyll a concentrations, phytoplankton production and bacterial biomass and production were low at ice covered stations. During the spring cruise, protozooplankton, phytoplankton, and bacterioplankton reached high concentrations in a welldeveloped ice edge bloom 100 km north of the receding ice edge. During the autumn cruise, the highest concentrations of biomass were in open water well-separated from the ice edge. Integrated protozoan biomass was <12% of the biomass of phytoplankton during the spring cruise and in the autumn the percentages at some stations were >20%. Bacterial biomass exceeded protozooplankton biomass at ice covered stations but in open water stations during the fall cruise, protozooplankton biomass reached twice that of bacteria in the upper 100m of the water column. The biomass of different protozoan groups was positively correlated with primary production, chlorophyll a concentrations and bacterial production and biomass, suggesting that the protozoan abundances were largely controlled by prey availability and productivity. Population grazing rates calculated from clearance rates in the literature indicated that protozooplankton were capable of consuming significant portions of the daily phyto- and bacterioplankton production.     

Calanoid copepod grazing on phytoplankton: seasonal experiments on natural communities

Calanoid copepods are major components of most lacustrine ecosystems and their grazing activities may influence both phytoplankton biomass and species composition. To assess this we conducted four seasonal, in situ, grazing experiments in eutrophic Lake Rotomanuka, New Zealand. Ambient concentrations of late stage copepodites and adults of calanoid copepods (predominantly Calamoecia lucasi, but with small numbers of Boeckella delicata) were allowed to feed for nine days on natural phytoplankton assemblages suspended in the lake within 1160 litre polyethylene enclosures. The copepods reduced the total phytoplankton biomass of the dominant species in all experiments but were most effective in summer (the time of highest grazer biomass) followed by spring and autumn. In response to grazing pressure the density of individual algal species showed either no change or a decline. There were no taxa which increased in density in the presence of the copepods. The calanoid copepods suppressed the smallest phytoplankton species (especially those with GALD (Greatest Axial Linear Dimension) < µm) and there appeared to be no selection of algae on the basis of biovolume. Algal taxa which showed strong declines in abundance in the presence of the copepods include Cyclotella stelligera, Coelastrum spp., Trachelomonas spp., Cryptomonas spp., and Mallomonas akrokomos. Calanoid copepods are considered important grazers of phytoplankton biomass in this lake. The study supports the view that high phytoplankton:zooplankton biomass ratios and large average algal sizes characteristic of New Zealand lake plankton may, at least partly, be caused by year round grazing pressure on small algae shifting the competitive balance in favour of larger algal species.     

An Under-appreciated Component of Biodiversity in Plankton Communities: The Role of Protozoa in Lake Michigan (A Case Study)

Recent technological advances have led to the discovery that free-living, planktonic protozoa are ubiquitous in nature and appear to be important components of pelagic food webs (e.g., fluorescent straining, flow cytometry). Despite this, limited information exists tying their seasonality to rate processes that drive succession patterns. The abundance, and seasonal growth and grazing loss of an entire protozoan assemblage were evaluated in Lake Michigan. The protozoan assemblage was species-rich (100 taxa) and abundant throughout the year in Lake Michigan. Nano-sized protozoa (Hnano and Pnano, <20 μm in size) ranged in abundance from 10² to 10³ cells ml⁻¹, while micro-protozoa (Hmicro and Pmico, >20 and <200 μm in size) ranged in abundance from 4 to 17 cells ml⁻¹. The biomass of Hnano and Hmicro by itself represented more than 70–80% of crustacean zooplankton biomass, while Pnano and Pmicro constituted nearly 50% of phytoplankton biomass. Protozoa exhibited growth rates comparable to other components of the plankton in Lake Michigan, and some populations grew at rates similar to maximum rates determined in the laboratory (rates of 1–2 day⁻¹). Overall, it appears that macro-zooplankton predation is a major loss factor counter-balancing growth with only small differences between the two rate processes (<0.1 day⁻¹). Discrepancies between growth and grazing loss in the spring were likely attributed to sedimentation losses for larger species of tintinnids and dinoflagellates (Codonella, Tintinnidium, and Gymnodinium) that can account for their occurrence in the deep chlorophyll layer. In the summer, carnivory among similar sized species (Chromulina and small ciliates) may be additional loss factors impinging on the protozoan assemblage.     

The influence of mesozooplankton on phytoplankton nutrient limitation: a mesocosm study with northeast Atlantic plankton

We used marine phytoplankton from mesocosms seeded with different zooplankton densities to study the impact of mesozooplankton on phytoplankton nutrient limitation. After 7 d of grazing (copepod mesocosms) or 9 d (appendicularian mesocosms) phytoplankton nutrient limitation was studied by enrichment bioassays. After removal of mesozooplankton, bioassay bottles received either no nutrients, phosphorus or nitrogen alone, or a combination of nitrogen and phosphorus and were incubated for 2 d. Phytoplankton reproductive rates in the bottles without nutrient addition were calculated after correction for grazing by ciliates and indicated increasing nitrogen limitation with increasing copepod abundance. No nutrient limitation was found in the appendicularian mesocosms. The increase of nutrient limitation with increasing copepod density seems to be mainly the result of a trophic cascade effect: Copepods released nanoplankton from ciliate grazing pressure, and thereby enhanced nitrogen exhaustion by nanophytoplankton and reduced nitrogen excretion by ciliates. Nitrogen sequestration in copepod biomass, the mechanism predicted by the ecological stoichiometry theory, seems to have been a weaker effect because there was only little copepod growth during the experiment.     

Seasonal dynamics of zooplankton and grazing impact of microzooplankton on phytoplankton in Sanmen Bay, China

The species composition, biomass, abundance and species diversity of zooplankton were determined for samples collected from 12 stations in Sanmen Bay, China, in four cruises from August 2002 to May 2003. Growth of phytoplankton and grazing rates of microzooplankton were measured using the dilution technique. The spatial and temporal variation of zooplankton and its relationship with environmental factors were also analyzed. The results showed that a total of 89 species of zooplankton belonging to 67 genera and 16 groups of pelagic larvae were found in Sanmen Bay. The coastal low-saline species was the dominant ecotype in the study area, and the dominant species were Calanus sinicus, Labidocera euchaeta, Tortanus derjugini, Acartia pacifica, Pseudeuphausia sinica and Sagitta bedoti. Maximum biomass was recorded in August, followed by November and May, and the lowest biomass was recorded in February. Similarly, the highest abundance of zooplankton was observed in August, followed by May, November, and February. Grazing pressure of microzooplankton on phytoplankton in Sanmen Bay existed throughout the year, although the grazing rate of microzooplankton on phytoplankton varied with the season. Estimates for growth rate of phytoplankton ranged from 0.25 d⁻¹ to 0.89 d⁻¹, whereas grazing rate of microzooplankton ranged between 0.18 d⁻¹ and 0.68 d⁻¹ in different seasons. The growth rate of phytoplankton exceeded the grazing rate of microzooplankton in all the seasons. Grazing pressure of microzooplankton on phytoplankton ranged from 16.1% d⁻¹ to 49.1% d⁻¹, and the grazing pressure of microzooplankton on primary production of phytoplankton ranged from 58.3% d⁻¹ to 83.6% d⁻¹ in different seasons.     

Protistan bacterivory in an oligomesotrophic lake: Importance of attached ciliates and flagellates

Seasonal and depth variations of the abundance, biomass, and bacterivory of protozoa (heterotrophic and mixotrophic flagellates and ciliates) were determined during thermal stratification in an oligomesotrophic lake (Lake Pavin, France). Maximal densities of heterotrophic flagellates (1.9 × 10³ cells ml^–1) and ciliates (6.1 cells ml^–1) were found in the metalimnion. Pigmented flagellates dominated the flagellate biomass in the euphotic zone. Community composition of ciliated protists varied greatly with depth, and both the abundance and biomass of ciliates was dominated by oligotrichs. Heterotrophic flagellates dominated grazing, accounting for 84% of total protistan bacterivory. Maximal grazing impact of heterotrophic flagellates was 18.9 × 10⁶ bacteria 1^–1h^–1. On average, 62% of nonpigmented flagellates were found to ingest particles. Ciliates and mixotrophic flagellates averaged 13% and 3% of protistan bacterivory, respectively. Attached protozoa (ciliates and flagellates) were found to colonize the diatom Asterionella formosa. Attached bacterivores had higher ingestion rates than free bacterivorous protozoa and may account for 66% of total protozoa bacterivory. Our results indicated that even in low numbers, epibiotic protozoa may have a major grazing impact on free bacteria. Correspondence: C. Amblard.     

Size‐Fractionated Phytoplankton and Relationships with Metazooplankton in a Newly Flooded Reservoir

In order to yield some insights into the planktonic food web structure of new reservoirs, size‐fractionated biomass and productivity of phytoplankton were examined from 1996 to 1997 (following the 1995 flooding of the Sep Reservoir, Puy‐de‐Dôme, France), in relation to nutrients (P, N) and metazooplankton (Rotifers, Cladocera, Copepods). Autotrophic nanoplankton (ANP, size class 3–45 μm) dominated the phytoplankton biomass (as Chlorophyll a) and production, while autotrophic picoplankton (APP, 0.7–3 μm) exhibited the lowest and relatively constant biomass and production. Cells of the autotrophic microplankton (AMP, >45 μm) were considered inedible for planktonic herbivores. The production‐biomass diagram for the different size classes and the positive correlation between APP production and ANP + AMP production suggested that grazing was potentially more important than nutrients in shaping the phytoplankton size structure. Metazooplankton biomass was low compared to other newly flooded reservoirs or to natural lakes with phytoplankton biomass similar to that of the Sep Reservoir. This resulted in low ratios (metazooplankton to edible phytoplankton) both in terms of production (average 0.43% in 1996 and 0.76% in 1997) and biomass, suggesting that only a small fraction of phytoplankton was directly consumed by metazooplankton. We suggest that the observed low ratios in the Sep Reservoir, reflect possible low metazooplankton inputs in the main influents, changes in hydrologic conditions and a high potential role of microheterotrophs. The latter role was supported by (i) the positive inter‐annual correlation between ciliates and phytoplankton, (ii) the significant and negative correlations between ciliates and metazooplankton, and (iii) the significant and negative correlations between total metazooplankton biomass and total phosphorus (TP), whereas neither TP nor total metazooplankton biomass was correlated with phytoplankton variables.     

Grazer control and nutrient limitation of phytoplankton biomass in two Australian reservoirs

1. Grazer and nutrient controls of phytoplankton biomass were tested on two reservoirs of different productivity to assess the potential for zooplankton grazing to affect chlorophyll/phosphorus regression models under Australian conditions. Experiments with zooplankton and nutrients manipulated in enclosures, laboratory feeding trials, and the analysis of in-lake plankton time series were performed. 2. Enclosures with water from the more productive Lake Hume (chlorophyll a = 3–17.5 μg l^–1), revealed significant zooplankton effects on chlorophyll a in 3/6, phosphorus limitation in 4/6 and nitrogen limitation in 1/6 of experiments conducted throughout the year. Enclosures with water from the less productive Lake Dartmouth (chlorophyll a = 0.8–3.5 μg l^–1), revealed significant zooplankton effects in 5/6, phosphorus limitation in 5/6 and nitrogen limitation in 2/6 of experiments. 3. While Lake Hume enclosure manipulations of the biomass of cladocerans (Daphnia and Diaphanosoma) and large copepods (Boeckella) had negative effects, small copepods (Mesocyclops and Calamoecia) could have positive effects on chlorophyll a. 4. In Lake Hume, total phytoplankton biovolume was negatively correlated with cladoceran biomass, positively with copepod biomass and was uncorrelated with total crustacean biomass. In Lake Dartmouth, total phytoplankton biovolume was negatively correlated with cladoceran biomass, copepod biomass and total crustacean biomass. 5. In both reservoirs, temporal variation in the biomass of Daphnia carinata alone could explain more than 50% of the observed variance in total phytoplankton biovolume. 6. During a period of low phytoplankton biovolume in Lake Hume in spring–summer 1993–94, a conservative estimate of cladoceran community grazing reached a maximum of 0.80 day^–1, suggesting that Cladocera made an important contribution to the development of the observed clear-water phase. 7. Enclosure experiments predicted significant grazing when the Cladocera/Phytoplankton biomass ratio was greater than 0.1; this threshold was consistently exceeded during clear water phase in Lake Hume. 8. Crustacean length had a significant effect on individual grazing rates in bottle experiments, with large Daphnia having highest rates. In both reservoirs, mean crustacean length was negatively correlated with phytoplankton biovolume. The observed upper limit of its variation was nearly twice as high compared to other world lakes.     

Erratum to: How do UV radiation, temperature, and zooplankton influence the dynamics of alpine phytoplankton communities?

Plankton in mountain lakes are confronted with generally higher levels of incident ultraviolet radiation (UVR), lower temperatures, and shorter growing seasons than their lower elevation counterparts. The direct inhibitory effects of high UVR and low temperatures on montane phytoplankton are widely recognized. Yet little is known about the indirect effects of these two abiotic factors on phytoplankton, and more specifically whether they alter zooplankton grazing rates which may in turn influence phytoplankton. Here, we report the results of field microcosm experiments that examine the impact of temperature and UVR on phytoplankton growth rates and zooplankton grazing rates (by adult female calanoid copepods). We also examine consequent changes in the absolute and relative abundance of the four dominant phytoplankton species present in the source lake (Asterionella formosa, Dinobryon sp., Discostella stelligera, and Fragilaria crotonensis). All four species exhibited higher growth rates at higher temperatures and three of the four species (all except Dinobryon) exhibited lower growth rates in the presence of UVR versus when shielded from UVR. The in situ grazing rates of zooplankton had significant effects on all species except Asterionella. Lower temperatures significantly reduced grazing rates on Fragilaria and Discostella, but not Dinobryon. While UVR had no effect on zooplankton grazing on any of the four species, there was a significant interaction effect of temperature and UVR on zooplankton grazing on Dinobryon. Discostella and Dinobryon increased in abundance relative to the other species in the presence of UVR. Colder temperatures, the presence of zooplankton, and UVR all had consistently negative effects on rates of increase in overall phytoplankton biomass. These results demonstrate the importance of indirect as well as direct effects of climate forcing by UVR and temperature on phytoplankton community composition in mountain lakes, and suggest that warmer climates and higher UVR levels may favor certain species over others.     

Phytoplankton community growth-rate response to nutrient pulses in a shallow turbid estuary, Galveston Bay, Texas

Phytoplankton growth is a physiological process often limitedby temperature, nutrients or light, while biomass accumulationis a function of growth rates, grazing and deposition. Althoughprimary productivity measurements are usually used to assessresponses to limiting factors, the rates are proportional tobiomass and inversely related to grazing pressure during experimentalincubations. Alternatively, carbon-specific growth-rate determinationsprovide insights into physiological responses without the confoundingeffects of biomass and grazing. The objective of this studywas to quantify the growth-rate responses of phytoplankton toenhanced nutrient availability (nitrate and phosphate) overa range of in situ irradiances. Growth rates were determinedbased on chlorophyll a-specific ¹⁴C-uptake rates by phytoplankton.Phytoplankton demonstrated high (24 h) growth rates when exposedto increased concentrations of limiting nutrients, independentof the surface irradiances (12–41%). Growth-rate responseswere also compared with the biomass (chlorophyll a) responsesand community composition. Observed and estimated phytoplanktonbiomass changes during the incubations differed, emphasizingthe structural role of grazers on the phytoplankton community.The phytoplankton community in Galveston Bay has the potentialto instantaneously respond to nutrient pulses, facilitatingdiatom biomass accumulations in spring and summer and small,flagellated species and cyanobacteria during periods of lownutrient inputs. Thus, Galveston Bay phytoplankton biomass andcommunity composition reflect a dynamic balance between thefrequency of nutrient pulsing and grazing intensity.