Net pelagic heterotrophy in mesotrophic and oligotrophic basins of a large, temperate lake

Understanding the effects of trophic status and dissolved organic carbon concentration (DOC) on lake carbon cycling is essential for accurate ecosystem carbon models. Using isotopically labelled substrates we assessed spatial and temporal variability in bacterial respiration (BR) and algal primary production (PP) in two trophically, morphometrically and hydrologically different basins in Loch Lomond, a large temperate lake in Scotland. GIS modelling was used to construct a whole lake balance for bacterial production/respiration and PP, and from this the proportion of heterotrophy fuelled by allochthonous carbon was estimated. We tested the hypotheses that trophic status and DOC concentration affect the balance between PP and BR and examined which is the more significant driving factor. Additionally we estimated the percentage of BR that is fuelled by terrestrial carbon. PP varied seasonally and showed inter-basin homogeneity. BR was greatest in the mesotrophic south basin in autumn, which corresponded to measured peak DOC input, though over an annual cycle no relationship was observed between BR and DOC concentration. The PP:BR ratio was 0.37 ± 0.30 and 0.3 ± 0.45 in the north and south basins, respectively, assuming a bacterial growth efficiency of 0.1. We have found that allochthonous carbon potentially supports a substantial quantity of pelagic production, even during periods of high photosynthesis. Less productive systems are thought to be dominated by heterotrophic processes. However, we have found that the mesotrophic basin of a large lake to be as heterotrophic as its neighbouring oligotrophic basin, an observation that has implications for our understanding of modelling of the role of lakes in linking the terrestrial-atmospheric carbon cycle.     

Internal heterotrophy following the switch from macrophytes to algae in Lake Apopka, Florida

Measurements of photosynthesis and community respiration in Lake Apopka, Florida, U.S.A. indicate that this lake may be heterotrophic, and that the source of extra organic carbon is internal rather than external to the lake. This large and shallow lake (area 124 km², mean depth 1.7 m) was dominated by macrophytes until hurricane-associated winds disrupted the plants in 1947, and the lake switched to a turbid, algal state. A layer of flocculent, organic sediments covers the lakebed to an average depth of 45 cm and winds regularly resuspend the upper portion into the water column. We used the diel oxygen curve method to estimate production and respiration and also reanalyzed the results of five past studies of production in the lake. The production measurements did not support the hypothesis that the flocculent layer represented excess algal production since 1947. Community respiration exceeded gross production on 60 out of 76 days sampled with statistically significant negative net production found in two of the three studies using the light and dark bottle oxygen method. External supplies of organic carbon are relatively small and are balanced by losses through the outlet. If the lake is heterotrophic, the excess respiration is most likely supported by the remains of macrophytes deposited in the sediments prior to the switch to an algal state. Similar sediment oxidation probably occurs in other shallow lakes that switch from the macrophyte to the algal state.     

Microbial carbon processing in oligotrophic Lake Lucerne (Switzerland): results of in situ <Superscript>13</Superscript>C-labelling studies

Although lakes play a major role in the storage of organic carbon, processes involved are not yet very well characterized, especially for oligotrophic lakes. Whether a lake functions as a net source or sink for carbon depends on relative rates of primary production, inputs of terrestrial organic matter and respiration. The microbial community will affect the efficiency of carbon cycling and thereby potential carbon storage. Because the organic matter fluxes are smaller in oligotrophic lakes they have been studied less intensively with respect to their carbon cycling compared to eutrophic lakes. Whether they play an appreciable role in freshwater carbon cycling relies on unraveling primary and secondary production. Here we present the results from such a study in oligotrophic Lake Lucerne, Switzerland. Based on in situ carbon isotopic labelling experiments using dark, glucose-labelled and transparent, DIC-labelled bottles positioned at different depths in the water column, we conclude that even though the photic zone was very deep, integrated primary productivity was consistently low. The carbon processing efficiency of the heterotrophic producers was such that photosynthesized organic matter was fully consumed, even during times of maximum productivity. This implies that the heterotrophic producers were well adapted to rapidly respond to a temporary increase in primary productivity, which is in line with calculated bacterial growth efficiencies in the surface water layer. Highest glucose-based productivity, as a measure of the heterotrophic potential, was observed in the deepest parts of the water column. Chemoautotrophy was shown at 60 m water depth and is of relatively minor importance for overall fluxes. Mixotrophy was recognized as a strategy to keep up production when light conditions become less favorable for autotrophic growth. A mesocosm experiment earlier in the year indicated lower primary production, which agrees well with the timing of this experiment preceding the annual spring bloom. During the low-productivity season the coupling between phytoplankton and bacterial production was much weaker and potentially more organic matter could escape recycling at that time, although quantitatively fluxes remained very low.     

The photosynthetic and growth rate responses of two freshwater angiosperms in lakes of different trophic status: responses to light and dissolved inorganic carbon

SUMMARY 1. Two rooted freshwater macrophytes ( Lagarosiphon major (Ridley) Moss; Myriophyllum triphyllum Orchard) were grown in two lakes of differing trophic status on sediments collected from an eutrophic lake. In the two experiments (summer and winter) in the oligotrophic lake (Lake Taupo, New Zealand), the relative growth rates of both species were approximately double those recorded in the experiments in the eutrophic take (Lake Rotorua, New Zealand). These growth responses occurred even though the light and temperature regimes were similar at the experimental sites in both lakes and the concentrations of inorganic nutrients were higher in the eutrophic lake water.
2. Dissolved inorganic carbon (DIC) analysis of both lake waters indicated that the oligotrophic water had approximately 10-fold more DIC than the eutrophic water. This corresponded, at natural pH, to a 2-fold higher concentration of free-CO₂ in the oligotrophic lake water (49 μM) than in the eutrophic lake water (23 μM). Photosynthetic experiments demonstrated that the maximum rates of photosynthesis were 2.46 and 2.25-fold higher in L. major and M. triphyllum , respectively, when the shoots were incubated in Lake Taupo water at Lake Taupo levels of free-CO₂ relative to Lake Rotorua water at Lake Rotorua levels of free-CO₂.
3. It is concluded that these differences in photosynthetic rates would lead approximately to a 2-fold difference in growth rate, thus explaining the growth responses observed in the field experiments.     

Budding and prosthecate bacteria from freshwater habitats of various trophic states

Budding and prosthecate bacteria were enumerated in spring and summer by viable counting procedures in several freshwater habitats in Australia including oligotrophic lakes, a mesotrophic lake, and eutrophic ponds.Caulobacter spp. were the most numerous type encountered. They were present in the highest concentrations (exceeding 1000/ml) in the mesotrophic lake during the summer. Their proportion to total viable heterotrophic bacteria was also highest (35.1 to 37.7) in this habitat. From 17 to 330/mlCaulobacter spp. were counted in the eutrophic habitats where their proportion to total viable numbers was less than 1.0%. In the oligotrophic lakes they varied from 5 to 23/ml and comprised greater than 5% of the total viable count.Hyphomicrobium- like bacteria were also numerous in the mesotrophic lake and in one oligotrophic lake during the summer sampling period.Ancalomicrobium spp. occurred in high concentrations (130/ml) in the mesotrophic lake. Budding bacteria of thePlanctomyces-Pasteuria group were most numerous in the eutrophic habitats where as many as 240/ml were counted; their proportion to total heterotrophs remained relatively constant regardless of trophic state, however. A similar pattern was observed withProsthecobacter spp.     

The role of meiobenthos in lake ecosystems

It is shown that meiobenthos plays an important role in the secondary production by zoobenthos in lakes, as well as in the degradation of organic matter. In large lakes (Lake Ladoga, Lake Onega, Lake Päijänne, Lake Constance), the ratio of meiobenthic production to the production of macrobenthos is on average 50–61%. In the small Latgalian lakes (Latvia), this proportion is different: in the profundal of these lakes it varies from 92.5% in a naturally clean mesotrophic lake to 0.0004% in the most eutrophic lake, and in the littoral of lakes – from 578–1476% in mesotrophic lakes to 148–306% in eutrophic ones. The level of production of littoral meiobenthos does not depend on the trophic status of the lake, and can be equally high both in undisturbed mesotrophic lakes and in strongly eutrophicated lakes. The intensity of production of the littoral meiobenthos in oligotrophic and mesotrophic lakes, on the one hand, and in eutrophic lakes on the other, are not reliably distinguished. There is a clear tendency for a decrease of the role of profundal meiobenthos with regard to the transformation of energy flows in lake ecosystems, both with an increase in eutrophication and with an increase in the amount of organic matter in the benthal available from phytoplankton.     

Purple Sulfur Bacteria Control the Growth of Aerobic Heterotrophic Bacterioplankton in a Meromictic Salt Lake

In meromictic Mahoney Lake, British Columbia, Canada, the heterotrophic bacterial production in the mixolimnion exceeded concomitant primary production by a factor of 7. Bacterial growth rates were correlated neither to primary production nor to the amount of chlorophyll a. Both results indicate an uncoupling of bacteria and phytoplankton. In the chemocline of the lake, an extremely dense population of the purple sulfur bacterium Amoebobacter purpureus is present year round. We investigated whether anoxygenic phototrophs are significant for the growth of aerobic bacterioplankton in the overlaying water. Bacterial growth rates in the mixolimnion were limited by inorganic phosphorus or nitrogen most of the time, and the biomass of heterotrophic bacteria did not increase until, in autumn, 86% of the cells of A. purpureus appeared in the mixolimnion because of their reduced buoyant density. The increase in heterotrophic bacterial biomass, soluble phosphorus concentrations below the detection limit, and an extraordinarily high activity of alkaline phosphatase in the mixolimnion indicate a rapid liberation of organically bound phosphorus from A. purpureus cells accompanied by a simultaneous incorporation into heterotrophic bacterioplankton. High concentrations of allochthonously derived dissolved organic carbon (mean, 60 mg of C(middot)liter(sup-1)) were measured in the lake water. In Mahoney Lake, liberation of phosphorus from upwelling purple sulfur bacteria and degradation of allochthonous dissolved organic carbon as an additional carbon source render heterotrophic bacterial production largely independent of the photosynthesis of phytoplankton. A recycling of inorganic nutrients via phototrophic bacteria also appears to be relevant in other lakes with anoxic bottom waters.     

Occurrence of mixotrophic flagellates in relation to bacterioplankton production, light regime and availability of inorganic nutrients in unproductive lakes with differing humic contents

1. Field data from five unproductive Swedish lakes were used to investigate the occurrence of mixotrophic flagellates in relation to bacterioplankton, autotrophic phytoplankton, heterotrophic flagellates and abiotic environmental factors. Three different sources of data were used: (i) a 3‐year study (1995–97) of the humic Lake Örträsket, (ii) seasonal measurements from five lakes with widely varying dissolved organic carbon (DOC) concentrations, and (iii) whole lake enrichment experiments with inorganic nutrients and organic carbon. 2. Mixotrophic flagellates usually dominated over autotrophic phytoplankton in Lake Örträsket in early summer, when both bacterial production and light levels were high. Comparative data from the five lakes demonstrated that the ratio between the biomasses of mixotrophic flagellates and autotrophic phytoplankton (the M/A‐ratio) was positively correlated to bacterioplankton production, but not to the light regime. Whole lake carbon addition (white sugar) increased bacterial biomass, and production, reduced the biomass of autotrophs by a factor of 16, and increased the M/A‐ratio from 0.03 to 3.4. Collectively, the results indicate that the dominance of mixotrophs among phytoplankton was positively related to bacterioplankton production. 3. Whole lake fertilisation with nitrogen (N) and phosphorus (P) demonstrated that the obligate autotrophic phytoplankton was limited by N. N‐addition increased the biomass of the autotrophic phytoplankton but had no effect on mixotrophic flagellates or bacteria, and the M/A‐ratio decreased from 1.2 to 0.6 after N‐enrichment. Therefore, we suggest that bacteria under natural conditions, by utilising allochthonous DOC as an energy and carbon source, are able to outcompete autotrophs for available inorganic nutrients. Consequently, mixotrophic flagellates can become the dominant phytoplankters when phagotrophy permits them to use nutrients stored in bacterial biomass. 4. In Lake Örträsket, the biomass of mixotrophs was usually higher than the biomass of heterotrophs during the summer. This dominance could not be explained by higher grazing rates among the mixotrophs. Instead, ratios between mixotrophic and heterotrophic biomass (the M/H‐ratio) were positively related to light availability. Therefore, we suggest that photosynthesis can enable mixotrophic flagellates to outcompete heterotrophic flagellates.     

Relationships between picophytoplankton and environmental variables in lakes along a gradient of water colour and nutrient content

SUMMARY 1. Biomass and production of picophytoplankton, phytoplankton and heterotrophic bacterioplankton were measured in seven lakes, exhibiting a broad range in water colour because of humic substances. The aim of the study was to identify environmental variables explaining the absolute and relative importance of picophytoplankton. In addition, two dystrophic lakes were fertilised with inorganic phosphorus and nitrogen, to test eventual nutrient limitation of picophytoplankton in these systems.
2. Picophytoplankton biomass and production were highest in lakes with low concentrations of dissolved organic carbon (DOC), and DOC proved the factor explaining most variation in picophytoplankton biomass and production. The relationship between picophytoplankton and lake trophy was negative, most likely because much P was bound in humic complexes. Picophytoplankton biomass decreased after the additions of P and N.
3. Compared with heterotrophic bacterioplankton, picophytoplankton were most successful at the clearwater end of the lake water colour gradient. Phytoplankton dominated over heterotrophic bacteria in the clearwater systems possibly because heterotrophic bacteria in such lakes are dependent on organic carbon produced by phytoplankton.
4. Compared with other phytoplankton, picophytoplankton did best at intermediate DOC concentrations; flagellates dominated in the humic lakes and large autotrophic phytoplankton in the clearwater lakes.
5. Picophytoplankton were not better competitors than large phytoplankton in situations when heterotrophic bacteria had access to a non-algal carbon source. Neither did their small size lead to picophytoplankton dominance over large phytoplankton in the clearwater lakes. Possible reasons include the ability of larger phytoplankton to float or swim to reduce sedimentation losses and to acquire nutrients by phagotrophy.     

Increased Diversity of Predacious <Emphasis Type="Italic">Bdellovibrio</Emphasis>-Like Organisms (BLOs) as a Function of Eutrophication in Kumaon Lakes of India

Predation by Bdellovibrio-like organisms (BLOs) results in bacterial community succession in aquatic ecosystems. The effects of nutrient loading on the distribution and phylogeny of BLOs remain largely unknown. To this end, we present our findings on BLO diversity from four north-Indian lakes that are variable in their trophic status; Nainital is eutrophic, both, Bhimtal and Naukuchiatal are mesotrophic and Sattal remains oligotrophic, respectively. Initially, total heterotrophic bacteria and BLOs were quantified by most probable number (MPN) analyses using Pseudomonas putida and Escherichia coli as prey bacteria. Total bacterial numbers were at least two-logs higher in the eutrophic lake samples compared with oligotrophic lake. Similarly, BLO numbers were approximately 39-fold higher using Pseudomonas sp., which is likely the preferred prey within these lakes. Conversely, significant differences were not observed between mesotrophic and oligotrophic BLO numbers when E. coli was used as the prey. PCR-RFLP of small subunit rDNA (SSU rDNA) of BLOs, followed by cloning, sequencing, and taxonomic categorization revealed distinct differences such that, eutrophic lake consisted of higher BLO diversity compared with mesotrophic and oligotrophic lake, most likely due to both, higher numbers and availability of a diverse population of prey bacteria resulting from nutrient loading in this ecosystem.     

The toxic effects of rubber contaminants on microbial food webs and zooplankton of two lakes of different trophic status

Rubber is commonly used in recreational equipment and devicesforsampling in lakes, but there have been few studies of theeffectsof rubber on planktonic organisms. We investigated the toxiceffects of rubber on the microbial food webs of a mesotrophiclakeand a eutrophic lake. Lake water was collected by pumping via,(i) a polyvinylchloride hose and, (ii) a rubber hose. Samplesoflake water collected by each method were incubated insitu in4.25 l enclosures for four days. The lake water was sampledbeforeand after incubation to determine the concentrations ofinorganicnutrients, chlorophyll a, microorganisms (bacteria,picophytoplankton, flagellates, ciliates) and zooplankton.In the mesotrophic lake, momentary exposure (ten seconds) oflakewater to the rubber hose significantly lowered theconcentrationsof chlorophyll a, bacteria, picophytoplankton and somespecies of zooplankton (Boeckella hamata, Bosmina,androtifers), relative to those in water exposed to the plastichose;flagellates, ciliates and Ceriodaphnia dubia were notsignificantly affected. In the eutrophic lake, the effects oftherubber hose on components of the microbial food web were muchlesssevere, and were consistent with the lake's high levels ofdissolved organic carbon (DOC), which is known to chelatetoxicmetals in water.     

Links between Terrestrial Primary Production and Bacterial Production and Respiration in Lakes in a Climate Gradient in Subarctic Sweden

We compared terrestrial net primary production (NPP) and terrestrial export of dissolved organic carbon (DOC) with lake water heterotrophic bacterial activity in 12 headwater lake catchments along an altitude gradient in subarctic Sweden. Modelled NPP declined strongly with altitude and annual air temperature decreases along the altitude gradient (6°C between the warmest and the coldest catchment). Estimated terrestrial DOC export to the lakes was closely correlated to NPP. Heterotrophic bacterial production (BP) and respiration (BR) were mainly based on terrestrial organic carbon and strongly correlated with the terrestrial DOC export. Excess respiration over PP of the pelagic system was similar to net emission of CO₂ in the lakes. BR and CO₂ emission made up considerably higher shares of the terrestrial DOC input in warm lakes than in cold lakes, implying that respiration and the degree of net heterotrophy in the lakes were dependant not only on terrestrial export of DOC, but also on characteristics in the lakes which changed along the gradient and affected the bacterial metabolization of allochthonous DOC. The study showed close links between terrestrial primary production, terrestrial DOC export and bacterial activity in lakes and how these relationships were dependant on air temperature. Increases in air temperature in high latitude unproductive systems might have considerable consequences for lake water productivity and release of CO₂ to the atmosphere, which are ultimately determined by terrestrial primary production.     

Photoheterotrophy and light-dependent uptake of organic and organic nitrogenous compounds by Planktothrix rubescens under low irradiance

1. Planktothrix rubescens is the dominant photoautotrophic organism in Lake Zürich, a prealpine, deep, mesotrophic freshwater lake with an oxic hypolimnion. Over long periods of the year, P. rubescens accumulates at the metalimnion and growth occurs in situ at irradiance near the photosynthesis compensation point. Experiments were conducted to evaluate the contribution of photoheterotrophy, heterotrophy and light‐dependent uptake of nitrogenous organic compounds to the carbon and nitrogen budget of this cyanobacterium under conditions of restricted availability of light quanta. 2. We used both purified natural populations of P. rubescens from the depth of 9 m and an axenic culture grown under low irradiance at 11 μmol m^?2 s^?1 on a light : dark cycle (10 : 14 h) to determine the uptake rates of various amino acids, urea, glucose, fructose, acetate and inorganic carbon. The components were added to artificial lake water in low amounts that simulated the naturally occurring potential concentrations. 3. The uptake rates of acetate and amino acids (glycine, serine, glutamate and aspartate) were strongly enhanced at low irradiance as compared with the dark. However, no difference was observed in the uptake of arginine, which was taken up at high rates under both treatments. The uptake rates of glucose, fructose and urea were very low under all conditions. Similar results were obtained for both axenic P. rubescens and for purified natural populations of P. rubescens that were separated from bacterioplankton and other phytoplankton. 4. Metalimnetic P. rubescens that was stratified at low irradiance for weeks exhibited much higher uptake rates than filaments that were entrained in the deepening surface mixed layer and experienced higher irradiance. The added organic compounds contributed up to 62% to the total carbon uptake of metalimnetic P. rubescens. On the basis of a molar C : N ratio of 4.9, the nitrogen uptake as organic compounds satisfied up to 84% of the nitrogen demand. 5. The experiments indicate that photoheterotrophy and light‐dependent uptake of nitrogenous organic compounds may contribute significantly to the carbon and nitrogen budget of filaments at low irradiance typical for growth of P. rubescens in the metalimnion and at the bottom of the surface mixed layer.     

Bioturbation potential of chironomid larvae for the sediment–water phosphorus exchange in simulated pond systems of varied nutrient enrichment

Chironomid larvae (2.0 individuals/cm²) were introduced in sediment–water microcosms of 3.0 l capacity to assess the impact of bioturbation on phosphorus flux across sediment–water interface, under different nutrient-enriched conditions. Recruitment of chironomid resulted in 21% and 19% increase in aquatic orthophosphate and nitrate quanta, respectively, with concomitant decrease in nutrient concentration in the sediment compared to macrofauna-free controls under mesotrophic condition. It implied that cost of fertilizer for biological production could be curtailed by at least 19–21% by recovering nutrients stored in the sediment pool. Bioturbation-induced orthophosphate flux under chironomid impacted mesotrophic treatment was 2.3- and 1.8-fold greater than that under bioturbated eutrophic treatment, suggesting that the macrofaunal impact was reduced in the presence of higher nutrient load perhaps due to physicochemical stressors under eutrophic condition. Nevertheless, chironomid larvae can further accelerate nutrient enrichment in the eutrophic system that may invite a “snow ball effect” towards a hypereutrophic one. The counts of both heterotrophic and phosphate solubilizing bacteria show strong positive correlation with orthophosphate concentration in water and the correlation also exists between organic carbon concentration in sediment and phosphate in overlying water. This implied that the accelerated phosphate flux was the result of coordinated eco-engineering activities of chironomid larvae and microbe-mediated mineralization of organic matter.     

Trophic status determines the nirS-denitrifier community in shallow freshwater lakes

To explore whether trophic status is involved in identifying the denitrifier communities in shallow freshwater lakes, comparative studies regarding the variation of nirS-denitrifier communities were performed in water columns of six shallow freshwater lakes, ranging from mesotrophic to hypereutrophic status, in Jiangsu province of China. Restriction fragment length polymorphism (RFLP) analysis and cloning, followed by sequence analysis of selected samples were performed to examine the nirS-denitrifier communities. Results showed that a same predominant nirS unique operational taxonomic unit (OTU) appeared in mesotrophic, light eutrophic and middle eutrophic lakes, while a different nirS OTU appeared in the hypereutrophic lake. The nirS dendrogram could be divided into four clusters, and suggested a habitat-specific observation. The nirS libraries from lakes having mesotrophic and light eutrophic statuses grouped together, while those from lakes having middle eutrophic and hypereutrophic statuses diverged. The hypereutrophic lake had the lowest richness and evenness of nirS. Thus, the observed variation of nirS distribution and diversity was mainly affected by the trophic status, via some environmental factors such as pH, dissolved oxygen, nitrite, nitrate and chlorophyll-a concentrations.     

Seasonal changes in temperature and nutrient control of photosynthesis, respiration and growth of natural phytoplankton communities

1. To investigate the influence of elevated temperatures and nutrients on photosynthesis, respiration and growth of natural phytoplankton assemblages, water was collected from a eutrophic lake in spring, summer, autumn, winter and the following spring and exposed to ambient temperature and ambient +2, +4 and +6 °C for 2 weeks with and without addition of extra inorganic nutrients. 2. Rates of photosynthesis, respiration and growth generally increased with temperature, but this effect was strongly enhanced by high nutrient availability, and therefore was most evident for nutrient amended cultures in seasons of low ambient nutrient availability. 3. Temperature stimulation of growth and metabolism was higher at low than high ambient temperature showing that long‐term temperature acclimation of the phytoplankton community before the experiments was of great importance for the measured rates. 4. Although we found distinct responses to relatively small temperature increases, the interaction between nutrient availability, time of the year and, thus, ambient temperature was responsible for most of the observed variability in phytoplankton growth, photosynthesis and respiration. 5. Although an increase in global temperature will influence production and degradation of organic material in lakes, the documented importance of ambient temperatures and nutrient conditions suggests that effects will be most pronounced during winter and early spring, while the remaining part of the growth season will be practically unaffected by increasing temperatures.     

Characterization of the water quality of dam lakes on Shikoku Island, Japan

This study clarifies the distribution and variation of bioelements in dam lakes of the Shikoku region, Japan. The survey was carried out from 2002 to 2003 on three dam lakes in the Shikoku Mountains with high precipitation levels and on three dam lakes in the Sanuki mountain range with low precipitation levels. By the conventional classification for eutrophication, the dam lakes in the Shikoku Mountains were classified as oligotrophic to mesotrophic, whereas those in the Sanuki mountain range were classified as mesotrophic to eutrophic. Furthermore, for the dam lakes in the Sanuki mountain range, it was indicated that the amount of precipitation directly and indirectly affected lake water quality, such as high nitrogen concentration, low dissolved oxygen concentration, and increase in primary production. Especially, in a hypertrophic dam lake, during the period of low precipitation the surface water flowed out and the water under the thermocline, with no dissolved oxygen, appeared on the lake surface. As a result, the anaerobic environment extended across all the layers. Because a compulsory water-level operation was carried out in the dam lake, it was considered that the effect of the hydrological environment in the catchment area on the dam lake was relatively large in comparison with that in natural lakes.     

On the importance of dissolved organic matter in the nutrition of zooplankton in some lake waters

Summary A steady state, radiotracer technique was used to study the original source of the carbon in zooplankton. The experiments were started in filtered lake water with added inorganic radiocarbon. At the beginning of the experiments, a proportionally insignificant volume of unfiltered water was introduced into the culture, together with some ovigerous zooplankton individuals. Since the radioactivity: carbon ratio in the dissolved inorganic carbon was kept constant, a similar ratio would be expected to develop in the autotrophic phytoplankton. The same ratio would then be expected to develop in the zooplankton, if its sole carbon source was autotrophic phytoplankton.According to the results of this approach dissolved organic matter seems to be an important food resource for zooplankton, particularly in highly humic lakes. This conclusion was confirmed by the finding that zooplankton from these lakes was able to grow and reproduce in experiments started with filtered lake water and conducted in complete darkness.The development of algae was followed over the course of one experiment in highly humic water. The same micro-flagellates reproduced equally well in both light and darkness, which indicates the importance of heterotrophic metabolism in their nutrition. Although there are no direct observations about the food of zooplankton in our experiments, it appears likely that heterotrophic flagellates play an important role as a food of zooplankton in humic waters.The importance of dissolved organic matter in the nutrition of aquatic organisms would seem to be much greater than has generally been recognized. Consequently the prevailing concepts of the structure and functioning of planktonic ecosystem should be thoroughly re-evaluated.     

湖泊生态系统碳汇特征及其潜在碳中和价值研究

为了应对气候挑战,达成碳达峰远景目标,需要正确评估自然资源碳中和价值。湖泊作为具有独特生态、人文价值的地理单元,因碳循环强度高、碳排放总量大,是传统意义上的碳源。通过梳理近期相关研究成果,对比不同类型湖泊碳汇/源状况,湖泊生态系统以一系列碳汇特征表现出潜在的碳中和价值。强烈的光合作用可以使水体CO₂欠饱和,但由呼吸-光合作用、碳酸盐岩溶蚀作用带来的水体碱度、CO₂分压pCO₂提高也有利于湖泊碳汇增益。CO₂在水体中大量溶解,积极参与到湖泊碳循环,将pCO₂高于40 Pa作为判断湖泊为碳源的依据可能忽视了水体碱度上升带来的碳汇。在湖泊沉积物中有机碳的累积受到生态系统光合-呼吸作用的影响,当异养微生物群落能及时分解沉入湖底的衰亡组织、有机质时,沉积物中有机碳不会大量累积,当呼吸对光合的相对滞后,有机碳才会大量累积。湖泊生态系统的生产力决定了固碳能力,是湖泊发挥碳汇效益的重要“碳库”。由水生植物固定下的CO₂总量不如浮游植物,但在过程中发挥了“压舱石”般的稳定作...     

Phytoplankton structure in different lake types in central Finland

Phyloplankton structure and its relation to physical and chemical properties of the water was studied in 58 central Finnish lakes. The biomass ranged from 0.2 to 14.2 g m⁻³ and the number of taxa per sample ranged from 33 to 152. The lakes were grouped into 5 types according to their trophic state: eutrophic, dyseutrophic, mesotrophic, oligotrophic, and acid oligotrophic lakes. The average biomass in eutrophic lakes was 5.57 g m⁻³, in dyseutrophic 3.54 g m⁻³, 1.23 g m⁻³ in mesotrophic, 0.52 g m⁻³ in oligotrophic and 0.39 g ⁻³ in acid oligotrophic lakes. The average number of taxa per sample in the corresponding lake types were 109. 1, 79.3, 97.9, 90.9 and 43.8, respectively. The phytoplankton communities in eutrophic lakes were characterized by blue-green algae (21.2% of total biomass) and green algae (18.7% of total biomass). In dyseutrophic lakes the proportion of green algae was much smaller (7.2% of total biomass) than in eutrophic lakes, whereas the proportion of diatoms and cryptophytes was higher (28.2 and 20.4% of total biomass, respectively). Chrysophytes dominated in the oligotrophic and mesotrophic lakes (27.3–39.9% of total biomass). The contribution of dinoflagellates to the total biomass was highest in the most oligotrophic acidified lakes and in those lakes the relative proportions of blue-green and green algae were much higher than in the typical oligotrophic lakes. The lakes were also grouped into 8 community types according to the dominating algal group. Cyanophyceae- and Chlorophyceae-types characterized the eutrophic lakes, whereas Chrysophyceae-Dinopheceae-type was typical for most oligotrophic lakes. The other 5 types occurred in mesotrophic and oligotrophic lakes but the physical and chemical properties of these lakes did not differ much.