Seasonal coupling between phyto- and bacterioplankton in a sand pit lake (Créteil lake,France)

Phyto- and bacterioplankton biomass and activity were simultaneously measured during the course of one year in the shallow Créteil Lake (France).Phytoplankton was dominated, during the whole year, by small-sized organisms (10 to 25 µm). Bacteria were in a majority small coccoids (<0.3 µm). Phyto -and bacterioplankton abundances averaged respectively 3.3 × 10⁶ cells l^–1 and 6 × 10⁹ cells l^–1.The phasing of the activity and biomass periods suggest a close coupling between phyto- and bacterioplankton. There were two distinct periods of high activity and biomass. Maximal values were observed in summer but an early increase occurred also in winter. Low or undetectable phytoplankton excretion rates, when heterotrophic activity was maximum, indicated a bacterial uptake of up to 100% of the released algal products during the incubation period. Heterotrophic uptake measurements with both glucose and amino acids revealed a seasonal change of the substrates in the lake, glucose uptake being associated more with the maximum activity of the algae, while the amino acids uptake was relatively higher during their decline.The maximal photosynthetic rate averaged 21.5 mgC m^–3 h^–1 and mean Vmax values were 0.056 and 0.050 mgC m^–3 h^–1 respectively for glucose and amino acids uptake.     

Primary production and phytoplankton composition in relation to DOC input and bacterioplankton production in humic Lake Örträsket

1. The biomass and production of picophytoplankton, large phytoplankton and heterotrophic bacterioplankton were measured in humic Lake Örträsket, northern Sweden during four consecutive summers.
2. High flow episodes, carrying fresh dissolved organic carbon (DOC) into the lake, always stimulated heterotrophic bacterial production at the expense of primary production. Primary production never exceeded bacterial production for approximately 20 days after such an episode had replenished epilimnial DOC. We suggest that allochthonous DOC is an energy source that stimulates bacterioplankton that, because of their efficient uptake of inorganic nutrients, are then able to outcompete phytoplankton. After the exhaustion of readily available DOC, phytoplankton were able to dominate epilimnion production in Lake Örträsket.
3. Biomass production was higher when dominated by phytoplankton than by bacterioplankton, despite a similar utilization of nutrients in the epilimnion throughout the summer. We propose that different C : N : P ratios of bacterioplankton and phytoplankton permit the latter to produce more carbon (C) biomass per unit of available inorganic nutrients than bacterioplankton.     

Microbial activity under the ice cover of the shallow Neusiedler See (Austria, Central Europe)

In an attempt to assess bacterioplankton production and growth yieldunder low temperature conditions and to compare bacterioplankton withphytoplankton production in the ice-covered water column of the shallowNeusiedler See, outdoor measurements under near in situ conditions wereperformed during the winter of 1995/96. During the investigation period,mean chlorophyll (Chl) a concentration was 21.03 ± 14.95 µg Chla l^-1. Phytoplankton primary production integrated over thewater column ranged from 1.35 to production integrated over the water columnranged from 1.35 to 4.23 mg C m^-2 d^-1 (mean± SD = 2.46 ± 1.06 mg C m^-2d^-1). Bacterial abundance varied from 20 to 40×10⁵ ml^-1 for most of the investigationperiod and increased by the end of March concomitantly with the increase intemperature from 1.3 to 6.3 °C within 5 days. Mean bacterial productionwas 15.3 ± 12.8 µg C l^-1 d^-1(range: 3.0 to 41.7 µg C l^-1 d^-1) and meanbacterial growth rate 0.23 ± 0.16 d^-1 following closelythe pattern in bacterial production. DOC concentration declined linearlyfrom 20.7 mg C l^-1 to 16.45 mg C l^-1 over the 4months period of ice cover. The contribution of humic substances to thetotal DOC pool declined from 43.6% at the end of November to37.3% at the end of March. Calculated on an area basis, phytoplanktonproduction amounted to only 16% of bacterial production which makesit unlikely that phytoplankton supply substrate for bacterioplankton growthin significant quantities when the lake is ice covered. From the observeddecline in DOC over the investigation period and assuming only negligibleinput of DOC from other sources we calculated an average DOC uptake by thebacterioplankton community of 47.5 µg C l^-1d^-1 resulting in a bacterial growth efficiency of 15.9%for the ice covered conditions. Based on the growth efficiency we estimatethat pelagic primary production amounts to 2.8% of the bacterialcarbon demand. This might indicate that the bacterioplankton in NeusiedlerSee sustain their high growth rates at low temperatures (<2°C formost of the investigation period) by using probably the DOC originating fromthe previous season. This DOM stems most likely from the decay of the reedPhragmites australis and its epiphytes and, probably of minor importance,from phytoplankton leachates.     

Grazing of bacteria and phytoplankton by heterotrophic nanoflagellates in a Baltic Sea sample

Grazing by heterotrophic nanoflagellates on bacteria and phytoplankton was studied in a laboratory experiment, using a natural pelagic community originating from the Tvärminne sea area off the southern coast of Finland. Water was prescreened to remove larger grazers. Four experimental treatments were used: light and dark, with and without added nutrients. The growth of the large heterotrophic flagellates was stimulated by increased production of < 3 m phytoplankton. Clearance rates for heterotrophic nanoflagellates were estimated and were found to be within the range of previously reported values.     

Seasonal Variation of Virioplankton in a Eutrophic Shallow Lake

Lake Donghu is a typical eutrophic freshwater lake in which high abundance of planktonic viruses was recently revealed. In this study, seasonal variation of planktonic viruses were observed at three different trophic sites, hypertrophic, eutrophic, and mesotrophic regions, and the correlation between their abundances and other aquatic environmental components, such as bacterioplankton, chlorophyll a, burst size, pH, dissolved oxygen, and temperature, was analyzed for the period of an year. Virioplankton abundance detected by transmission electron microscope (TEM) ranged from 5.48 × 10⁸ to 2.04 × 10⁹ ml⁻¹ in all the sites throughout the study, and the high abundances and seasonal variations of planktonic viruses were related to the trophic status at the sampled sites in Lake Donghu. Their annual mean abundances were, the highest at the hypertrophic site (1.23×10⁹ ml⁻¹), medium at the eutrophic site (1.19×10⁹ ml⁻¹), and the lowest at the mesotrophic site (1.02×10⁹ ml⁻¹). The VBR (virus-to-bacteria ratio) values were high, ranging from 49 to 56 on average at the three sampled sites. The data suggested that the high viral abundance and high VBR values might be associated with high density of phytoplankton including algae and cyanobacteria in this eutrophic shallow lake, and that planktonic viruses are important members of freshwater ecosystems.     

Temporal plankton dynamics in an oligotrophic maritime Antarctic lake

• 1 The population density, diversity and productivity of the microbial plankton in an oligotrophic maritime Antarctic lake were studied for a 15‐month period between December 1994 and February 1996.
• 2 In the lake, concentrations of nutrients and dissolved organic carbon were uniformly low, temperature varied over a small annual range of 0.1–3 °C, and the surface was ice‐covered except during a period of approximately 6 weeks in summer.
• 3 The total of 57 morphotypes of protozoa observed during the study is a higher taxonomic diversity than previously reported from continental Antarctic lakes, but lower than that found in more eutrophic maritime Antarctic lakes. Likewise, planktonic abundance and productivity were lower than has been reported in other lakes on Signy Island, but generally higher than those of lakes on the Antarctic continent.
• 4 There were marked seasonal and interannual variations in planktonic population density.
• 5 Chlorophyll a concentrations ranged from undetectable to 4.2 µg L^‐1 and the greatest rate of primary productivity measured was 4.5 mg C m^‐3 h^‐1. The phytoplankton was dominated by small chlorophytes and chrysophytes, with phototrophic nanoflagellate abundance ranging from 1.1 × 10³ to 1.2 × 10⁷ L^‐1.
• 6 Bacterial densities of 3.6 × 10⁸ to 1.9 × 10¹⁰ L^‐1 were recorded and bacterial productivity reached a peak of 0.36 µg C L^‐1 h^‐1. Numbers of heterotrophic nanoflagellates between 5.0 × 10⁴ and 1.8 × 10⁷ L^‐1, and of ciliates from undetectable to 1.1 × 10⁴ L^‐1 were observed. Naked amoebae were usually rare, but occasionally reached peaks of up to 1.5 × 10³ L^‐1.

     

Factors influencing the phytoplankton steady state assemblages in a drinking-water reservoir (Ömerli reservoir, Istanbul)

In situ growth of heterotrophic nanoflagellates (HNF) in Lake Donghu, a eutrophic shallow lake in mainland China, was studied from January 1999 to March 2000 using a modified Weisse protocol. The study results indicated that the growth rates of HNF showed pronounced seasonal variation (–0.37–1.25 d^–1), reaching the maximum during spring to early summer. When the water temperature was higher than 25.5°C, HNF growth was inversely proportional to water temperature. There was an effect by bacterial abundance and autotrophic picoplankton on HNF growth that depended on location. HNF biomass was the highest in late spring, and the HNF production ranged from –2.25 to 35.45 mg l^–1 d^–1 with mean of 3.17 mg l^–1d^–1. When considered in the context of biomass and production data for zooplankton in Lake Donghu, it was evident that HNF contributed significantly to the total zooplankton production in Lake Donghu. These in situ studies indicate that temperature and food supply are the major determinants of HNF abundance and productivity.     

Are zooplankton food resources poor in the vegetated littoral zone of shallow lakes?

1. The distribution of zooplankton in shallow lakes is negatively related to macrophyte density. However, the abundance of their food along density gradients of macrophytes is unknown. A common but untested assumption is that food quantity and quality for pelagic zooplankton is poor in the littoral zone owing to the deleterious influence of macrophytes on phytoplankton. 2. We tested this assumption with a combination of a field survey and laboratory experiments. We collected seston samples from the littoral and pelagic zones of four shallow temperate lakes and related food quantity (phytoplankton biovolume) and quality to macrophyte abundance (per cent volume infested). Seston food quality was assessed in three ways: N/C and P/C ratios, polyunsaturated fatty acid content and phytoplankton community composition. In the laboratory, we measured the growth and reproduction of Daphnia pulicaria on diets consisting of seston from the littoral and pelagic zones in one lake. 3. In our four study lakes, food quantity was not significantly influenced by macrophyte abundance, and food quality was generally high. Laboratory experiments showed increased juvenile growth, but no significant change in D. pulicaria reproduction, when feeding on littoral resources compared to pelagic resources. 4. Our results suggest that there is no nutritional cost to pelagic zooplankton inhabiting the littoral zone. Therefore, it is likely that other factors (e.g. predation, abiotic factors) are involved in determining zooplankton habitat use.     

The role of periphytes in the shift between macrophyte and phytoplankton dominated systems in a shallow, eutrophic lake (Lake Taihu, China)

Based on experiments of periphyte response to different trophic levels and their impact on macrophyte production, it was found that the periphyte biomass increased with the nutrient concentrations. Increased trophic level and periphyte biomass resulted in decreased macrophyte photosynthesis. It was suggested that the periphytes could cause resilience and hysteresis in the system shifts between macrophyte and phytoplankton domination. Other factors, such as fish farming, storm induced waves and mechanical destruction, and high water levels could be the perturbations during the system shifts, but these are not the key factors. Instead, the nutrient loading and periphyte abundance could determine the shift in lake ecosystem between macrophyte and phytoplankton domination. This finding could theoretically elucidate the mechanism of ecosystem shifts between macrophyte and phytoplankton domination.     

Top–down control of benthic heterotrophic nanoflagellates by oligochaetes and microcrustaceans in a littoral freshwater habitat

1. We investigated trophic interactions between benthic heterotrophic nanoflagellates (HNF) and oligochaetes and microcrustaceans (cladocerans and copepods) transferred from a silty, littoral freshwater habitat to laboratory microcosms. With a newly adapted experimental design we were able to compute (i) predation rates on benthic HNF by the tested metazoan organisms and (ii) growth rates of the natural benthic HNF population when losses because of this predation were excluded. 2. The experiments covered a temperature range of 4–27 °C and a fivefold variation of predator densities (September 2000–February 2002). For 60% of these experiments, significant predation of oligochaetes and microcrustaceans on benthic HNF was revealed. Predation rates on HNF ranged from 0 to 0.256 day^?1. Growth rates of the benthic HNF assemblage varied from ?0.098 to 0.353 day^?1; they were used to estimate the significance of the measured losses in comparison with possible other loss factors. 3. The data suggested that during the major part of the year a high percentage of the HNF production was consumed in the surficial sediment of the investigated system, resulting in a relatively constant and low HNF standing stock and an uncoupling of benthic bacteria and their protistan grazers. Top–down control by microcrustaceans and oligochaetes was identified as one significant, frequently prevailing regulatory factor, while other parameters responsible for the control of benthic HNF densities remain to be examined.     

Phytoplankton of the extremely acidic mining lakes of Lusatia (Germany) with pH ≤3

Most of the flooded, open-cast lignite mining lakes of Lusatia (Germany) impacted by the oxidation of iron sulphides (pyrite and marcasite) are extremely acidic. Of 32 lakes regularly studied from 1995 to 1998, 14 have a pH <3 (median pH 2.3–2.9). These lakes are typically buffered by high concentrations of Fe (III) and have high conductivity (1000–5000 S cm^–1). Concentrations of dissolved inorganic carbon (DIC) and phosphorus are typically extremely low. These factors result in a very different environment for algae than found in neutral and acid-rain impacted lakes. The planktonic algal flora is generally dominated by flagellates belonging to genera of Chlorophyta (Chlamydomonas), Heterokontophyta of the class Chrysophyceae (Ochromonas, Chromulina), Cryptophyta (Cyathomonas) and Euglenophyta (Lepocinclis, Euglena mutabilis). Near-spherical non-motile Chlorophyta (Nanochlorum sp.), Heterokontophyta of the class Bacillariophyceae (Eunotia exigua, Nitzschia), Dinophyta (Gymnodinium, Peridinium umbonatum), other Chlorophyta (Scourfieldia cordiformis) and Cryptophyta (Rhodomonas minuta) are also found.     

Effects of aquatic macrophytes on water quality and phytoplankton communities in shallow lakes

We investigated aquatic macrophytes, water quality, and phytoplankton biomass and species composition in three shallow lakes with different levels of vegetation cover and nutrient concentration in Kushiro Moor, during August 2000. Trapa japonica can live in a wide range of nutrient levels. This species forms an environment with a steeper extinction of light, higher concentrations of dissolved organic carbon (DOC), lower concentrations of dissolved oxygen (DO) near the bottom, and lower concentrations of nitrate+nitrite and soluble reactive phosphorus (SRP) than other vegetation types. The pH was much higher in a Polygonum amphibium community, and the DO near the bottom did not decrease compared to a T.japonica community in the summer. The relationship between chlorophyll a and the limiting nutrient (total phosphorus (TP) when total nitrogen (TN):TPis 10 and TN/10 when TN:TP is <10) significantly differed between lakes with and without submerged vegetation. The chlorophyll a concentrations at a given nutrient level were significantly lower in water with submerged macrophytes than in water without them. Correspondence analysis showed that the difference in phytoplankton community structure across sites was largely due to the presence or absence of submerged macrophytes, and the ordination of phytoplankton species in the lakes with submerged macrophytes is best explained by environmental gradients of TN, chlorophyll, pH and SRP.     

Experimental evidence of the dynamic effect of free-floating plants on phytoplankton ecology

1. There is an increasing concern to understand the role of free-floating plants (FFP) on the functioning and structure of shallow lakes, particularly the mechanisms by which their dominance is self-stabilizing and how they may outcompete phytoplankton.
2. In a field experiment with mesocosms, we simulated three commonly encountered scenarios in warm temperate shallow lakes: FFP dominance, FFP fluctuation and FFP absence. We explored the effects of several key processes, triggered by FFP dynamics, on the composition, diversity and production of phytoplankton, and on the physicochemical conditions.
3. The effects of persistent floating mats on light interception triggered a complex response by the phytoplankton: species were lost and biomass was low, yet high diversity and productivity potential were maintained. A contrasting phytoplankton response characterized mesocosms lacking FFP, where light was sufficient but nitrogen was limiting. Fluctuating FFP cover brought periodic shifts between these two limiting resources for the phytoplankton, which most probably explain the maintenance of richness, diversity and production in these shallow lakes.
4. These results support the recently proposed framework of alternative stable states, providing experimental field evidence of the mechanisms resulting from the shifts between floating plants and phytoplankton dominance.     

Productivity of high-latitude lakes: climate effect inferred from altitude gradient

Climate change is predicted to be dramatic at high latitudes. Still, climate impact on high latitude lake ecosystems is poorly understood. We studied 15 subarctic lakes located in a climate gradient comprising an air temperature difference of about 6°C. We show that lake water productivity varied by one order of magnitude along the temperature gradient. This variation was mainly caused by variations in the length of the ice‐free period and, more importantly, in the supply of organic carbon and inorganic nutrients, which followed differences in terrestrial vegetation cover along the gradient. The results imply that warming will have rapid effects on the productivity of high latitude lakes, by prolongation of ice‐free periods. However, a more pronounced consequence will be a delayed stimulation of the productivity following upon changes of the lakes terrestrial surroundings and subsequent increasing input of elements that stimulate the production of lake biota.     

Diel dynamics of bacterioplankton activity in eutrophic shallow Lake Võrtsjärv,Estonia

The diel dynamics of bacterio- and phytoplankton as main compartments in the pelagic foodweb were followed in order to assess the coupling between algal photosynthesis and bacterial growth during a diel cycle in Lake Võrstjärv, Estonia. Three diurnal studies were carried out, on July 12th–13th, 1994; on June 25th–26th, 1995 and on July 17th–18th, 1995 with a sampling interval of 3–4 hours. Diel variations in bacterial number, biomass and productivity, in phytoplankton primary production and extracellular release of photosynthetic products, in ciliate number and biomass were followed. Phytoplankton was dominated by filamentous species: Limnothrix redekei, Oscillatoria sp., Aulacoseira (Melosira) ambigua and Planktolyngbya limnetica. The abundance of bacteria ranged from 4.1 to 14.6 · 1012 cells m-2 (median 9.88). The production of heterotrophic bacteria varied from 0.6 to 11 mgC m-2 h-1 (median 3.65), the variation during diel cycle was high. Depth integrated values of particulate (PPpart) and extracellular primary production (PPdiss) ranged from 6 to 55 and from 17 to 90 mgC m- 2 h-1, respectively. About 50 ciliate taxa were identified among them more abundant were bacterivores, bacterivores- herbivores and omnivores. Biomass of bacterivorous ciliates (TCbact) varied from 8 to 427 mgC m-2. Bacterioplankton production constituted not more than 20% of total primary production (particulate + released), dynamics of bacterial production was related to the primary production, the correlation was negative with PPpart and positive with PPdiss. Different types of potential controlling factors of bacterioplankton (N and P nutrient control, bottom-up control by food and top-down control) are discussed.     

Seasonal dynamics of phytoplankton and phytobenthos,and associated chemical interactions,in a shallow upland lake (Malham Tarn,northern England)

Seasonal changes of phytoplankton were followed over 3 years (1985–87) in a shallow, unstratified and calcareous upland lake.The phytoplankton was of low to moderate abundance and generally dominated by phytoflagellates. Seasonality involved a winter minimum of abundance, a spring maximum of diatoms, and often brief increases in summer that included blue-greens, especially the colonial Gloeotrichia echinulata. Some components were of benthic origin. Seasonal growth of the main component of the phytobenthos, Chara globularisvar. virgata, caused a regular summer depletion in lake water of Ca²⁺ and HCO₃ ^- (alkalinity) by associated CaCO₃ deposition, and a more extreme (and unusual) depletion of K⁺. Chemical analysis of Chara biomass and of underlying sediments indicated a large benthic nutrient stock, much surpassing that represented by the phytoplankton. Growth in this biomass, and the magnitude of water-borne inputs, influenced the removals of Ca²⁺, K⁺ and inorganic N. The phytoplankton was probably limited by a low-P medium, to which co-precipitation of phosphate with CaCO₃ may have contributed. A vernal depletion of Si was probably limiting to diatom growth, and appeared to be mainly induced by benthic rather than planktonic diatoms. Examples of long-term change in composition of the phytoplankton and phytobenthos are noted and discussed in relation to the interaction of these components, nutrient enrichment, and possible alternative stable states.