Overturn Period Research Articles

Phytoplankton assemblage shifts with dissolved organic matter properties: A functional perspective for sentinel lakes

Abstract Climate change is altering both dissolved organic matter (DOM) properties and phytoplankton dynamics in lakes. High‐altitude lakes are good sentinels of global change. However, their value as sentinels depends on how well we understand their functioning. This study analysed physical, chemical and biological data during three pre‐defined periods running from complete ice‐cover right to the end of the ice‐free season, in a high‐altitude oligotrophic lake. A functional approach was used to assess how relationships between DOM and the planktonic community vary over time. Phytoplankton functional change was found to occur with shifts in DOM. During the ice‐influenced period, from February to the end of the thaw, the phytoplankton community was dominated by small autotrophs and mixotrophic flagellates and DOM from sediment and terrestrial origin dominated the DOM pool of the lake. Phytoplankton diversity and richness increased during the post‐snowmelt overturn period, when terrestrial DOM dominated the DOM pool. Finally, large siliceous autotrophs, competitive under low nitrogen concentrations and high temperature, dominated almost exclusively during the late summer period (LSP). Increased phytoplankton biomass meant that phytoplankton‐derived DOM was dominant during the LSP. These phenological changes in the phytoplankton community resulted in functional shifts at the base of the food web. Based on the relationships between the variables in the study, it can be deduced that the nature of the relationship between phytoplankton and bacteria progressively shifted from strong top‐down control exerted by phytoplankton over bacteria towards predominantly bottom‐up control at the end of the ice‐free season. Synthesis. Using a field survey starting from complete ice‐cover and lasting right to the end of the ice‐free season in a sentinel lake, we show seasonal shifts in the link between DOM properties and plankton community traits. Differences in environmental conditions and DOM origin explained variation in phytoplankton community structure and function, pointing to seasonal shifts in microbial food web interactions. Our research suggests that, under climate change, the balance between mixotrophy and autotrophy and the control exerted by phytoplankton over bacterioplankton will change in oligotrophic high‐altitude lakes with large, vegetated catchments.

Microbial methane oxidation efficiency and robustness during lake overturn

AbstractMany seasonally stratified lakes accumulate substantial amounts of the greenhouse gas methane in the anoxic zone. Methane oxidizing bacteria in the water column act as a converter, oxidizing methane into carbon dioxide and biomass before it reaches the atmosphere. Current observations and estimates of this methane oxidation efficiency are diverging, especially for the lake overturn period. Here, we combine a model of turbulent mixing, gas exchange, and microbial growth with a comprehensive data set for autumn mixing to quantify the relevant physical and microbial processes for a 16 m deep, wind‐sheltered Swiss lake. Scenario analysis suggests that the methane converter is efficient and robust under a large range of mixing velocities and only rare events of pronounced surface cooling can trigger substantial outgassing. This case study combines in situ observation and a deterministic physically based model and suggests that the frequency of storms may strongly impact methane emissions for similar temperate lakes.

Temporal and spatial variations of CO2 diffuse volcanic degassing on Cuicocha Caldera Lake – Ecuador

Cuicocha Caldera is the youngest eruptive center of Cotacachi-Cuicocha Volcanic Complex, located at the north of Ecuador. The caldera contains a lake of 3.95 km2 surface, and a maximum depth of 148 m. Cuicocha Lake is characterized by the presence of CO2 gaseous diffuse emissions, perceptible as bubbling zones. Since 2011, CO2 diffuse flux measurements have been performed in this lake using the accumulation chamber method. The data obtained from twenty surveys were processed by means of the Graphical Statistical Approach and the Sequential Gaussian Simulation. The results reveal that Cuicocha lake has released a total estimated amount of ~400 kt of CO2 in the period between March 2011 and May 2019, with an average rate of 135 t/day. Furthermore, the spatial and temporal analysis of the data made possible the understanding of the processes occurring in the lake: 1) Lake stratification caused by the seasons seem to favor CO2 accumulation in the hipolimnion and its posterior releasing. Minimum total flux values of ~50 t/day have been estimated during “warm” stratified periods and maximum flux values of ~170 t/day have been recorded during “cold” overturn periods. Additionally, at least two anomalous degassing episodes were identified in 2012–2013, seemingly associated to changes in the volcanic activity also detected through seismicity. 2) Cuicocha CO2 degassing seems to be controlled by the existence of diffuse degassing structures at the lake bottom, which correspond to high permeability zones resulting from the intersection between ~NE-SW and ~WNW-ESE oriented structures. We propose a conceptual model to explain the systematic apparition of CO2 anomalies on specific areas of the lake surface.

Dynamics of dissolved greenhouse gas response to seasonal water mixing in subtropical reservoirs

Although indispensable, significant uncertainty still exists in the underlying processes of the formation, dynamics, and emission of greenhouse gases (GHGs), the critical elements needed for the accurate estimation of greenhouse gas fluxes in inland lakes and reservoirs. Seasonal changes in water thermal stratification and turbulence strongly influence the concentration and emission of dissolved GHGs in water columns. Here, we studied the stratification and overturn processes of water column in the subtropical Lianhe Reservoir during different seasons and determined the dynamics of dissolved CO2, CH4, and N2O in the reservoir. Observation of temperature and analysis of chlorofluorocarbons (CFCs) clearly suggested that stratification of water column occurred in summer, but not in winter. The results showed that while dissolved oxygen (DO) was high in the top 5-m layer (the upper epilimnion layer), it dropped considerably especially below 10 m, resulting in an increase in concentration of CO2 and CH4. The high concentrations of dissolved N2O and CH4 were related to the decomposition of organic matter in the hypolimnion layer under anaerobic conditions after stratification. In winter overturn period, vertical circulants of water not only homogenized the concentration of DO in the water column, but also potentially moved CO2, CH4, and N2O from the bottom to the surface of the reservoir. The estimated GHG flux from the reservoir was - 7.13 mmol m-2 day-1 in summer and 2.14 mmol m-2 day-1 in winter. There was the potential that CO2 fluxes from subtropical lakes and reservoirs are overestimated by traditional geochemical models.

Methane distribution patterns along a transect of Lake Fuxian, a deep oligotrophic lake in China.

Freshwater ecosystems are recognized as one of the important natural methane (CH4) sources, but little is known about the emission hotspots and the effects of algal blooms on CH4 production in deep lakes. In this study, carried out from the littoral (S1), pelagic (S2-S4), and the deepest site (S5), water samples from different depths and sediment cores were collected along the transect of Lake Fuxian, a deep monomictic lake to investigate the spatial-temporal variations of CH4. Dissolved methane concentration observed at the oxic metalimnion was 37.5% and 19.5% higher than that those observed at the epilimnion and at the layer between 80 and 100 m depth, respectively. During the overturn period, the vertical distribution of CH4 in the water column was uniform, with an average concentration of 0.031 ± 0.007 μM in S2-S5. Statistical analysis indicated that the CH4 concentration in the water column was significantly higher in S1 than other sites along the transect during both sampling periods. Sediment CH4 concentration and methane production potential (MPP) were also significantly higher in S1 than in other sites. Along the sediment depth, the maximum MPP was observed at 6-8 cm in S1, but it moved up to the surface layer in S2-S5 in both sampling periods. In addition, stable carbon isotope analysis indicated that the surface sediments in the pelagic zone (S2-S5) mainly comprised autochthonous organic matters. In this zone, MPP had a significantly positive correlation with sediment total organic carbon (TOC) (R2 = 0.401, p < 0.01). In summary, we described the spatial and temporal distributions of CH4 in deep Lake Fuxian, littoral zones are CH4 emission hotspots that can contribute to the CH4 accumulation in the oxic metalimnion layer during the stratification period. In the pelagic zone, autochthonous organic matter was transported into the surface sediment after a massive algal bloom, representing another hotspot for CH4 production.

Numerical study on the response of the largest lake in China to climate change

Abstract. Lakes are sensitive indicators of climate change. There are thousands of lakes on the Tibetan Plateau (TP), and more than 1200 of them have an area larger than 1 km2; they respond quickly to climate change, but few observation data of lakes are available. Therefore, the thermal condition of the plateau lakes under the background of climate warming remains poorly understood. In this study, the China regional surface meteorological feature dataset developed by the Institute of Tibetan Plateau Research, Chinese Academy of Sciences (ITPCAS), MODIS lake surface temperature (LST) data and buoy observation data were used to evaluate the performance of lake model FLake, extended by simple parameterizations of the salinity effect, for brackish lake and to reveal the response of thermal conditions, radiation and heat balance of Qinghai Lake to the recent climate change. The results demonstrated that the FLake has good ability in capturing the seasonal variations in the lake surface temperature and the internal thermal structure of Qinghai Lake. The simulated lake surface temperature showed an increasing trend from 1979 to 2012, positively correlated with the air temperature and the downward longwave radiation while negatively correlated with the wind speed and downward shortwave radiation. The simulated internal thermodynamic structure revealed that Qinghai Lake is a dimictic lake with two overturn periods occurring in late spring and late autumn. The surface and mean water temperatures of the lake significantly increased from 1979 to 2012, while the bottom temperatures showed no significant trend, even decreasing slightly from 1989 to 2012. The warming was the strongest in winter for both the lake surface and air temperature. With the warming of the climate, the later ice-on and earlier ice-off trend was simulated in the lake, significantly influencing the interannual and seasonal variability in radiation and heat flux. The annual average net shortwave radiation and latent heat flux (LH) both increase obviously while the net longwave radiation and sensible heat flux (SH) decrease slightly. Earlier ice-off leads to more energy absorption mainly in the form of shortwave radiation during the thawing period, and later ice-on leads to more energy release in the form of longwave radiation, SH and LH during the ice formation period. Meanwhile, the lake–air temperature difference increased in both periods due to shortening ice duration.

Year-round CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt; and CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; flux dynamics in two contrasting freshwater ecosystems of the subarctic

Abstract. Lakes and wetlands, common ecosystems of the high northern latitudes, exchange large amounts of the climate-forcing gases methane (CH4) and carbon dioxide (CO2) with the atmosphere. The magnitudes of these fluxes and the processes driving them are still uncertain, particularly for subarctic and Arctic lakes where direct measurements of CH4 and CO2 emissions are often of low temporal resolution and are rarely sustained throughout the entire year. Using the eddy covariance method, we measured surface–atmosphere exchange of CH4 and CO2 during 2.5 years in a thawed fen and a shallow lake of a subarctic peatland complex. Gas exchange at the fen exhibited the expected seasonality of a subarctic wetland with maximum CH4 emissions and CO2 uptake in summer, as well as low but continuous emissions of CH4 and CO2 throughout the snow-covered winter. The seasonality of lake fluxes differed, with maximum CO2 and CH4 flux rates recorded at spring thaw. During the ice-free seasons, we could identify surface CH4 emissions as mostly ebullition events with a seasonal trend in the magnitude of the release, while a net CO2 flux indicated photosynthetic activity. We found correlations between surface CH4 emissions and surface sediment temperature, as well as between diel CO2 uptake and diel solar input. During spring, the breakdown of thermal stratification following ice thaw triggered the degassing of both CH4 and CO2. This spring burst was observed in 2 consecutive years for both gases, with a large inter-annual variability in the magnitude of the CH4 degassing. On the annual scale, spring emissions converted the lake from a small CO2 sink to a CO2 source: 80 % of total annual carbon emissions from the lake were emitted as CO2. The annual total carbon exchange per unit area was highest at the fen, which was an annual sink of carbon with respect to the atmosphere. Continuous respiration during the winter partly counteracted the fen summer sink by accounting for, as both CH4 and CO2, 33 % of annual carbon exchange. Our study shows (1) the importance of overturn periods (spring or fall) for the annual CH4 and CO2 emissions of northern lakes, (2) the significance of lakes as atmospheric carbon sources in subarctic landscapes while fens can be a strong carbon sink, and (3) the potential for ecosystem-scale eddy covariance measurements to improve the understanding of short-term processes driving lake–atmosphere exchange of CH4 and CO2.

Importance of the autumn overturn and anoxic conditions in the hypolimnion for the annual methane emissions from a temperate lake.

Changes in the budget of dissolved methane measured in a small temperate lake over 1 year indicate that anoxic conditions in the hypolimnion and the autumn overturn period represent key factors for the overall annual methane emissions from lakes. During periods of stable stratification, large amounts of methane accumulate in anoxic deep waters. Approximately 46% of the stored methane was emitted during the autumn overturn, contributing ∼80% of the annual diffusive methane emissions to the atmosphere. After the overturn period, the entire water column was oxic, and only 1% of the original quantity of methane remained in the water column. Current estimates of global methane emissions assume that all of the stored methane is released, whereas several studies of individual lakes have suggested that a major fraction of the stored methane is oxidized during overturns. Our results provide evidence that not all of the stored methane is released to the atmosphere during the overturn period. However, the fraction of stored methane emitted to the atmosphere during overturn may be substantially larger and the fraction of stored methane oxidized may be smaller than in the previous studies suggesting high oxidation losses of methane. The development or change in the vertical extent and duration of the anoxic hypolimnion, which can represent the main source of annual methane emissions from small lakes, may be an important aspect to consider for impact assessments of climate warming on the methane emissions from lakes.

Preservation potential of seasonal laminated deposits as a useful tool for environmental analysis in mesotrophic Lake Kizaki, central Japan

In this paper discusses the aquatic environment at Lake Kizaki, central Japan, during the last ca. 60years and describes the importance of laminated sediments in environmental analyses. The flux of total algal carbon increased between 1965 and 1996 while the BSi flux, which can be used in diatom production, showed a larger increase between 1960 and 1998 than typically occurs on average. The flux of dinosterol, an indicator of dinoflagellate production, also increased from 1966 to 1998, and the sedimentary dinoflagellate record correlated with observations of freshwater red tides caused by dinoflagellate blooms. Parallel laminated sediment was determined to have appeared between 1960 and 1995, corresponding to a period of high total algal carbon levels and BSi fluxes. For lamina formation and its preservation process, the thickness of the oxygen-poor layer depended on excessive total algal production during stagnant periods and appeared to be important because nutrients were diffused and accumulated in the oxygen-poor layer from bottom sediments. This could result in diatom blooms at the next overturn period. The development of the thick oxygen-poor layer appeared to be triggered primarily by the presence of excessive nutrients generated by human activities. In general, the occurrence of laminated sediments can provide a useful tool for understanding the eutrophic state in Lake Kizaki.

Methane Emissions from a Small Wind Shielded Lake Determined by Eddy Covariance, Flux Chambers, Anchored Funnels, and Boundary Model Calculations: A Comparison

Lakes are large sources of methane, held to be responsible for 18% of the radiative forcing, to the atmosphere. Periods of lake overturn (during fall/winter) are short and therefore difficult to capture with field campaigns but potentially one of the most important periods for methane emissions. We studied methane emissions using four different methods, including eddy covariance measurements, floating chambers, anchored funnels, and boundary model calculations. Whereas the first three methods agreed rather well, boundary model estimates were 5-30 times lower leading to a strong underestimation of methane fluxes from aquatic systems. These results show the importance of ebullition as the most important flux pathway and the need for continuous measurements with a large footprint covering also shallow parts of lakes. Although fluxes were high, on average 4 mmol m(-2) d(-1) during the overturn period, water column microbial methane oxidation removed 75% of the methane and only 25% of potential emissions were released to the atmosphere. Hence, this study illustrates second the importance of considering methane oxidation when estimating the flux of methane from lakes during overturn periods.

Seasonal increase of methane in sediment decreases δ13C of larval chironomids in a eutrophic shallow lake

Recent studies have shown that larval chironomids assimilate 13C-depleted carbon derived from biogenic methane by feeding on methane-oxidizing bacteria (MOB). The dietary contribution of MOB is known to be maximized in the autumn overturn period or winter in eutrophic dimictic lakes due to the increase of MOB biomass following the supply of oxygenated water, but in polymictic lakes, such seasonal variability has not been revealed. We investigated the seasonal patterns of larval δ13C and methane concentrations in the sediment of a eutrophic polymictic lake, Izunuma, Japan. Larval δ13C decreased in late summer and autumn. Methane concentrations above a 6 cm depth peaked in late summer or autumn, while those in the 10–11- and 20–21-cm layers peaked in October. Negative correlations between methane concentrations in the 5–6/10–11-cm layers and larval δ13C were found. This suggests that an increase in the supply of methane stimulated the activity of MOB in a polymictic lake, where water above the lake bottom rarely became anoxic because of frequent overturn, thus increasing the dietary contribution of MOB to larval chironomids.

Size-related photosynthesis of algae in a strongly stratified humic lake

We studied the size-specific phytoplankton production and chlorophyll-based biomass in Lake Valkea-Kotinen, where the epilimnetic nutrient concentrations during stratification are low and the phytoplankton community mainly consists of flagellates. We hypothesized that in this humic lake, where large diatoms are few, small species are favoured. For a complete size range, autotrophic picoplankton (APP) were included in the study. We sampled the pristine headwater lake in 2005 and in addition to size-fractionated production and biomass, we also identified the species and calculated the biovolume-based biomass estimates. In contrast to our hypothesis, most of the production was composed of a few large cell size taxa with flagella, e.g. Gonyostomum semen during stratification and Peridinium spp. during overturn periods. The large micro-plankton, mainly G. semen, were the dominant primary producers (46―72% of the total primary production) from late June until the late September. G. semen was also superior in terms of chlorophyll-specific photosynthetic rate, whereas APP were less efficient and thus less successful. Our results indicate that in this lake, the large-sized flagellated phytoplankton could largely overcome nutrient limitation and thus solar irradiance was presumably the main driver for the production of phytoplankton.

Autochthonous origin of semi-labile dissolved organic carbon in a large monomictic lake (Lake Biwa): carbon stable isotopic evidence

Semi-labile dissolved organic carbon (DOC) plays an important role in the transport and hypolimnetic remineralization of carbon in large freshwater lakes. However, sources of semi-labile DOC in lakes remain unclear. This study used a carbon stable isotope approach to examine relative contributions of autochthonous and allochthonous sources to semi-labile DOC. Vertical and seasonal variations in the concentration and carbon stable isotope ratio (δ13C) of DOC were determined in large (surface area 674 km2; maximum depth 104 m), monomictic Lake Biwa. A sharp vertical gradient of δ13C of DOC (δ13C-DOC) during the stratification period [mean ± standard error (SE) −25.5 ± 0.1 and −26.0 ± 0.0‰ in the epi- and hypolimnion, respectively] indicated the accumulation of 13C-rich DOC in the epilimnion. Vertical mixing explained the intermediate values of δ13C-DOC (−25.7 ± 0.0‰) measured throughout the water column during the overturn period. Both DOC concentration and δ13C-DOC decreased in the hypolimnion during stratification, indicating selective remineralization of 13C-rich DOC. Using a two-component mixing model, we estimated the δ13C value of semi-labile DOC to be −22.2 ± 0.3‰, which was close to the δ13C of particulate organic carbon collected in the epilimnion during productive seasons (−22.7 ± 0.7‰) but much higher than the δ13C-DOC in river waters (−26.5 ± 0.1‰). Semi-labile DOC appeared to be mainly autochthonous in origin, produced by planktonic communities during productive seasons. The spatiotemporal uncoupling between production and remineralization of semi-labile DOC implies that hypolimnetic oxygen consumption may be affected by pelagic primary production during productive seasons of the preceding year.

Coupling relationships between brachiopods and Girvanella during the Late Devonian F-F transition in Guilin, South China

The abundance curves derived from area proportions of brachiopods and Girvanella in the thin sections from the Yangdi section, South China, exhibits complex relationships during the Late Devonian Frasnian-Famennian transition. The living activity of brachiopods such as grazing and borrowing did great damage to the growth of Girvanella. However, there was more to just a mere a survival competing relationship between them, Girvanella actually improved marine environments by oxygenating the dysoxic ambient sea-water through photosynthesis in the lower Kellwasser Horizon. Profited from this improvement brachiopods’ abundance increased subsequently and suppressed Girvanella again. Nonetheless, without Girvanella’s photosynthesis, brachiopods were wiped out by the farther anoxic environments in the upper Kellwasser Horizon. The complex relationships between Girvanella and brachiopods may be a key to unlocking the relationships between geomicrobes and metazoans in the geological overturn periods.

Experimental d13C evidence for a contribution of methane to pelagic food webs in lakes

We tested the hypothesis that low stable carbon isotope (δ13C) values commonly observed for zooplankton in humic lakes are due to their feeding on isotopically light methane‐oxidizing microbes, and thus that methane‐derived carbon is important in the food webs of these lakes. In replicate laboratory cultures, Daphnia longispina, a common crustacean zooplankter in humic lakes, were fed microbial suspensions with or without enrichment by biogenic methane. The δ13C values of Daphnia indicated consumption of 13C‐depleted methanotrophic bacteria, while growth rates, survival, and reproduction of Daphnia in cultures enriched with methane were equal to or greater than those in nonenriched cultures. Results from lake enclosures during the autumn overturn period revealed a decrease in δ13C of adult Daphnia from ‐40.5% to ‐50.3%, reflecting extensive consumption of 13C‐depleted methanotrophic bacteria. Methane‐derived carbon is a more important contribution to carbon flux through lake pelagic food webs than has previously been suspected.

The life history of Cyclocypris ovum (Ostracoda) in a permanent karstic lake

We studied the life history of Cyclocypris ovum, a small candonid ostracod, in a Spanish mountain karstic lake (Lake La Cruz). Samples were collected from the littoral area of the lake between June 1996 and December 1998 and the relative abundances of all developmental instars were analysed. Hatching took place between November and May and juvenile development lasted around 6 months; adult life-span was 6-8 months. In summer, a sharp decline in adult population densities was observed, and juveniles completely disappeared until the next hatching period. This population decline is probably a result of mortality, and the commencement of dormancy in immature adults, which may move towards the bottom to enter dormancy buried in the sediments. These processes may be triggered by the combined effects of summer whiting and an increase in water temperature above 20°C. Densities of mature adults progressively increase in autumn, suggesting that the end of dormancy is stimulated by decreasing temperatures and turbulence. In this way, females lay eggs that will hatch mainly during the overturn period. In permanent waters, dormancy of an immature adult stage which optimises reproduction and growth under favourable conditions, could be more adaptive than resting egg production (as commonly occurs in ostracods inhabiting temporary waters). Further research is needed to know to what extent temperature variability and calcium carbonate precipitation affect the life history of this small, nektonic ostracod, which is common in northern Europe but rather rare at low altitudes in the Iberian Peninsula.

The phytoplankton community as a bioindicator of health conditions of Atezca Lake, Mexico

Atezca Lake is located in a mountainous zone of North–eastern Mexico. It is a small subtropical monomictic system with a winter circulation period, and stratification in spring and summer. The lake was first studied monthly during two annual cycles in 1981 and 1983; biannual sampling was then conducted in the stratification and circulation periods for 10 years (1984–1994) in order to determinate the seasonal and annual succesional patterns of the phytoplankton community.Thermal behaviour was found to influence both physical and chemical features. A high concentration of nitrates and phosphates occurred at the beginning of the stratification period and decreased toward late stratification. During the overturn period these nutrients increased. These dynamics affected the phytoplankton assemblage because bacillariophyceans and chlorophyceans were dominant in early stratification, dinophyceans and cyanophyceans in late stratification and bacillariophyceans and chrysophyceans were abundant at circulation.The long term changes showed an increase in the thickness of the anoxic hypolimnion; surface nitrates also diminished, whereas phosphates tended to increase. From 1981 to 1986 the phytoplankton community was dominated by chlorphyceans; however from 1990 to 1994 they were replaced by cyanophyceans, especially Microcystis aeruginosa. The results indicate that the lake has shown severe signs of eutrophication in recent years. The prospects for recovery depend upon sewage input control, hyplomnetic discharges and conservation of the forest in the catchment area.

Dynamics of dissolved methane and methane oxidation in dimictic Lake Nojiri during winter

Changes in the vertical distribution of dissolved CH4 were monitored during the autumnal lake overturn period in mesotrophic Lake Nojiri, Japan (4.4 km2 in area and 9.4 X 107 m3 in vol). A survey in 1992 revealed that the surface CH4 concentration was highest in December, when the lake overturned. During the following two winters (1993–1994 and 1994–1995) we carried out detailed sampling during the autumnal overturn period. As a result of lake overturn in mid‐December, CH4 that had accumulated in the hypolimnion during the stratification period mixed rapidly throughout the water column. Increased CH4 in the epilimnion quickly disappeared after the overturn as a result of the rising of CH4 oxidation activity throughout the water column. The in situ‐specific CH4 oxidation rate peaked at 0.274, 0.235, and 1.01 d−1 at 0.5, 20, and 36 m, respectively, during the overturn, and then declined the following month. During this period, the diffusive flux of methane across the air‐water interface increased but was not the dominant sink (avg rate of 4.5 kg lake−1 d−1); instead, methane oxidation in the water column was the dominant CH4 sink (avg rate of 67.8 kg lake−1 d−1), removing ~94% of the CH4 during the overturn period. A significant methane flux from the bottom sediments throughout the overturn period was confirmed but decreased gradually as the overturn proceeded. The production of organic carbon as a result of CH4 oxidation in the water column by methanotrophs was comparable in extent to that generated by primary production at that time.

The phytoplankton community as a bioindicator of health conditions of Atezca Lake, Mexico

Abstract Atezca Lake is located in a mountainous zone of North–eastern Mexico. It is a small subtropical monomictic system with a winter circulation period, and stratification in spring and summer. The lake was first studied monthly during two annual cycles in 1981 and 1983; biannual sampling was then conducted in the stratification and circulation periods for 10 years (1984–1994) in order to determinate the seasonal and annual successional patterns of the phytoplankton community. Thermal behaviour was found to influence both physical and chemical features. A high concentration of nitrates and phosphates occurred at the beginning of the stratification period and decreased toward late stratification. During the overturn period these nutrients increased. These dynamics affected the phytoplankton assemblage because bacillariophyceans and chlorophyceans were dominant in early stratification, dinophyceans and cyanophyceans in late stratification and bacillariophyceans and chrysophyceans were abundant at circulation. The long term changes showed an increase in the thickness of the anoxic hypolimnion; surface nitrates also diminished, whereas phosphates tended to increase. From 1981 to 1986 the phytoplankton community was dominated by chlorphyceans; however from 1990 to 1994 they were replaced by cyanophyceans, especially Microcystis aeruginosa. The results indicate that the lake has shown severe signs of eutrophication in recent years. The prospects for recovery depend upon sewage input control, hyplomnetic discharges and conservation of the forest in the catchment area.

Ecology and morphological variability of Aulacoseira granulata (Bacillariophyceae) in Spanish reservoirs

The ecology of Aulacoseira (formerly Melosira) granulata and its different forms in ~30 reservoirs scattered throughout Spain has been analysed. Nutrient concentration explained a significant proportion of the variation in cell characteristics. In general, there is a tendency to find large cells with less densely areolated valves in nutrient-rich waters. Ecological differences between the type and its variety angustissima mainly concern calcium concentrations Aulacoseira granulata showed a unimodal response to the concentration of calcium, and preferred lower concentrations of calcium than the variety angustissima. The ecological differences between the a and -y forms (statuses) of A.granulata were also the object of our investigation. The y status occurred in all the reservoirs studied in summer, but in only 3 of 28 reservoirs studies in the overturn period. Light and turbulence explained the ecological differences observed between the two forms. The y status showed a certain preference for less turbulent waters compared with the a status, which is a more cosmopolitan form in its requirements.