The influence of plant-associated filter feeders on phytoplankton biomass: a mesocosm study

Low phytoplankton biomass usually occurs in the presence of submerged macrophytes, possibly because submerged macrophytes enhance top-down control of phytoplankton by offering a refuge for efficient grazers like Daphnia against fish predation. However, other field studies also suggest that submerged macrophytes suppress phytoplankton in the absence of Daphnia. In order to investigate these mechanisms further, we conducted an outdoor mesocosm experiment to study the effect of submerged macrophytes (Elodea nuttallii) on phytoplankton and zooplankton biomass. The experiment combined four nutrient addition levels (0, 10, 100, and 1000 μg P l⁻¹; N/P ratio: 16) with three macrophyte levels (no macrophytes, artificial macrophytes, and real macrophytes). We inoculated the tanks with species-rich inocula of phytoplankton and zooplankton but excluded fish or macro-invertebrates. Probably due to the lack of predators in the mesocosms, potential grazing rates of pelagic zooplankton (estimated from zooplankton biomass) did not differ between the macrophyte treatment combinations. Compared to the treatment combinations without macrophytes, lower phytoplankton biomass occurred in the treatment combinations with real macrophytes at all the nutrient addition levels and in those with artificial macrophytes at all the nutrient levels except the highest. Significantly, higher abundances of plant-associated filter feeders (Simocephalus vetulus and Ceriodaphnia spp.) occurred in the treatment combinations with real and artificial macrophytes. The estimated potential grazing rate of these plant-associated filter feeders indicated that these filter feeders could be responsible for the lower phytoplankton biomass in the presence of real and artificial macrophytes. Our results suggest that the plant-associated filter feeders may be significant grazers in vegetated shallow lakes.     

Contributions of Different Organic Carbon Sources to <Emphasis Type="Italic">Daphnia</Emphasis> in the Pelagic Foodweb of a Small Polyhumic Lake: Results from Mesocosm DI<Superscript>13</Superscript>C-Additions

Abstract Freshwater ecosystems derive organic carbon from both allochthonous and autochthonous sources. We studied the relative contributions of different carbon sources to zooplankton in a small, polyhumic, steeply stratified lake, using six replicate surface-to-sediment enclosures established during summer and autumn 2004. We added ¹³C-enriched bicarbonate to the epilimnion of half the enclosures for three weeks during each season and monitored carbon stable isotope ratios of DIC, DOC, POC and Daphnia, along with physical, chemical and biological variables. During summer, ¹³C-enriched DIC (δ¹³C up to 44 ± 7.2‰) was soon taken up by phytoplankton (δ¹³C up to −5.1 ± 13.6‰) and was transmitted to Daphnia (δ¹³C up to −1.7 ± 7.2‰), demonstrating consumption of phytoplankton. In contrast, during autumn, ¹³C-enriched DIC (δ¹³C up to 56.3 ± 9.8‰) was not transmitted to Daphnia, whose δ¹³C became progressively lower (δ¹³C down to −45.6 ± 3.3‰) concomitant with decreasing methane concentration. Outputs from a model suggested phytoplankton contributed 64–84% of Daphnia diet during summer, whereas a calculated pelagic carbon mass balance indicated only 30–40% could have come from phytoplankton. Although autumn primary production was negligible, zooplankton biomass persisted at the summer level. The model suggested methanotrophic bacteria contributed 64–87% of Daphnia diet during autumn, although the calculated carbon mass balance indicated a contribution of 37–112%. Thus methanotrophic bacteria could supply virtually all the carbon requirement of Daphnia during autumn in this lake. The strongly ¹³C-depleted Daphnia values, together with the outputs from the models and the calculated carbon mass balance showed that methanotrophic bacteria can be a greater carbon source for Daphnia in lakes than previously suspected.     

Feeding at different plankton densities alters invasive bighead carp (<Emphasis Type="Italic">Hypophthalmichthys nobilis</Emphasis>) growth and zooplankton species composition

Invasive Asian carps Hypophthalmichthys spp. are an ecological threat to non-native aquatic ecosystems throughout the world, and are poised to enter the Laurentian Great Lakes. Little is known about how these filter-feeding planktivores grow and impact zooplankton communities in mesotrophic to oligotrophic systems like the Great Lakes. Our purpose was to determine how different plankton densities affect bighead carp H. nobilis biomass and how bighead carp affect zooplankton species composition. We conducted a 37-day indoor mesocosm experiment (volume = 678 l) with high and low plankton treatments (zooplankton dry mass ≈ 1,900 and 700 μg l⁻¹; chlorophyll a = 25 and 14 μg l⁻¹, respectively) in the presence and absence of juvenile bighead carp (mean = 5.0 g, 8.5 cm). Carp lost weight in the low plankton treatment and gained weight in the high plankton treatment, suggesting that food availability may be a limiting factor to bighead carp growth in regions of low plankton densities. In the presence of carp, zooplankton shifted from Daphnia to copepod dominance, while in the absence of carp, Daphnia remained dominant. Chydorids and ostracods increased in the presence of carp, but only in the low plankton treatment, suggesting that the impact of bighead carp on zooplankton species composition may vary with zooplankton density. Chlorophyll was higher in the absence of carp than in the presence. Chlorophyll and zooplankton densities in many Great Lakes ecosystems are substantially lower than our low treatment conditions, and thus our results suggest that Asian carp establishment in these regions may be unlikely. Handling editor: S. Declerck     

The Status and Characteristics of Eutrophication in the Yangtze River (Changjiang) Estuary and the Adjacent East China Sea, China

Eutrophication has become increasingly serious and noxious algal blooms have been of more frequent occurrence in the Yangtze River Estuary and in the adjacent East China Sea. In 2003 and 2004, four cruises were undertaken in three zones in the estuary and in the adjacent sea to investigate nitrate (NO₃–N), ammonium (NH₄–N), nitrite (NO₂–N), soluble reactive phosphorus (SRP), dissolved reactive silica (DRSi), dissolved oxygen (DO), phytoplankton chlorophyll a (Chl a) and suspended particulate matter (SPM). The highest concentrations of DIN (NO₃–N+NH₄–N+NO₂–N), SRP and DRSi were 131.6, 1.2 and 155.6 μM, respectively. The maximum Chl a concentration was 19.5 mg m⁻³ in spring. An analysis of historical and recent data revealed that in the last 40 years, nitrate and SRP concentrations increased from 11 to 97 μM and from 0.4 to 0.95 μM, respectively. From 1963 to 2004, N:P ratios also increased from 30–40 up to 150. In parallel with the N and P enrichment, a significant increase of Chl a was detected, Chl a maximum being 20 mg m⁻³, nearly four times higher than in the 1980s. In 2004, the mean DO concentration in bottom waters was 4.35 mg l⁻¹, much lower than in the 1980s. In comparison with other estuaries, the Yangtze River Estuary was characterized by high DIN and DRSi concentrations, with low SRP concentrations. Despite the higher nutrient concentrations, Chl a concentrations were lower in the inner estuary (Zones 1 and 2) than in the adjacent sea (Zone 3). Based on nutrient availability, SPM and hydrodynamics, we assumed that in Zones 1 and 2 phytoplankton growth was suppressed by high turbidity, large tidal amplitude and short residence time. Furthermore, in Zone 3 water stratification was also an important factor that resulted in a greater phytoplankton biomass and lower DO concentrations. Due to hydrodynamics and turbidity, the open sea was unexpectedly more sensitive to nutrient enrichment and related eutrophication processes.     

Arctic cyanobacteria and limnological properties of their environment: Bylot Island, Northwest Territories, Canada (73°N, 80°W)

Cyanobacteria were a major constituent of phototrophic communities in the lakes, ponds and streams of Bylot Island, in the Canadian high Arctic. The waters spanned a range of temperatures (1.8–16.8°C in late July), pH regimes (6.2–9.2) and conductivities (1.5–1700 μS cm⁻¹) but nutrient concentrations were consistently low (< 1 μg dissolved reactive P l⁻¹ at all sites; < 10 μg NO₃-N l⁻¹ at most sites). Picoplanktonic species (Synechococcus spp.) were often the numerical dominants in the plankton, and periphytic filamentous species (Oscillatoriaceae) commonly formed thick (5–50 mm) benthic mats. Bloom-forming species of cyanobacteria were either absent or poorly represented even in Chla-rich ponds. The total community biomass ranged from 0.1 to 29.8 μg Chla l⁻¹ in the plankton and from 1.1 to 34.8 μg Chla cm⁻² in the benthos. The in vivo absorbance characteristics of isolates from these environments indicated a genetically diverse range of species in each group of Arctic cyanobacteria. Growth versus irradiance relationships were determined for each of the isolates and similarly revealed large genetic differences (maximum growth rates from 0.17 to 0.61 day⁻¹), even between morphologically identical taxa. A comparison of nutrients, pigment concentrations and species composition underscores the strong similarities between freshwater ecosystems in the north and south polar zones. Received: 3 June 1996 / Accepted: 3 November 1996     

Spatial variability in the abundance of pelagic invertebrate predators in relation to depth and turbidity

The abundance of pelagic invertebrate predators in relation to turbidity and depth gradients in Lake Hiidenvesi (southern Finland) were studied. In the shallow (<5 m) and the most turbid (up to 75 NTU) part of the lake, the community of invertebrate predators consisted of cyclopoid copepods (max biomass >500 μg dw l⁻¹) and Leptodora kindtiii (Focke) (17 μg dw l⁻¹), while in the less turbid (10–40 NTU) stratifying area Chaoborus flavicans (Meigen) dominated (max 146 μg dw l⁻¹). In the temporarily stratifying and moderately turbid basin Chaoborus and small-bodied invertebrate predators co-existed. Mysis relicta (Lovén) occurred only in the stratifying area (max 15 μg dw l⁻¹). The results suggested that both water depth and turbidity contributed to the community structure of Chaoborus flavicans. Depth great enough for stratification was of special importance and its effect was amplified by elevated turbidity, while high turbidity alone could not maintain chaoborid populations. Mysis relicta also requires a hypolimnetic refuge but is more sensitive to low oxygen concentrations and may therefore be forced to the epilimnion where it is vulnerable to fish predation. Cyclopoids as rapid swimmers can take advantage at elevated turbidity levels and coexist in high biomass with fish even in shallow water. Leptodora kindtii can form high biomass despite planktivorous fish providing that turbidity exceeds 20 NTU. The results demonstrated that depth and water turbidity can strongly regulate the abundance and species composition of invertebrate predators. These factors must thus be taken into account when applying food web management, which aims to reduce phytoplankton biomass by depressing planktivorous fish.     

Phytoplankton and primary production in clear-vegetated,inorganic-turbid,and algal-turbid shallow lakes from the pampa plain (Argentina)

Shallow lakes often alternate between two possible states: one clear with submerged macrophytes, and another one turbid, dominated by phytoplankton. A third type of shallow lakes, the inorganic turbid, result from high contents of suspended inorganic material, and is characterized by low phytoplankton biomass and macrophytes absence. In our survey, the structure and photosynthetic properties (based on ¹⁴C method) of phytoplankton were related to environmental conditions in these three types of lakes in the Pampa Plain. The underwater light climate was characterized. Clear-vegetated lakes were more transparent (K _d 4.5–7.7 m⁻¹), had high DOC concentrations (>45 mg l⁻¹), low phytoplankton Chl a (1.6–2.7 μg l⁻¹) dominated by nanoflagellates. Phytoplankton productivity and photosynthetic efficiency (α ~ 0.03 mgC mgChla ⁻¹ h⁻¹ W⁻¹ m²) were relatively low. Inorganic-turbid lakes showed highest K _d values (59.8–61.4 m⁻¹), lowest phytoplankton densities (dominated by Bacillariophyta), and Chl a ranged from 14.6 to 18.3 μg l⁻¹. Phytoplankton-turbid lakes showed, in general, high K _d (4.9–58.5 m⁻¹) due to their high phytoplankton abundances. These lakes exhibited the highest Chl a values (14.2–125.7 μg l⁻¹), and the highest productivities and efficiencies (maximum 0.56 mgC mgChla ⁻¹ h⁻¹ W⁻¹ m²). Autotrophic picoplankton abundance, dominated by ficocianine-rich picocyanobacteria, differed among the shallow lakes independently of their type (0.086 × 10⁵–41.7 × 10⁵ cells ml⁻¹). This article provides a complete characterization of phytoplankton structure (all size fractions), and primary production of the three types of lakes from the Pampa Plain, one of the richest areas in shallow lakes from South America. Handling editor: J. Padisak     

Phytoplankton biomass is mainly controlled by hydrology and phosphorus concentrations in tropical hydroelectric reservoirs

Phytoplankton is widely recognized as being regulated mainly by resources (nutrients and light) and predation by higher trophic levels. In reservoirs, these controls also can be modulated by hydrology, for example through the influence of flow pulses generated by the operation of the dam. In this study, we tested the influence of light, nutrients, and zooplankton grazing pressure, and also hydrology (as water residence time) on the phytoplankton biomass in eight tropical hydroelectric reservoirs, which differ in size, morphometry, location, trophic state, and water residence time. Our hypothesis was that, as these reservoirs are used for hydroelectric purposes, the control that would otherwise be exerted on phytoplankton biomass primarily by resource availability and grazing will also be modulated by hydrology. Low phytoplankton biomass (range of system medians = 12–299 μg C l⁻¹) occurred in most systems, except for one highly eutrophic reservoir (median = 1331 μg C l⁻¹). Our data showed that phosphorus was more often likely to be the limiting nutrient in these systems, as assessed through nutrient limitation indexes (nitrogen and phosphorus), based on concentrations and ratios. For most reservoirs, excluding the eutrophic system with high cyanobacteria biomass, seasonal water residence time was the variable that best explained phytoplankton variation among the several environmental variables analyzed in this study (P < 0.0001; adjusted r ² = 0.38). Hydrology was an important and additional factor modulating phytoplankton in these tropical reservoirs, directly removing phytoplankton populations and their potential zooplankton grazers by washout, and also affecting nutrient availability.     

A study of the diatom-dominated microplankton summer assemblages in coastal waters from Terre Adélie to the Mertz Glacier, East Antarctica (139°E–145°E)

In January 2004 the microplankton community from the coastal waters of Terre Adélie and Georges V Land (139°E–145°E) was studied. Results showed a diatom-dominated bloom with chlorophyll a levels averaging 0.64 μg l⁻¹ at 5 m depth (range 0.21–1.57 μg l⁻¹). Three geographic assemblages of diatoms were identified, based on principal diatom taxa abundances. The stratified waters near the Mertz Glacier presented highest phytoplankton biomasses (0.28–1.57 μg Chl a l⁻¹ at 5 m) and diatom abundances (6,507–70,274 cells l⁻¹ at 5 m), but low diversity, dominated by Fragilariopsis spp. Lower biomasses (0.38–0.94 μg Chl a l⁻¹ at 5 m) and abundances (394–9,058 cells l⁻¹ at 5 m) were observed in the mixed waters around the Astrolabe Glacier with a diverse diatom community characterised by larger species Corethron pennatum and Rhizosolenia spp. Finally an intermediate zone between them over the shallower shelf waters of the Adélie Bank represented by Chaetoceros criophilus, where biomasses (0.21–0.35 μg Chl a l⁻¹ at 5 m) and abundances (1,190–5,431 cells l⁻¹ at 5 m) were lowest, coinciding with the presence of abundant herbivorous zooplankton.     

Seasonality and controls of phytoplankton productivity in the middle Cape Fear River, USA

Our 1 year study was aimed at assessing seasonal patterns and controls on phytoplankton primary production (PPR) and biomass (chlorophyll a) in a fourth order section of the middle Cape Fear River in North Carolina, USA, and to determine the impact of three low-head lock and dam (LD) structures on these variables within the 70 km study reach of this coastal river. Mean concentrations of NO₃ ⁻–N, NH₄ ⁺–N and soluble reactive phosphorus (SRP) averaged 52.9, 6.0, and 3.6 μmol l⁻¹ in monthly sampling, while the average light attenuation coefficient was 2.4 m⁻¹. The average euphotic depth was 2.1 m. Nutrient concentrations and attenuation coefficients were not significantly different above versus below each LD, or along the entire study reach. Significantly higher concentrations of dissolved O₂ below versus above each LD were attributed to re-aeration during spillway transit. No seasonal pattern in physicochemical properties was apparent. Phytoplankton chlorophyll a concentrations ranged from <1 to 36 μg l⁻¹, while rates of primary production ranged from 18 to 2,580 mg C m⁻² day⁻¹, with values for both variables peaking in the spring and early summer. Chlorophyll a and primary productivity values were consistently higher above versus below each LD in May and June suggesting a seasonal effect, but values were otherwise similar such that overall means were not significantly different. Several factors point to light as the primary control on phytoplankton in the middle Cape Fear River: high nutrient concentrations; a low ratio of euphotic : mixing depth (0.46); progressive increases in chlorophyll a and radiocarbon uptake in all treatments in quarterly nutrient enrichment bioassays conducted at levels of irradiance elevated relative to in situ river values; and consistently low quarterly values of (maximum rate of chlorophyll-normalized C uptake; ≤3.7 mg C mg chl a⁻¹ h⁻¹) and I _k (light saturation parameter; ≤104 μmol photons m⁻² s⁻¹) for photosynthetic light–response (P–I) curves. Handling editor: L. Naselli-Flores     

Response of phytoplankton and bacteria to nutrients and zooplankton: a mesocosm experiment

Although both nutrient inputs and zooplankton grazing are importantto phytoplankton and bacteria in lakes, controversy surroundsthe relative importance of grazing pressure for these two groupsof organisms. For phytoplankton, the controversy revolves aroundwhether zooplankton grazers, especially large cladocerans likeDaphnia, can effectively reduce phytoplankton populations regardlessof nutrient conditions. For bacteria, little is known aboutthe balance between possible direct and indirect effects ofboth nutrients and zooplankton grazing. However, there is evidencethat bacteria may affect phytoplankton responses to nutrientsor zooplankton grazing through direct or apparent competition.We performed a mesocosm experiment to evaluate the relativeimportance of the effects of nutrients and zooplankton grazingfor phytoplankton and bacteria, and to determine whether bacteriamediate phytoplankton responses to these factors. The factorialdesign crossed two zooplankton treatments (unsieved and sieved)with four nutrient treatments (0, 0.5, 1.0 and 2.0 µgphosphorus (P) l^–1 day^–1 together with nitrogen(N) at a N:P ratio of 20:1 by weight). Weekly sieving with 300µm mesh reduced the average size of crustacean zooplanktonin the mesocosms, decreased the numbers and biomass of Daphnia,and increased the biomass of adult copepods. Nutrient enrichmentcaused significant increases in phytoplankton chlorophyll a(4–5x), bacterial abundance and production (1.3x and 1.6x,respectively), Daphnia (3x) and total zooplankton biomass (2x).Although both total phytoplankton chlorophyll a and chlorophylla in the <35 µm size fraction were significantly lowerin unsieved mesocosms than in sieved mesocosms, sieving hadno significant effect on bacterial abundance or production.There was no statistical interaction between nutrient and zooplanktontreatments for total phytoplankton biomass or bacterial abundance,although there were marginally significant interactions forphytoplankton biomass <35 µm and bacterial production.Our results do not support the hypothesis that large cladoceransbecome less effective grazers with enrichment; rather, the differencebetween phytoplankton biomass in sieved versus unsieved zooplanktontreatments increased across the gradient of nutrient additions.Furthermore, there was no evidence that bacteria buffered phytoplanktonresponses to enrichment by either sequestering P or affectingthe growth of zooplankton.     

Thresholds and Stability of Alternative Regimes in Shallow Prairie–Parkland Lakes of Central North America

Numerous studies have demonstrated alternative regimes in shallow lake ecosystems around the world, with one state dominated by submerged macrophytes and the other by phytoplankton. However, the stability of each regime, and thresholds at which lakes shift to the alternative regime, are poorly known. We used a cross-sectional analysis of 72 shallow lakes located in prairie and parkland areas of Minnesota, USA, during 2005 and 2006 to assess the occurrence of alternative regimes and shifts between them. Cluster analysis revealed two distinct groups of lakes characterized not only by different macrophyte abundance and chlorophyll a levels but also by different total phosphorus–chlorophyll a relationships. Thirty-nine lakes were macrophyte- and 23 lakes phytoplankton-dominated in both years, whereas 10 sites shifted sharply between those regimes. We failed to detect a universal shifting threshold in terms of chlorophyll a or total phosphorus. However, 95% of the lakes with chlorophyll a concentrations less than 22 μg l⁻¹ were in a clear-water regime, whereas 95% of the lakes with chlorophyll a higher than 31 μg l⁻¹ were in a turbid regime. Total phosphorus less than 62 μg l⁻¹ was an accurate predictor of lakes in a stable clear-water regime, whereas a large change in biomass of planktivores and benthivores between years was the only variable weakly related to regime shifts. Our results support the theoretical prediction that regime thresholds vary among lakes. We recommend that lake managers focus on improving resilience of clear regimes in shallow lakes by reducing nutrient loading, rather than attempting to identify and manage complex triggers of regime shifts. Author contributions KDZ, MAH, BRH, and MLK all contributed to the design of the study, performed the research, analyzed data, and helped write the article.     

Abundance and composition of the summer phytoplankton community along a transect from the Barguzin River to the central basin of Lake Baikal

The abundance and composition of phytoplankton were investigated at six stations along a transect from the Barguzin River inflow to the central basin of Lake Baikal in August 2002 to clarify the effect of the river inflow on the phytoplankton community in the lake. The water temperature in the epilimnion was high near the shore at Station 1 (17.3°C), probably due to the higher temperature of the river water, and gradually decreased offshore at Station 6 (14.5°C). Thermal stratification developed at Stations 2–6, and a thermocline was observed at a 17–22-m depth at Stations 2–4 and an 8–12-m depth at Stations 5 and 6. The concentrations of nitrogen and phosphorus nutrients in the epilimnion at all stations were <1.0 μmol N l⁻¹ and <0.16 μmol P l⁻¹, respectively. Relatively high concentrations of nutrients (0.56–7.38 μmol N l⁻¹ and 0.03–0.28 μmol P l⁻¹) were detected in the deeper parts of the euphotic zone. Silicate was not exhausted at all stations (>20 μmol Si l⁻¹). The chlorophyll a (chl. a) concentration was high (>10 μg l⁻¹) near the shore at Station 1 and low (<3 μg l⁻¹) at five other stations. The <2 μm fraction of chl. a in Stations 2–6 ranged between 0.80 and 1.85 μg l⁻¹, and its contribution to total chl. a was high (>60%). In this fraction, picocyanobacteria were abundant at all stations and ranged between 5 × 10⁴ and 5 × 10⁵ cells ml⁻¹. In contrast, chl. a in the >2 μm fraction varied significantly (0.14–11.17 μg l⁻¹), and the highest value was observed at Station 1. In this fraction, the dominant phytoplankton was Aulacoseira and centric diatoms at Station 1 and Cryptomonas, Ankistrodesmus, Asterionella, and Nitzschia at Stations 2–6. The present study demonstrated the dominance of picophytoplankton in the pelagic zone, while higher abundance of phytoplankton dominated by diatoms was observed in the shallower littoral zone. These larger phytoplankters in the littoral zone probably depend on nutrients from the Barguzin River.     

Can Simple Empirical Equations Describe the Seasonal Dynamics of Bacterioplankton in Lakes: An Eight-Year Study in Shallow Hypertrophic and Biologically Highly Dynamic Lake Søbygård,Denmark

Abstract Seasonal variation in bacterioplankton abundance, biomass, and bacterioplankton production was studied over eight years in hypertrophic Lake S?byg?rd. Biologically, the lake is highly variable; this is due mainly to large interannual variation in fish recruitment. Bacterioplankton production was low during winter, typically 1–3 × 10⁷ cells l⁻¹ h⁻¹, and high during summer, albeit greatly fluctuating with maximum rates typically ranging from 60 to 90 × 10⁷ cells l⁻¹ h⁻¹ (or 0.4 to 0.6 mg C l⁻¹ day⁻¹). Less pronounced variations were found in bacterioplankton abundance, which typically ranged from 3–8 × 10⁹ cells l⁻¹ in winter to 15–30 × 10⁹ cells l⁻¹ during summer. The specific growth rate of bacterioplankton varied from 0.02–0.2 d⁻¹ in winter to 0.5–2.3 day⁻¹ during summer. Interpolated mean bacterioplankton production, in terms of carbon, ranged from 0.08 to 0.16 mg C l⁻¹ day⁻¹, corresponding to 1.6–5.5% of the phytoplankton production, while biomass ranged from 0.28 to 0.36 mg C l⁻¹, corresponding to 1.9–4.6% of the phytoplankton biomass. We conducted regression analysis, relating the bacterioplankton variables to a number of environmental variables, and evaluated the interannual parameter variability. Chlorophyll a and phytoplankton production contributed less to the variation in the bacterioplankton variables than in most previous analyses using data from less eutrophic systems. We suggest that the proportion of phytoplankton production that is channelized through bacterioplankton in lakes decreases with increasing trophic state and decreasing mean depth. This probably reflects a concurrent increase in fish predation on macrozooplankton and loss by sedimentation. An important part of the residual variation in the equations hitherto proposed in the literature could be explained by variation in macrozooplankton biomass and pH > 10.2. A negative effect of high pH on bacterioplankton production was confirmed by laboratory experiments. The impact of different zooplankton varies considerably, with Daphnia seeming to have a negative impact on bacterioplankton abundance and, thereby, indirectly on bacterioplankton production, while Bosmina, rotifers, and cyclopoid copepods seem to stimulate both abundance and production. Bosmina apparently also stimulate the bacterioplankton specific growth rate. Received: 8 February 1996; Accepted: 16 July 1996     

Field and experimental studies on the combined impacts of cyanobacterial blooms and small algae on crustacean zooplankton in a large,eutrophic, subtropical,Chinese lake

Field and experimental studies were conducted to evaluate the combined impacts of cyanobacterial blooms and small algae on seasonal and long-term changes in the abundance and community structure of crustacean zooplankton in a large, eutrophic, Chinese lake, Lake Chaohu. Seasonal changes of the crustacean zooplankton from 22 sampling stations were investigated during September 2002 and August 2003, and 23 species belonging to 20 genera were recorded. Daphnia spp. dominated in spring but disappeared in mid-summer, while Bosmina coregoni and Ceriodaphnia cornuta dominated in summer and autumn. Both maximum cladoceran density (310 ind. l⁻¹) and biomass (5.2 mg l⁻¹) appeared in autumn. Limnoithona sinensis, Sinocalanus dorrii and Schmackeria inopinus were the main species of copepods. Microcystis spp. were the dominant phytoplankton species and formed dense blooms in the warm seasons. In the laboratory, inhibitory effects of small colonial Microcystis on growth and reproduction of Daphnia carinata were more remarkable than those of large ones, and population size of D. carinata was negatively correlated with density of fresh large colonial Microcystis within a density range of 0–100 mg l⁻¹ (r = −0.82, P< 0.05). Both field and experimental results suggested that seasonal and long-term changes in the community structure of crustacean zooplankton in the lake were shaped by cyanobacterial blooms and biomass of the small algae, respectively, i.e., colonial and filamentous cyanobacteria contributed to the summer replacement of dominant crustacean zooplankton from large Daphnia spp. to small B. coregoni and C. cornuta, while increased small algae might be responsible for the increased abundance of crustacean zooplankton during the past decades.     

Epipelic and pelagic primary production in Alaskan Arctic lakes of varying depth

We compared on eight dates during the ice-free period physicochemical properties and rates of phytoplankton and epipelic primary production in six arctic lakes dominated by soft bottom substrate. Lakes were classified as shallow ( < 2.5 m), intermediate in depth (2.5 m <  < 4.5 m), and deep ( > 4.5 m), with each depth category represented by two lakes. Although shallow lakes circulated freely and intermediate and deep lakes stratified thermally for the entire summer, dissolved oxygen concentrations were always >70% of saturation values. Soluble reactive phosphorus and dissolved inorganic nitrogen (DIN = NO₃ ⁻–N + NH₄ ⁺–N) were consistently below the detection limit (0.05 μmol l⁻¹) in five lakes. However, one lake shallow lake (GTH 99) periodically showed elevated values of DIN (17 μmol l⁻¹), total-P (0.29 μmol l⁻¹), and total-N (33 μmol l⁻¹), suggesting wind-generated sediment resuspension. Due to increased nutrient availability or entrainment of microphytobenthos, GTH 99 showed the highest average volume-based values of phytoplankton chlorophyll a (chl a) and primary production, which for the six lakes ranged from 1.0 to 2.9 μg l⁻¹ and 0.7–3.8 μmol C l⁻¹ day⁻¹. Overall, however, increased resulted in increased area-based values of phytoplankton chl a and primary production, with mean values for the three lake classes ranging from 3.6 to 6.1 mg chl a m⁻² and 3.2–5.8 mmol C m⁻² day⁻¹. Average values of epipelic chl a ranged from 131 to 549 mg m⁻² for the three depth classes, but levels were not significantly different due to high spatial variability. However, average epipelic primary production was significantly higher in shallow lakes (12.2 mmol C m⁻² day⁻¹) than in intermediate and deep lakes (3.4 and 2.4 mmol C m⁻² day⁻¹). Total primary production (6.7–15.4 mmol C m⁻² day⁻¹) and percent contribution of the epipelon (31–66%) were inversely related to mean depth, such that values for both variables were significantly higher in shallow lakes than in intermediate or deep lakes. Handling editor: L. Naselli-Flores     

A reliable protocol for plant regeneration from callus culture of <Emphasis Type="Italic">Phalaenopsis</Emphasis>

Summary Callus of Phalaenopsis Nebula was induced from seed-derived protocorms on 1/2 Murashige and Skoog (MS) basal medium plus 0–1.0 mg l⁻¹ (0–4.52 μM) N-phenyl-N′-1,2,3,-thiadiazol-5-yl urea (TDZ) and/or 0–10 mg l⁻¹ (0–45.24 μ M) 2,4-dichlorophenoxyacetic acid (2,4-D). Protocorms 2 mo. old performed better than 1-mo.-old protocorms for callus induction. More calluses formed on 1/2 MS basal medium supplemented with 0.1–1.0 mg l⁻¹ (0.45–4.52 μM) TDZ. These calluses could be maintained by subculturing every month with basal medium supplemented with 0.5 mg l⁻¹ (2.27 μM) TDZ and 0.5 mg l⁻¹ (2.26 μM) 2,4-D. Protocorm-like bodies were formed, and plants regenerated from these calluses on 1/2 MS basal medium alone or supplemented with 0.1–1.0 mg l⁻¹ (0.45–4.52 μM) TDZ. Plantlets were then potted on sphagnum moss in the greenhouse and grew well. No chromosomal abnormalities were found among the root-tip samples of 21 of the regenerated plantlets that were successfully acclimatized.     

Changes in bacterial and ciliate densities with trophic status in Mediterranean shallow lakes

Ciliate and bacterial densities and their link with eutrophication were studied in fourteen shallow lakes in northwest Spain. Total phosphorus (TP) in these lakes varied between 30 μg l⁻¹ and 925 μg l⁻¹ and chlorophyll a concentration (chla) between 0.5 μg l⁻¹ and 107 μg l⁻¹. Bacterial abundance ranged from 1 × 10⁶ to 14 × 10⁶ cells ml⁻¹, while ciliate abundance ranged from 0.6 cells ml⁻¹ to 229 cells ml⁻¹. Lakes were classified into three trophic types from their TP and chla concentrations. Bacterial abundance was significantly correlated with trophic type, as well as with TP and with chla separately, whereas ciliate abundance was only correlated with chla. No significant relationship could be established between bacterial and ciliate abundance across the trophic gradient. A general pattern was observed in the ratios of bacterial abundance to TP and chla concentrations, of decreasing ratios with increases in the nutrient loading. This pattern was not found for ciliates. The dominant zooplankton group in 13 of the 14 lakes studied was Rotifera, which accounted for a mean of 71% of total zooplankton abundance (41% of zooplankton biomass). The positive correlation between bacteria and ciliates with this group, and the absence of any relationship with Cladocera suggest that top down control by cladocerans was weaker in our lakes than previously shown in northern European shallow lakes. Rotifers could be important predators of bacteria in the high-nutrient lakes of our study. Higher slopes of regressions on bacterial abundance towards the hypertrophic range indicate that top-down control was weaker in our lakes than in northern European shallow lakes.     

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.     

Phytoplankton biomass in the sub-Antarctic area of the Straits of Magellan (53°S), Chile during spring-summer 1997/1998

In order to provide a better understanding of the dynamics of phytoplankton in the coastal regions of high latitudes, a study was carried out to estimate the dynamics of carbon biomass of autotrophic and heterotrophic algal groups over the austral spring-summer 1997/1998 period. At a fixed station located in the central basin (Paso Ancho) of the Straits of Magellan (53°S), surface water samples were collected at least once a week from September 1997 (early spring) to March 1998 (late summer). Quantitative analysis of biomass of phytoplankton was estimated from geometric volumes, using non-linear equations, and converted to biomass. The pattern of chlorophyll a showed a strong temporal variability, with maximum values (mean 2.8 mg m⁻³) at the austral spring phytoplankton increase or bloom (October/November) and minimum values during early spring (September: <0.5 mg m⁻³) and summer (January/March: 0.5–1.0 mg m⁻³). During the spring bloom, diatoms made up to 90% of the total phytoplankton carbon (0.01–189 μg l⁻¹), followed by a maximum of thecate dinoflagellates (0.08–34 μg l⁻¹), and sporadic high biomass of phytoflagellates during summer. Heterotrophic algal groups such as Gymnodinium and Gyrodinium spp. dominated (70%, in the 5- to 25-μm size range) shortly before the main diatom bloom, and small peaks were observed within spring and early summer periods (0–0.4 μg l⁻¹). Phytoflagellates dominated earlier (spring) with higher carbon biomass (8 μg l⁻¹) and post-bloom periods (summer) when carbon biomass ranged between 1 and 4 μg l⁻¹. Accepted: 6 September 2000