Facilitation of periphyton production by tadpole grazing: functional differences between species

1. This study examined how interactions between resources that vary in edibility, and herbivores that vary in ability to acquire resources, control primary productivity. In a northern California river, grazing on Cladophora glomerata , a relatively inedible filamentous green alga, and its more nutritious epiphytic diatoms, was manipulated by exposing cobbles to tadpoles ( Rana boylii or Hyla regilla ) or excluding tadpoles.
2. Rana indirectly facilitated Cladophora by removing diatoms, whereas Hyla did not significantly change biomass relative to controls. Algal ash-free dry mass on cobbles in Rana treatments was 65 and 72% greater than on controls in two years of investigation (1991 and 1993). Rana decreased epiphytic diatom biovolume by 56% and detritus by 87%.
3. Because nitrogen excretion rates of Hyla and Rana were similar, the differences in effect between the two species were probably due to their roles as consumers rather than as recyclers.
4. The net effect of Rana on periphyton was a 10% increase in areal specific primary productivity (mg O₂ h^–1 m^–2); Hyla caused an 18% decrease. Rana decreased biomass-specific productivity (mg O₂ h^–1 g^–1) 44%; Hyla had no effect.
5. In tadpole exclosures, grazers such as baetid mayfly larvae (mostly Centroptilum sp.) were 4.7 (1991) and 1.8 (1993) times more abundant, and midge larvae (Chironomidae) were 2.5 (1991) and 2 (1993) times more abundant than in Rana enclosures. Invertebrate assemblages in Hyla enclosures, however, were similar to exclosures. Few predatory insects and fish colonized Rana enclosures. Path analyses indicated that Rana affected macroinvertebrates via both interference and exploitation of epiphytic diatoms.     

Micro-environmental characteristics of filamentous algal communities in flowing freshwaters

SUMMARY. 1. Filamentous algae in flowing freshwaters can represent a spatially and temporally distinct sub-habitat for epiphytic diatom communities. This sub-habitat is described in a low discharge, spring-fed stream with extensive filamentous green algal mats, and in a tuft of Cladophora glomerata (L.) Kützing from a large river.
2. Oxygen micro-electrodes, a thermistor current velocity probe, a standard pH probe and water chemistry were employed to assess spatial heterogeneity. Temporal patterns of epiphyte colonization were evaluated on filamentous artificial substrates.
3. There were steep spatial gradients in the low discharge stream. At mid-day, O₂ ranged from 0–1.5 times air-saturated O₂ concentrations, pH varied from 7.25–8.0, and current velocity spanned 0–0.5 m s⁻¹. Areas near the surface of algal mats had high O₂, pH and current velocity. These patterns were correlated with epiphyte community structure.
4. In the interior of C. glomerata tufts O₂ concentration was raised and current velocity depressed compared to the surrounding water, even when external current velocity was as high as 0.4 m s⁻¹.
5. After thirty-five days of colonization of artificial substrate in the low discharge environment, epiphyte communities were similar to those on filamentous atgae. Epiphyte diversity on artificial substrates subsequently decreased compared to natural substrates as did the similarity between the types on substrates, suggesting that microscale renewal of epiphyte habitat (growth of filamentous algae) maintains high epiphytic diversity.     

The affect of temperature on the metabolism and behaviour of an endemic amphipod, Hyalella montezuma, from Montezuma Well, Arizona, U.S.A.

1. Hyalella montezuma is endemic to Montezuma Well, Arizona, and is exposed to minimal diel and seasonal temperature fluctuations in the pelagic zone (21 ± 4 °C). Juvenile H . montezuma feed in the pelagic zone during the day and migrate into the littoral vegetation at night, while adults remain primarily in the littoral vegetation.
2. Oxygen consumption ( V O₂) of adult and juvenile H . montezuma was measured at 20, 25 and 30 °C. The V O₂ of both adult and juvenile H . montezuma increased with temperature. However, the V O₂ of juveniles was significantly greater than that of adults at all temperatures, with greatest divergence at 30 °C where mean juvenile V O₂ (6.31 μl mg^–1 dry weight (DW) h^–1) was almost twice that of adults (3.60 μl mg^–1 DW h^–1).
3. Survivorship of juveniles was significantly lower (54%) at 30 °C than at 27.5 °C (95%) after 4 h, whereas adults showed at least a 93% survivorship at both temperatures.
4. Our data suggest that temperature may have been the proximate cue that elicited the diel horizontal migration of juvenile H . montezuma in Montezuma Well, with the behaviour maintained and enhanced by intensive invertebrate predation in the pelagic and littoral zones.     

The affect of temperature on the metabolism and behaviour of an endemic amphipod, Hyalella montezuma, from Montezuma Well, Arizona, U.S.A.

1. Hyalella montezuma is endemic to Montezuma Well, Arizona, and is exposed to minimal diel and seasonal temperature fluctuations in the pelagic zone (21 ± 4 °C). Juvenile H . montezuma feed in the pelagic zone during the day and migrate into the littoral vegetation at night, while adults remain primarily in the littoral vegetation.
2. Oxygen consumption ( V O₂) of adult and juvenile H . montezuma was measured at 20, 25 and 30 °C. The V O₂ of both adult and juvenile H . montezuma increased with temperature. However, the V O₂ of juveniles was significantly greater than that of adults at all temperatures, with greatest divergence at 30 °C where mean juvenile V O₂ (6.31 μl mg^–1 dry weight (DW) h^–1) was almost twice that of adults (3.60 μl mg^–1 DW h^–1).
3. Survivorship of juveniles was significantly lower (54%) at 30 °C than at 27.5 °C (95%) after 4 h, whereas adults showed at least a 93% survivorship at both temperatures.
4. Our data suggest that temperature may have been the proximate cue that elicited the diel horizontal migration of juvenile H . montezuma in Montezuma Well, with the behaviour maintained and enhanced by intensive invertebrate predation in the pelagic and littoral zones.     

A method for long-term measurement of respiration in intertidal fishes during simulated intertidal conditions

Aquatic and aerial respiration of the amphibious fishes Lipophrys pholis and Periophthalmus barbarus were examined using a newly designed flow-through respirometer system. The system allowed long-term measurements of oxygen consumption and carbon dioxide release during periods of aquatic and aerial respiration. The M o ₂ of L. pholis , measured at 15° C, was 2·1 μmol O₂ g^–1 h^–1 during aquatic and 1·99 μmol O₂ g^–1 h^–1 during aerial exposure. The corresponding values of the M co₂ were 1.67 and 1.59 μmol O₂ g^–1 h^–1 respectively, giving an aquatic respiratory exchange ratio (RER) of 0·80 and an aerial RER of 0·79. The M o₂ of P. barbarus , measured at 28°C, was 4·05 μmol O₂ g^–1 h^–1 during aquatic and 3·44 μmol O₂ g^–1 h^–1 during aerial exposure. The corresponding values of the Mco₂ were 3·29 μmol CO₂ g^–1 h^–1 and 2·63 μmol CO₂ g^–1 h^–1 respectively, giving an aquatic RER of 0·81 and an aerial RER of 0·77. While exposed to air for at least 10 h, both species showed no decrease in metabolic rate or carbon dioxide release. The RER of these fishes equalled their respiratory quotient. After re-immersion an increased oxygen consumption, due to the payment of an oxygen debt, could not be detected.     

Growth, diet and metabolism of common wolf–fish in the North Sea, a fast–growing population

The von Bertalanffy growth parameters for common wolf–fish Anarhichas lupus in the North Sea were: male: L ∞=111·2 cm, t ₀=–0·43 and K =0·12; and female: L ∞=115·1 cm, t ₀=–0·39 and K =0·11, making this the fastest growing stock reported. Resting metabolic rates (RMR±S.E.) and maximum metabolic rates (MMR±S.E.) for six adult common wolf–fish (mean weight, 1·39 kg) at 5° C were 12·18±1·6 mg O₂ kg^–1 h^–1 and 70·65±7·63 mg O₂ kg^–1 h^–1 respectively, and at 10° C were 25·43±1·31 mg O₂ kg^–1 h^–1 and 113·84±16·26 mg O₂ kg^–1 h^–1. Absolute metabolic scope was 53% greater at 10° C than at 5° C. The diet was dominated by Decapoda (39% overall by relative occurrence), Bivalvia (20%) and Gastropoda (12%). Sea urchins, typically of low energy value, occupied only 7% of the diet. The fast growth probably resulted from summer temperatures approximating to the optimum for food processing and growth, but may have been influenced by diet, and reduced competition following high fishing intensity.     

Metabolism of a desert stream

SUMMARY. Rates of photosynthesis and community respiration were determined for benthic assemblages in Sycamore Creek, a Sonoran Desert stream in Arizona. Benthos in this stream can be separated into (1) mats of Cladophora glomerata and associated epiphytes and (2) assemblages of epipelic diatoms and blue-green algae. Community respiration and net photosynthesis were measured for these assemblages using submerged light-dark chambers in situ . Multiple regression analysis was used to predict (1) gross photosynthesis as a function of photosynthetically active radiation, temperature and chlorophyll-α concentration; and (2) community respiration as a function of temperature and biomass.
Calculations suggest that Sycamore Creek is autotrophic during the summer ( P/R = 1.7) and that the rates of gross photosynthesis ( P =8.5 g O₂ m⁻² day⁻¹) and community respiration ( R = 5.1 g O₂ m⁻² day⁻¹) are high for a small stream. Considerable difference exists between the Cladophora mat assemblages, in which mean P is 12.5gO₂m⁻² day⁻¹and the P/R ratio is 2.3, and the epipelic assemblages in which mean P is 4.4 g O₂m⁻² day⁻¹ and P/R is 0.96. The high rate of gross photosynthesis, low litter inputs, high biomass of algae and the intermittent but severe floods that characterize Sycamore Creek indicate that this stream and other similar desert streams are net exporters of organic matter and are, thereby, truly autotrophic stream ecosystems.     

Early development of milkfish: effects of salinity on embryonic and larval metabolism, yolk absorption and growth

During embryogenesis of Chanos chanos , more than half of the yolk was consumed and the majority of it was converted into larval tissue. Salinity affected both yolk absorption and embryonic and larval growth. Larvae hatched in 20% had larger yolk reserves but were smaller and grew more slowly than larvae in 35 and 50%. Larvae hatched in 35 and 50% had equal amounts of yolk but those from 35% were larger. Oxygen consumption rates increased during development (from 0.06 ± 0.01 μl O₂ egg^–1 h^–1 by blastulae to 0.37 ± 0-01 μl O₂ egg^–1 h^–1 by prehatch embryos and 0–43 ± 0–03 μl O₂ larva ^–1 h ^–1 by newly-hatched larvae) and were significantly affected by salinity. Eggs and yolk-sac larvae incubated in 35% consumed more oxygen than those in the low and high salinities. Salinity affected both the rate and pattern of yolk utilization but salinity-related differences in metabolism, yolk absorption, and growth were not related directly to the osmotic gradient. Low salinity retarded yolk absorption while high salinity reduced yolk utilization efficiencies. Differences in oxygen consumption rates were probably related to variations in the relative amounts of metabolically active embryonic and larval tissue and/or higher activity levels rather than differential osmoregulatory costs. 35% is probably the most suitable salinity for incubation and larval rearing of milkfish.     

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

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

Spatial and temporal variation of microbial respiration rates in a blackwater stream

1. The extent of spatial and temporal variation of microbial respiration was determined in a first-order, sand-bottomed, blackwater stream on the coastal plain of south-eastern Virginia, U.S.A.
2. Annual mean respiration rates (as g O₂ m^–3 h^–1) differed significantly among substrata: leaf litter, 12.9; woody debris, 2.4; surface sediment, 0.8; hyporheic sediment, 0.4; water column, 0.003. Rates associated with wood were higher than those with leaves when expressed per unit surface area.
3. Highest respiration rates on leaves, wood and in the water column occurred during the summer, whereas rates in the sediments were greatest during the late autumn and winter. Water temperature, as well as particulate organic matter and nitrogen content of the substrata, was correlated positively with respiration rates.
4. A stepwise multiple regression showed that temperature and nitrogen content together explained 88% of the variation in respiration rates of leaves and wood. In contrast, particulate organic matter content and nitrogen content explained 89–90% of the variation in respiration in the sediments. Although water temperature was a significant factor in the sediment multiple regressions, its addition as an independent variable improved the regression models only slightly.
5. Annual mean respiration in the stream channel, based on the proportional amount of respiration occurring associated with each type of substratum during each month, was 1.1 kg O₂ m^–2 yr^–1. Seventy per cent of respiration in the stream occurred in the hyporheic zone, 8–13% occurred in the surface sediment, leaf litter or woody debris, and < 1% occurred in the water column. Approximately 16% of total detritus, or 40% of non-woody detritus, stored in the stream during the year was lost to microbial respiration.     

Spatial and temporal variation of microbial respiration rates in a blackwater stream

1. The extent of spatial and temporal variation of microbial respiration was determined in a first-order, sand-bottomed, blackwater stream on the coastal plain of south-eastern Virginia, U.S.A.
2. Annual mean respiration rates (as g O₂ m^–3 h^–1) differed significantly among substrata: leaf litter, 12.9; woody debris, 2.4; surface sediment, 0.8; hyporheic sediment, 0.4; water column, 0.003. Rates associated with wood were higher than those with leaves when expressed per unit surface area.
3. Highest respiration rates on leaves, wood and in the water column occurred during the summer, whereas rates in the sediments were greatest during the late autumn and winter. Water temperature, as well as particulate organic matter and nitrogen content of the substrata, was correlated positively with respiration rates.
4. A stepwise multiple regression showed that temperature and nitrogen content together explained 88% of the variation in respiration rates of leaves and wood. In contrast, particulate organic matter content and nitrogen content explained 89–90% of the variation in respiration in the sediments. Although water temperature was a significant factor in the sediment multiple regressions, its addition as an independent variable improved the regression models only slightly.
5. Annual mean respiration in the stream channel, based on the proportional amount of respiration occurring associated with each type of substratum during each month, was 1.1 kg O₂ m^–2 yr^–1. Seventy per cent of respiration in the stream occurred in the hyporheic zone, 8–13% occurred in the surface sediment, leaf litter or woody debris, and < 1% occurred in the water column. Approximately 16% of total detritus, or 40% of non-woody detritus, stored in the stream during the year was lost to microbial respiration.     

Denitrification in river sediments: relationship between process rate and properties of water and sediment

1. A survey was made of denitrification and nitrous oxide (N₂O) production in river sediments at fifty sites in north‐east England during one season in order to investigate the relationship between rates and environmental factors likely to influence these processes. The sites were chosen to represent a wide range of physical and chemical conditions. Denitrification rate and N₂O production were measured within 5 h of sampling using the slurry acetylene blockage technique.
2. Denitrification rate ranged from less than 0.005–260 nmol N g^–1 DW h^–1, tending to increase in a downstream direction. N₂O production ranged from negative values (net consumption) to 13 nmol N₂O‐N g^–1 DW h^–1 and accounted for 0–115% of the N gases produced.
3. Denitrification rate and N₂O concentration in the sediment were correlated positively with nitrate concentration in the water column, water content of the sediment and percentage of fine (< 100 μm) particles in the sediment.
4. The variation in denitrification rate was satisfactorily explained (64% total variance) by a model employing measurements of water nitrate and water content of sediments. No simple or multiple relationship was found for N₂O production.     

Impact of sediment and nutrient inputs on growth and survival of tadpoles of the Western Toad

1. Sediment and nutrient loading in freshwater systems are leading causes of aquatic habitat degradation globally. We investigated the impacts of fine-sediment and nutrient additions on the growth and survival of western toad ( Bufo boreas ) tadpoles and emergent metamorphs in mesocosm and exclosure experiments.
2. Mesocosm tanks received weekly pulses of fine sediments to create initial concentrations of 0, 130 and 260 mg L⁻¹ of suspended sediment and either bi-weekly additions of nutrients (N = 160 μg L⁻¹, P = 10 μg L⁻¹) or no additions in a factorial design. Within mesocosms, tadpole exclosures allowed for quantification of tadpole grazing pressure on periphyton biomass, chlorophyll- a and sediment deposition.
3. Tadpoles receiving sediment additions experienced slower growth rates and reduced survival to metamorphosis, although no effects of treatment were detected on size at metamorphosis or time to metamorphosis. Nutrient additions also lowered survival, but had no impact on other measured parameters of tadpole fitness. Dissections and gut content analysis revealed that tadpoles ingested sediment in large quantities altering the proportion of the organic content of ingested food.
4. Together these results suggest that although sediment was readily consumed by tadpoles, its presence in the larval environment had an overall negative effect on tadpole growth and survival, although not as severe as predicted.     

The seasonal biomass and productivity of the submerged macrophytes in a polluted Wisconsin stream

SUMMARY 1. We measured biomass and light/dark bottle productivity of macrophytes in a Wisconsin stream throughout one growing season. Except for a brief period in early spring when a Cladophora glomerata -filamentous algal community was dominant, Potamogeton pectinatus was the dominant macrophyte species in Badfish Creek.
2. Maximum community biomass was 710 g DW m⁻², with a maximum above ground biomass of 620 g DW m⁻² and a maximum below ground biomass of 120 g DW m⁻². Annual productivity was estimated at 1435 g DW m⁻² year⁻¹, with a calculated P/B of 2.01.
3. In situ net production averaged 2.83g C g AFDW⁻¹ h⁻¹ Net positive carbon gain by the P. pectinatus community occurred when water temperatures were above 15°C, and daylength at least 12h. This is correlated to the onset of tuber germination in spring, and the point of maximal biomass decline in autumn.     

Biomass and oxygen dynamics of the epiphyte community in a Danish lowland stream

SUMMARY. 1. We examined the abundance and oxygen metabolism of epiphytic organisms on the dominant macrophyte, Potamogeton pectinatus , in headwaters of the eutrophic River Suså. Microbenthic algae were abundant in the stream during spring and macrophytes during summer.
2. The low macrophyte biomass in spring supported a dense epiphyte cover whereas the high macrophyte biomass during summer had a thin epiphyte cover of 10–100-fold lower abundance per unit area of macrophyte surface. The epiphyte community was dominated by microalgae in spring and by heterotrophs, probably bacteria, during summer. This seasonal shift was shown by pronounced reductions of the chlorophyll a content (from 2–3% to 0.1–0.7% of organic DW), the gross photosynthetic rate (from 20–85 to 3–15 mg O₂, g-¹ organic DW h⁻¹) and the ratio of gross photosynthesis to dark respiration in the epiphyte community (from 5–18 to 1). The reduced contributions of epiphytic microalgae correlated with reduced light availability during summer.
3. Both the density and the photosynthetic activity of epiphytic algae were low on a stream area basis relative to those of microbenthic algae and macrophytes. Rapid variations in water velocity and extensive light attenuation in water and macrophyte stands probably constrained the development of epiphytic algae. The epiphyte community was more important in overall stream respiration, contributing c. 10% to total summer respiration and c. 20% to summer respiration within the predominantly heterotrophic microbial communities on sediments and macrophyte surfaces.     

Sediment microbial respiration in a synoptic survey of mid‐Atlantic region streams

1. The rate of microbial respiration on fine‐grained stream sediments was measured at 196 first‐ to third‐order sites in the mid‐Atlantic region of the United States.
2. Sample collection took place between April and July in 1993, 1994 and 1995.
3. Study streams were randomly selected from the United States Environmental Protection Agency's (USEPA) River Reach File (RF3) using the sample design developed by USEPAs Environmental Monitoring and Assessment Programme (EMAP).
4. Respiration rate ranged from 0 to 0.622 g O ₂ g^–1 ash‐free dry mass (AFDM) h^–1, and was significantly correlated with stream temperature, water chemistry and physical habitat.
5. Respiration rate was significantly higher in streams from the Chesapeake drainage area compared to those from the Ohio drainage area, and in streams from the Northern Appalachian Plateau and Uplands and Central Appalachian Valleys ecoregions compared with streams from the Blue Ridge and Central Appalachian Plateau ecoregions.
6. Respiration was not significantly different among stream orders or between years.     

Intraspecific stoichiometric variability and the ratio of nitrogen to phosphorus resupplied by zooplankton

1. An in situ experiment was carried out in a high-mountain oligotrophic lake (La Caldera) to quantify the specific release rates of ammonium and phosphorus in different phases of the seasonal succession of plankton. The zooplankton community was dominated by the calanoid copepod Mixodiaptomus laciniatus .
2. The rates of release of N and P (from non-detectable to 3.58 μg N mg^–1 DW h^–1 and from 0.26 to 2.29 μg P mg^–1 DW h^–1, respectively) and the N : P released ratio (not detectable to 4.77) fell within the range typical of oligotrophic lakes dominated by copepodite assemblages.
3. The values of the zooplankton N : P ratio (5 : 1 to 10 : 1 by weight) were lower than those established for other species of calanoids, and followed a well-established pattern of seasonal variation from year to year with higher values after ice thaw and lower values as individual size increased (ontogenetic development).
4. The elemental composition of the zooplankton depended on individual size, while the released N : P ratio was inversely related to the N : P ratio of the food. This pattern conforms to the predictions of the homeostasis theory of Sterner (1990).
5. A feedback regulation was established between the stoichiometric composition of the zooplankton, their food and the released N : P ratio that can help explain changes in pelagic community structure during the ice-free period.     

Relations between suspended bacteria, epiphytic bacteria and submerged vegetation over the spring growing season in a calcareous headstream

SUMMARY. 1. Quantitative observations on suspended bacteria, epiphytic bacteria and submerged vegetation, in a calcareous headstream in North Humberside, were made at regular intervals from January to June 1984.
2. Downstream increase in concentration of suspended bacteria was observed along a 135 m richly-vegetated section but not over a 150 m vegetation-free section, hence there was measurable drift loss of bacteria only from within the vegetated section.
3. The total number of epiphytic bacteria within the vegetated section was estimated from the density of epiphytic bacteria per unit dry weight of submerged vegetation and the total dry weight of submerged vegetation.
4. The total number of epiphytic baeteria within the vegetated section decreased frtim 2.5 × 10¹⁵ in January to 0.8 × l0¹⁵ March before increasing to 4.7 × 10¹⁵ by June. This pattern was related to deerease and subsequent increase in density of epiphytic bacteria per unit dry weight of vegetation. The total dry weight of submerged vegetation inereased throughout the study period.
5. The daily drift loss of bacteria from the vegetated section averaged 53% (range 9–132%) of the total epiphytic bacteria, a loss rate which might reasonably be supported by the epiphyte population.
6. The dimensions and volume of suspended bacteria increased, between the source and the downstream limit of the vegetated section, to resemble those of epiphytic bacteria.
7. The results suggested that released epiphytic bacteria might largely be the source of suspended bacteria in this headstream.     

Photosynthesis and respiration of a diatom biofilm cultured in a new gradient growth chamber

Abstract A diatom biofilm was grown in a chamber developed for culture of biofilms in chemical gradients. The diatoms grew on a polycarbonate membrane filter which separated a sterile reservoir, with added phosphate, from a reservoir without phosphate. Within 3 weeks of inoculation, a thick biofilm developed on the surface of the filter. The biofilms were homogeneous and therefore suitable for calculations of O₂ diffusion fluxes from concentration profiles of O₂. Profiles of O₂, pH, and gross photosynthesis at different light intensities and liquid medium concentrations of dissolved inorganic carbon and O₂ were measured with microelectrodes. Respiratory activity in a layer of the biofilm was determined as the difference between gross photosynthesis and outflux of O₂ from that layer. The photosynthetic activity in a well-developed biofilm grown at 360 μEinst m⁻² s⁻¹ and 2.4 mM HCO₃⁻ was limited by the supply of inorganic carbon. Exposure to light above 360 μEinst m⁻² s⁻¹ stimulated gross photosynthesis as well as respiratory processes without affecting net outflux of O₂. Higher concentrations of inorganic carbon, on the other hand, enhanced gross photosynthesis without concurrent increase in respiratory rate, resulting in an increased outflux of O₂. High concentrations of O₂ in the liquid medium decreased the net outflux of O₂ with little effect on the gross photosynthesis. The effects of inorganic carbon and O₂ on the metabolic activities of the biofilm were consistent with the presence of photorespiratory activity.     

Seasonal dynamics of free and attached heterotrophic nanoflagellates in an oligomesotrophic lake

1. The seasonal development of heterotrophic nanoflagellates (HNF), bacteria, rotiferans and crustacean zooplankton was studied in the epilimnion of Lake Pavin, an oligomesotrophic lake in the Massif Central of France.
2. HNF abundance varied from 0.1 to 2.5 × 10³ mL^–1. Free-living HNF reached their highest density in spring when the copepod Acanthodiaptomus denticornis dominated the metazooplankton. They were present in low numbers when rotifers and cladocerans were numerous.
3. Attached HNF, consisting of bicoecids and choanoflagellates, were fixed to large diatoms and to the colonial cyanobacterium Anabaena flos-aquae . The abundance of attached HNF was significantly correlated to bacterial abundance, which fluctuated between 1.1 and 2.7 × 10⁶ mL^–1. Highest abundance of these epiphytic protists was recorded when free-living heterotrophic nanoflagellates declined.
4. The comparison of the dynamics of heterotrophic nanoflagellates, bacteria, and the impact of zooplankton grazing suggested that prey abundance, the presence of suitable attachment sites and limited competition from the free-living forms were the main factors controlling the development of the epiphytic flagellate protists. In contrast, the low abundance of free-living forms during the period of rotiferan and cladoceran development suggests the prevalence of a top-down control by predation of the metazoopankton.