Period Of Thermal Stratification Research Articles

Thermal Stratification and the Potential for Enhanced Phosphorus Release from the Sediments in Lake Okeechobee, USA

We inferred the potential importance of enhanced phosphorus (P) release from the sediments during periods of thermal stratification in Lake Okeechobee USA, a large, shallow, eutrophic lake. This was accomplished by determining the frequency and duration of thermal stratification, using monitoring data collected during 1997 and 1999, with multiparameter sondes, which were positioned at a weather platform in the center of the lake. We also monitored dissolved oxygen, pH, specific conductance, turbidity and redox potential, to assess if any relationships existed between these variables and thermal stratification. Thermal stratification was infrequent, documented for 42 and 74 total hours, and occurred for an extrapolated period of at least one hour on approximately 18 days or 5% of both years. Thermal stratification occurred almost exclusively during the summer (May – September), and was brief, typically lasting for one to six hours per event, although there was one event during 1999, where 49 continuous hours of thermal stratification were documented. Dissolved oxygen (DO) concentrations and redox potential (ORP) measurements collected 0.5 m above the sediment surface suggested that the top of the sediment may not have ever become anoxic and was rarely reducing. Chemical characteristics during periods of thermal stratification, compared to those considered favorable for P release based on previous Lake Okeechobee sediment studies, indicate that thermal stratification is usually insufficiently long for chemical conditions (anoxic or low DO and ORP) to develop above the sediment surface that would enable enhanced P release from the sediments.

Inhibitory effect of light on methane oxidation in the pelagic water column of a mesotrophic lake (Lake Biwa, Japan)

Methane oxidation was studied in mesotrophic lake water (Lake Biwa, Japan) under thermally stratified conditions. Methane oxidation rates at in situ concentrations were very low in lake water from the epilimnion and thermocline but were high in hypolimnetic water. Incubation under light conditions ranging from 4.1 to 57 μmol photons m -2 s -1 resulted in decreased methane oxidation in hypolimnetic water. This inhibition was more severe as the light intensity increased. Addition of inorganic nitrogen (ammonium and nitrate) did not promote methane oxidation in the thermocline but inhibited it in the hypolimnion. Methane oxidation activity in the thermocline was observed after 1 month of incubation under dark conditions. Our results suggest that the inhibitory effect of light was bacteriostatic for the methanotrophic population. The different rates of methane oxidation between the hypolimnion and epilimnion/thermocline may explain the surface maximum of dissolved methane during the period of thermal stratification.

Seasonal changes in the abundance and composition of picophytoplankton in relation to the occurrence of ‘Kyucho’ and bottom intrusion in Uchiumi Bay, Japan

Uchiumi Bay experiences intermittent physical events of 'Kyucho' and bottom intrusion. A Kyucho is an intrusion of warm surface water from the Kuroshio in the Pacific Ocean. Bottom intrusion, which contains a large amount of nitrates, phosphates, and silicates, slips through just above the continental shelf. We investigated seasonal changes in the abundance of Prochlorococcus, Syne- chococcus, and eukaryotic picophytoplankton while monitoring Kyucho and bottom intrusion from March to October 2002. Kyucho and bottom intrusion frequently occurred from June to September. Relatively high concentrations of nitrate + nitrite (>0.8 µmol N l -1 ) and phosphate (>0.1 µmol P l -1 ) were found when bottom intrusion occurred. The cell densities of Prochlorococcus were relatively high (>1 × 10 4 cells ml -1 ) when Kyucho occurred. Those of Synechococcus were high (2 to 30 × 10 4 cells ml -1 ) during the period of thermal stratification except in July, when bottom intrusion occurred. The cell densities of eukaryotic picophytoplankton were high (2 to 8 × 10 4 cells ml -1 ) in May and July. To examine the effects on picophytoplankton growth of the nutrients supplied by bottom intrusion, we conducted nutrient-enrichment experiments. The growth rates of Prochlorococcus and Synechococcus were not stimulated by the addition of any kinds of nutrients. The growth rates of Prochlorococcus were negative in most cases. In July, the growth rate of eukaryotic picophytoplankton was stimulated by nitrate and phosphate additions. Thus, Prochlorococcus detected in Uchiumi Bay might have been transported by Kyucho from the Pacific Ocean and could therefore not grow vigorously. Synechococ- cus may have been flushed out by bottom intrusion, and its growth was not limited by the nutrient con- centrations. Eukaryotic picophytoplankton was abundant in spring, and its growth might have been limited by the nutrient concentrations in some cases. These results suggest that Kyucho and bottom intrusion have different effects on the abundance and growth rate of the 3 picophytoplankton groups.

Low consumption of virus-sized particles by heterotrophic nanoflagellates in two lakes of the French Massif Central

Seasonal and depth-related variability in the grazing activity of heterotrophic nano- flagellates (HNF) on viruses was examined in the oligo-mesotrophic Lake Pavin and in the eutrophic Lake Aydat, French Massif Central, between May and November 2000. Ingestion rates (IR) were determined using 50 nm diameter fluorescent microspheres, as virus analogues. In both lakes, high- est grazing activities on virus-sized particles were recorded in the metalimnion, at the beginning and the end of the thermal stratification period. Estimated IRs in Lake Pavin (mean = 0.4 viruses HNF -1 h -1 , CV = 38.0%) and in Lake Aydat (mean = 0.3 viruses HNF -1 h -1 , CV = 35.6%) were not signifi- cantly different, in contrast to clearance rates (CR), which were significantly higher in the oligome- sotrophic (2.3 × 10 -2 nl HNF -1 h -1 ) than in the eutrophic lake (0.7 × 10 -2 nl HNF -1 h -1 ). CRs for viruses were correlated with CRs for bacteria in Lake Aydat but not in Lake Pavin, suggesting a greater abundance within the HNF assemblages of virus-sized particle feeders in the less productive lake. We estimated that 4.1 and 0.8% of viral production were grazed by HNF in Lake Pavin and Lake Aydat, respectively. Finally, although viruses seem to represent a minor food source for HNF (i.e. compared to bacteria), they may not be inconsequential in their diet, especially in oligotrophic lakes.

Seasonal variation of phototrophic picoplankton in Lake Biwa (1994?1998)

Seasonal abundances of phototrophic picoplankton (PP) and heterotrophic nanoflagellates in Lake Biwa were studied from 1994 to 1998. Seasonal variation in cell volume and biomass of the phototrophic picoplankton were also studied. PP were counted using disposable glass microscopic plates, which gave superior accuracy to sample filtration onto membrane filters. Phycoerythrin-rich rod-shaped cyanobacteria (PEC), one of the major components of the picoplankton community, were sparse (about 104 cells ml −1) in winter and began to increase in April. Several PEC peaks were observed during the period of thermal stratification, and a rapid fall took place after October or November. In the northern basin, PEC peaked during late June and early July in 3 of the 5 years, and in late summer in the remaining years. Phycocyanin-rich rod-shaped cyanobacteria (PCC) were abundant in the southern basin and were present in smaller numbers in the eutrophic nearshore area of the northern basin; they peaked several times during the period from July to October. Seasonal variations of these two kinds of picoplankton were correlated with seasonal changes in water temperature. Phycoerythrin-rich cylinder-shaped cyanobacteria exhibited narrow peaks in July, their abundance declining as the year progressed. The density of heterotrophic nanoflagellates was greatest in early spring. Average cell volume of PEC was largest in winter, then decreased gradually to a minimum in late summer; after the fall, it recovered to the winter cell volume. This change can likely be attributed to the depletion of nitrogen in the warmer seasons.

A Short-Term Study of Vertical and Horizontal Distribution of Zooplankton During Thermal Stratification in Lake Kinneret, Israel

This study documents for the first time both vertical and horizontal distribution patterns of the zooplankton community in Lake Kinneret during the period of thermal stratification. The zooplankton distribution patterns were explored in relation to abiotic (temperature, oxygen) and biotic (picocyanobacteria, ciliates, flagellates, phytoplankton, fish) environmental gradients. Sampling was carried out on 6–7 July 1992 at five stations and six depths from nearshore to offshore. Zooplankton abundance and biomass varied from 5 to 267 ind. l−1(mean: 95 ind. l−1), and from 0.1 to 65 d.w. mg m−3(mean: 24 d.w. mg m−3). Zooplankton taxonomic groups (Rotifera, Cladocera, Cyclopoida, Calanoida) and size classes (micro-, meso- and macrozooplankton) showed peaks of maximal density and biomass in the epilimnetic and metalimnetic strata (5 and 14 m). Depth, accounting for 31–39% of total spatial variation, reflected the vertical distribution of zooplankton in relation to temperature and oxygen declines, and the higher concentration of food resources (protists and phytoplankton) in the epilimnion and metalimnion. Onshore–offshore distance, accounting for 17–22% of the total spatial variance, reflected different distribution patterns shown among zooplankton groups and size classes. The macrozooplankton (Copepoda, Cladocera) was more abundant offshore, whereas microzooplankton (Rotifera and nauplii) predominated nearshore. These horizontal distribution patterns were related to small increases in temperature and phytoplankton biomass, and higher concentrations of fish in the littoral zone. Although limited to a short temporal scale, our study indicated that zooplankton spatial distribution in Lake Kinneret during the period of thermal stratification was related to physicochemical, food and predation factors, manifested differently along the vertical and nearshore–offshore gradients.

Water management of Lake Yogo targeting internal phosphorus loading

AbstractLake Yogo is a small, eutrophic lake located 1.3 km north of Lake Biwa, Japan. The lake, which was originally a natural, closed lake, has a unique water management system. It is replenished with water pumped from Lake Biwa and supplies irrigation water to ≈ 5 km2 of agricultural fields. As a result, Lake Yogo functions as a water storage dam and is artificially controlled. As seen in most stratified lakes in temperate regions, large amounts of phosphate (PO4) are released from the sediment in the hypolimnetic water layer during the summer stratification period. In order to reduce the impacts of internal phosphorus (P) loading, a destratification aeration system was installed in the lake. Although the stratification was weakened concomitant with a temperature increase in the hypolimnetic layers, anoxic conditions were still observed because of the small scale of the aeration system, with cyanobacterial blooms continuing to occur during the summer and autumn of every year. Although the internal P load at the autumn turnover is probably immediately scavenged by its incorporation into ferric oxyhydroxide colloids, the phytoplankton present at that time would doubtless benefit from the temporary, increased PO4 supply because of their rapid uptake rates. Although the PO4 transported continually from the hypolimnetic to the epilimnetic layers by the destratification aeration system should be scavenged as ferric oxyhydroxide colloids, a portion of the PO4 is probably incorporated into the phytoplankton biomass. As a result, an insufficient level of artificial destratification might support phytoplankton growth during the period of thermal stratification in the lake.

Growth of cultured mussels (Perna canaliculus Gmelin 1791) at a deep-water chlorophyll maximum layer

We tested the hypothesis that yield of the mussel Perna canaliculus (Gmelin 1791) could be increased by placing culture ropes deep into the water column to take advantage of deep-water chlorophyll a maxima. The study site, in Pelorus Sound, New Zealand, showed periods of thermal stratification of the water column, causing a high concentration of phytoplankton in deeper water, at the thermocline. Mussels were grown simultaneously at 5 and 17 m, for up to 96 days. Mussel growth and condition index data showed no significant differences between the two depth treatments, indicating that in this system there may not be substantial mussel productivity benefits from lowering mussel farms to the deep-water chlorophyll maximum layer.

Partitioning of dissolved thallium by seston in Lakes Erie and Ontario

The fate of thallium (Tl) was traced in the epipelagic and coastal waters of Lake Erie (in 2000) and Lake Ontario (in 2001) sampled during the summer period of thermal stratification. Fractionation of 204Tl bound to particulate matter was conducted to trace the scavenging potential of various seston size ranges (0.2–2 µm, 2–20 µm, and 20–210 µm,). Added [204Tl]-Tl(I) scavenged by seston over 1–3 days was 0.34–4.05 % in Lake Erie and 0.09–0.21 % in Lake Ontario, where scavenging was correlated with suspended particulate matter content and chlorophyll a concentration. There was no consistent trend with respect to Tl scavenging by a particular size fraction, except at coastal sites in Lake Erie where most Tl was scavenged by the 2–20 µm seston. Partition coefficients (Kd) of Tl scavenging by seston in Lake Ontario were 938–1483 L·kg–1. At the same pelagic site in Lake Ontario, [204Tl]-dimethylthallium partitioned into seston (Kd = 807 ± 258) to a lesser degree than added Tl(I) (Kd = 998 ± 297), a finding consistent with laboratory assays of these two Tl species. These results suggest that the plankton food web plays a role in the geochemical cycle of this toxic metal in the water column of the lower Great Lakes.

Autotrophic picoplankton in Lake Baikal: Abundance, dynamics, and distribution

The autotrophic picoplankton of Lake Baikal was discovered more than 30 years ago; the productivity of the endemic member of the group, Synechocystis limnetica, is of particular interest. In the course of long-term investigations, the abundance of this species was established using the sedimentation technique. Maximum abundance was reached at 104 cells per ml in summer. In addition to estimating the distribution of this species throughout a defined lake area during the spring-summer period, the character of interrelations with large phytoplankton was revealed. Baikalian autotrophic picoplankton was studied from 1997 to 2000 using epifluorecsence and scanning electron microscopy. The study of seasonal and annual dynamics of picoplankton abundance was conducted monthly at the monitoring station in southern Baikal. The spatial distribution of autotrophic picoplankton was observed in the period when Lake Baikal was free of ice; the lake area was defined by 50 stations. Microscopic examination of planktonic samples revealed that the phycoerythrin-rich cyanobacteria of genus Synechococcus generated the largest biomass of autotrophic picoplankton in Lake Baikal. Morphotype diversity of the representatives of genus Synechecoccus was revealed in the study: Synechococcus spp. of coccoid and ellipsoid forms were dominant by abundance. The coccoid and ellipsoid forms had diameters of 0.9 and 0.7 to 1.3 μm, respectively. The portion of picoplanktonic algae in the Baikalian picoplankton community is not high and does not exceed 10% in the summer time when growth is highest. The autotrophic picoplankton abundance throughout the year in the 0–50 m layer varies by more than one hundred times. Maximal concentration of picocyanobacteria occurs in late July–August at 10 to 25 m depths where it reached 106 cells ml−1. The minimal concentration (103 cells ml−1) was observed in December at the same depths. A small peak of the picoplankton growth occurred in April of each year when the total numbers of picocyanobacteria increased 2 to 4 times after winter. In 1999 the peak rose to 105 cells ml−1 . During the investigation period the highest indices of autotrophic picoplankton numbers were observed in 1999 and the lowest in 1997. The spatial distribution of this group in the whole Baikal aquatorium was characteristically heterogeneous; its abundance varied by 2 to 10 times in different parts of the lake. The largest fluctuation occurred in August and the least in September. The maximum picoplankton abundance was observed in the epilimnion during the periods of thermal stratification. In the course of homothermy a comparably homogeneous distribution of picocyanobacteria was revealed up to 300 m depth, but the highest numbers were found in the upper 30 m layer.

Analyses of physico-chemical characteristics and phytoplankton communities of Lake Nasser during the last two decades

Lake Nasser is one of the largest man-made lakes in the world. The creation of Lake Nasser, due to the impoundment of the River Nile in Southern Egypt, was accompanied by alterations in the environmental conditions that consequently affect the biota. Such alterations lead to corresponding changes in the diversity, abundance and distribution of the phytoplankton community. Therefore, the phytoplankton populations were followed and investigated in Lake Nasser. The phytoplankton community structure revealed a floristical diversity and is composed of various planktonic algal taxa appertaining to the divisions: Chlorophyta, Bacillariophyta (diatoms), Cyanophyta (cyanobacteria), Dinophyta (Dinoflagellates) and Euglenophyta. Chlorophyta contribute more taxa to the phytoplankton than any other groups. However, diatoms and cyanobacteria are numerically the main components and alternate in dominance of the community. Dinoflagellates persist as frequent forms and the euglenoid algae are very scarcely represented. Remarkable spatial (horizontal and vertical) as well as temporal (seasonal) variations are recorded in the distribution of phytoplankton. Pronounced variations in the vertical distribution of the phytoplankton standing crop appear during the periods of thermal stratification (late spring, summer and early autumn). However, diatoms and cyanobacteria remain the most influential groups also in the vertical distribution of the phytoplankton. Water blooms are occasionally observed in limited areas of the southern region of Lake Nasser mainly due to the florishment of the cyanobacterium Microcystis aeruginosa.

Virioplankton and microbial communities in aquatic systems: a seasonal study in two lakes of differing trophy

SUMMARY1. The seasonal and vertical distribution of the abundance of virus‐like particles (VLPs), together with the abundances of other microbial organisms (bacteria, unpigmented and pigmented nanoflagellates and ciliates), were determined in an oligomesotrophic lake (Pavin, France) and in a eutrophic lake (Aydat, France) between March and December 2000.2. The abundance of the viral plankton and those of other microbial taxa were significantly higher in the more productive system. The same was for the virus‐to‐bacteria quotient (VBQ), which averaged seven in Lake Pavin and nine in Lake Aydat.3. The abundance of viruses increased during the period of thermal stratification in both lakes, with the highest values being recorded at the end of summer/early autumn in the epi‐ and the metalimnion. The seasonal pattern of abundance of viruses in both lakes in the surface layer was similar, indicating that the dynamics of viruses may be controlled by environmental factors such as light conditions.4. There was no correlation between the abundance of viruses and protists. We found correlations between viruses and heterotrophic bacteria in the whole water column in Lake Pavin, but only in the dark bottom waters in Lake Aydat.5. Overall, the empirical findings in this study lead us to speculate that the weaker correlation between bacteria and viruses in Lake Aydat than in Lake Pavin, as well as the higher VBQ in the former, is a consequence of the increasing relative abundance of non‐bacteriophage VLPs along the trophic gradient of aquatic systems.

Comparison of thermal stratification, light attenuation, and chlorophyll-a dynamics between the ends of Lake Tanganyika

Thermal structure, light attenuation, and chlorophyll-a dynamics within the upper 100 m at the northern and southern ends of Lake Tanganyika were measured from August 1995–July 1996. Pronounced variability of thermal structure in time, depth and region were observed. During the dry windy season (June–September), the water column in the south was largely isothermal down to 100 m, while in the north stratification was detectable. Subsequently, in October, cessation of winds coincided with a reestablishment and strengthening of thermal stratification in the south, and a distinct rise of a weakened thermocline in the north was accompanied by an increase in epilimnetic concentrations of dissolved solids. The depth of photic zone was highly variable with the average position of 1% of incident photosynthetically active radiance found between 20 and 70 m, sometimes showing vertical shifts of 25 m a week. Weekly observations showed that concentrations of chlorophyll-a were rather similar at both ends of the lake. At both stations, periods of unstable or absent thermal stratification were accompanied by increased concentrations of chlorophyll-a, although in the north chlorophyll-a also peaked during periods of deep thermal stratification and low rates of mixing. Concentrations of chlorophyll-a did not show significant relationship with the available amount of underwater available photosynthetically active radiance (Im from 11–173 μEm−2 s−). We suggest that in Lake Tanganyika chlorophyll-a maxima can occur whenever growth rates exceed spatial mixing rates, that is, if mixing is less than a critical turbulence. Our observations underscore the consequences of hydrodynamic processes on biological productivity either by securing internal nutrients or by controlling the timing and magnitude of phytoplankton biomass production.

Light climate and plankton in the deep chlorophyll maxima in North Patagonian Andean lakes

The light climate at the deep chlorophyll maxima (DCM) was analysed in a set of lakes of the North Patagonian Andean region. Apparent and inherent optical properties in relation to the chlorophyll a vertical distribution were investigated in seven lakes including deep (Z max > 90 m) and shallow (Z max < 12 m) ones. Sampling was carried out during the thermal stratification period (summer) of the deep lakes since in the shallower lakes no stable thermal stratification was detected. The large deep lakes presented very low diffuse attenuation coefficients of photosynthetically active radiation (K dPAR ), and a DCM situated at 0.98-2% of surface PAR irradiance, coinciding with the maximum abundance of the mixotrophic ciliate Ophrydium naumanni and autotrophic picoplankton. Both fractions seemed to be favoured by dim light conditions of particular wavelengths, since at these DCM layers mainly green and blue wavelengths prevailed (<600 nm). In contrast, shallow lakes showed higher K dPAR values, with a higher concentration of dissolved yellow substances, which caused substantial differences in the spectral quality that may have contributed to explain the absence of this ciliate population in these lakes.

Estimation of the fate of dissolved DNA in thermally stratified lake water from the stability of exogenous plasmid DNA

The fate of the exogenous plasmid DNA, pEGFP, was traced in lake waters collected from the epilimnion and the hypolimnion during a period of marked thermal stratification. Both gel electrophoresis analysis and dot-blot hybridization analysis demonstrated that the seeded pEGFP was completely degraded in epilimnion water within 170 h of incubation, while no pEGFP degrada- tion was observed in hypolimnion water. Removal of microorganisms from the lake waters of the epi- limnion and hypolimnion attenuated the degradation of pEGFP but did not stop it. Addition of EDTA to the lake water samples resulted in complete termination of the pEGFP degradation and indicated the involvement of a dissolved enzyme for DNA degradation. A measurable release of phosphate during pEGFP degradation indicated the existence of DNA-mediated phosphate regeneration in epilimnion water. In contrast, no DNA degradation in hypolimnion water suggested the possibility that it is less nutritionally important in the hypolimnion. Transformability of the pEGFP was not altered in hypolimnion water during the incubation, which suggested stability of genetic information that gradually decreased in epilimnion water. Dissolved DNA (dDNA) is commonly observed in aquatic environments and is thought to be an important nutritional source for microorganisms. How- ever, our results suggest that dDNA in the hypolimnion environment may not be so important as a nutritional source for microbes, but might be more important as a genetic pool for further natural transformation.

The role of light and concentration gradients in the vertical stratification and seasonal development of phototrophic bacteria in a meromictic lake.

The seasonal population dynamics of phototrophic sulphur bacteria and their vertical stratification in Lake La Cruz were studied over two consecutive summers. Development of purple sulphur bacteria occurred just below the oxic-anoxic boundary (the plate located at 14-15 m depth). The principal constituent was a species of Amoebobacter, containing okenone as its main carotenoid and which reached maximal densities at the end of summer (2.2 × 10 6 cells/ml and BChl.-a concentrations of 111 μg/l in September 1987). Green phototrophic bacteria developed below the Amoebobacter layer at a mean depth of 16 m. Pelodictyon clathratiforme was the dominant species, reaching maximal cell concentrations of 3.1 × 10 6 cells/ml and BChl.-d concentrations of 362 μg/l, also in September. Growth of these purple and green sulphur bacteria was light-limited, and sulphide was also present in very low concentrations owing to the low sulphate content of the lake. The two thermal stratification periods under study differed noticeably as a consequence of altered meteorological conditions and structural differences in the water column, with a shallower oxic-anoxic boundary and a major development of algal populations in the upper oxic layer during the second period. This reduced the light available to deep phototrophic bacteria, which were unable to attain the biomass achieved in the earlier period. The monimolimnion of Lake La Cruz acts as a refuge for phototrophic bacteria during the mixing period and this is considered to be a key factor in understanding the ecology of these microorganisms.

Erratum: An investigation into iron-stimulated phytoplankton productivity in epipelagic Lake Erie during thermal stratification using trace metal clean techniques

We tested the hypothesis that phytoplankton productivity in pelagic Lake Erie is limited by low Fe bioavailability during the period of thermal stratification. Iron enrichment (20 and 200 nM Fe) of...

An investigation into iron-stimulated phytoplankton productivity in epipelagic Lake Erie during thermal stratification using trace metal clean techniques

We tested the hypothesis that phytoplankton productivity in pelagic Lake Erie is limited by low Fe bioavailability during the period of thermal stratification. Iron enrichment (20 and 200 nM Fe) of water sampled from the eastern basin surface water (5 m depth) in July 1996 revealed a dramatic 180 and 30% increase in the standing crop of the picoplankton (0.2-2 µm) and nanoplankton (2-20 µm) size fractions, respectively. Light-saturated rates of photosynthesis for picoplankton increased 2.8 times that of the controls within 24 h. In contrast, simultaneous P and Fe enrichment experiments carried out in July 1997 and August 1998 with water samples from three pelagic stations revealed that P enrichment alone stimulated phytoplankton growth, although at several stations, phytoplankton yield was greater in combined P- and Fe-amended experiments relative to P-enriched treatments. Periodogram analysis of meteorological buoy wave height data and heat budgets calculated from thermal profiles in the eastern basin revealed important interyear differences in precruise conditioning of the epilimnion. Low-frequency, turbulent mixing events that characterized the 30-day precruise period in the years 1997 and 1998 relative to 1996 may have prevented Fe-stressed conditions in the phytoplankton community studied in 1997 and 1998.

Methane emissions from an Amazon floodplain lake: Enhanced release during episodic mixing and during falling water

Methane fluxes to the troposphere were measured inLake Calado, a dendritic floodplain lake located inthe central Amazon Basin. Methane concentrations inthe surface water of the lake were less than 0.5 µM during a period of high and rising water inApril and May, except when episodic, deep mixingoccurred and surficial concentrations reached as highas 4.8 µM. Diffusive fluxes ranged from ca.2–20 mg CH4 m−2d−1 (mean: 6.6 mg CH4 m−2d−1) normally, but increased up to 220 mg CH4 m−2d−1 during the passageof a rare cold front. During six weeks of risingwater, the volume-weighted methane content of the lakeincreased during periods of stable thermalstratification (accumulation rates ranging from16–1411 mg CH4 m−2d−1) and decreasedduring episodes of partial mixing (rates of decreaseranging from 63–1792 mg CH4 m−2d−1). Surface water methane concentrations during low andfalling water in September varied by more than anorder of magnitude (range, 0.3–9 µM), anddiffusive fluxes ranged from 3–158 mg CH4m−2d−1 (mean, 54 mg CH4 m−2d−1). During falling water in September, total methane flux from the lake to the troposphere measured with a floating chamber rangedfrom 53–328 mg CH4 m−2d−1 (mean, 163 mgCH4 m−2d−1). Ebullitive flux averaged 69% of total flux. Total methane flux was highest when thermal stratification was lost due to decreasing lake depth.

Studies of the water quality and algae of Llangorse Lake

1. Despite the importance of Llangorse Lake as the largest natural body of fresh water in South Wales, no long-term limnological project has ever been planned for the lake. 2. A mass of data is available from sampling carried out at intervals since 1961, but these visits were for various reasons and used widely differing levels of expertise. However, from these data some indications of seasonal variations, effects of climate and human interference on the lake system over 30 years have been inferred. 3. Depth soundings revealed the general shallowness of the lake with two deeper basins. Wind action oxygenated the water and stirred up sediments releasing nutrients and resting algal stages into the water column. Periods of thermal stratification were transient in summer. Secchi disc and light readings showed greatest light penetration in April/May. The lake water remained alkaline throughout. Dissolved ammonia values were low in the 1960s, increased in the 1970s and decreased again in the 1980s. Nitrite- and nitrate-nitrogen showed a converse pattern over the years. Phosphate (PO4P) and silica (SiO2) values were highest in the 1960s, showed a decrease in the 1970s and early-1980s, and rose again towards the end of the decade, as did the chloride values. Seasonal fluctuations of all nutrients were observed. 4. Diatoms dominated the phytoplankton, with highest production in summer and early autumn. The Cyanobacteria (blue–green algae) produced blooms in the 1960s and 1970s, but Chlorophyta (green algae) increased over the years and dominated in the 1980s after a diversion of sewage from the lake. Diatom production followed a vernal and late summer periodicity common in temperate lakes. Results from diatom stratigraphy of the deep sediment cores indicated a change from epiphytic to planktonic species related to land-use change and a subsequent increase in soil erosion and turbidity. Copyright © 1999 John Wiley & Sons, Ltd.