Impacts of bleak (Alburnus alburnus) and roach (Rutilus rutilus) on water quality,sedimentation and internal nutrient loading

Lake Vesijärvi, southern Finland, suffered sewere eutrophication by sewage effluent from the city of Lahti during the 1960's and the early 1970's. The municipal sewage loading was diverted from the lake in 1976 and the lake started to recover. However, in the 1980's blue-green algal blooms increased again and the recovery of the lake faded. Enclosure experiments demonstrated that high roach (Rutilus rutilus) biomass is one of the key factors in the fading recovery of the lake. In this study, the influence of roach and another cyprinid fish species (bleak, Alburnus alburnus) to planktonic algal productivity and biomass in Lake Vesijärvi was examined. Enclosure experiments in the field showed the impacts of planktivorous bleak on water quality; in an enclosure with a density of 1 fish m^–2 average daily algal production (1370 mg C m^–2) and chlorophyll-a concentration (50–90 µg 1^–1) were more than twice that in an enclosure without fish. Laboratory experiments showed that the availability of planktonic food affects the foraging behaviour of roach and consequently the internal nutrient loading from the sediment into the water. Roach caused the highest phosphorus loading and turbidity when there was no zooplanktonic food available in the water. The possible interactions between planktivorous and omnivorous fish species are discussed.     

Studies on the primary production of the river Shatt Al-Arab at Basrah,Iraq

The study was conducted on the Shatt Al-Arab River at Basrah, Iraq from September 1976 to August 1977 at three stations located at the upstream, middle and lowest parts of Basrah city. There was a bimodal seasonal variation of chlorophyll-a, the concentration of which ranged between 0.52–3.25 mg/m³. The gross primary production ranged between 6.03–37.02 mgC/m³/hr and showed a unimodal seasonal variation with a maximum in August. From the concentration of chlorophyll-a and from measurement of primary productivity it was clear that the section of the river at the upstream end of Basrah city was poorest and that at the middle of the Basrah city below Ashar Channel was the richest. A positive corelation between primary productivity and chlorophyll-a.     

Dynamics and distribution of microphytobenthic chlorophyll-a in the Western Scheldt estuary (SW Netherlands)

The temporal dynamics and spatial distribution of microphytobenthic chlorophyll-a in the layer 0–1 cm were determined in the Western Scheldt estuary over the period 1991–1992. Connections between the annually averaged benthic chlorophyll-a and station elevation and sediment composition (as a measure of the hydrodynamic energy caused by currents and waves) were also examined.Microphytobenthic chlorophyll-a showed one main peak in early summer and a smaller peak in autumn. The mean chlorophyll-a concentration of 113 mg Chl-a m^–2 in the upper centimeter is of the same order of magnitude as in other estuarine areas. The average annual primary production of the microphytobenthos has been estimated at 136 g C m^–2 y^–1 The primary production of sediment inhabiting microalgae is at least 17% of the total primary production in the estuary.Considerable differences in annually averaged chlorophyll-a emerges between the stations. These differences are related mainly to the interaction between station elevation and clay content of the sediment.     

Phytoplankton primary production in a shallow,well- mixed,hypertrophic South African lake

This paper reports on a two-year analysis of the wind climateand its effect on phytoplankton primary production in ashallow (mean depth = 1.9 m), hypertrophic South Africancoastal lake, Zeekoevlei. The lake is subject to continuousmixing of the euphotic zone (Z _eu = 0.8 m), andcomplete mixing of the water column to the mean depth on adaily basis. Median annual wind speeds, prevailing fromeither the north or the south, were 6.4 m s^–1. There wasan almost total absence of calms, measured as hourly meanwind speeds of <1 m s^–1. Notwithstanding the highfrequency of mixing, the lake supports a dense population ofphytoplankton, dominated by Cyanophyte and Chlorophytespecies. Mean concentrations of chlorophyll-a were240 g l^–1. The attenuation of photosyntheticallyavailable radiation, PAR, was high, with mean K _dvalues of 6.4 m^–1 and water transparencies of <0.5 m.Levels of primary productivity, determined using the lightand dark bottle oxygen method, were very high, comparable toor exceeding that of the most productive systems yet studied.Maximum volumetric productivity ranged from 525 to 1524 mg Cm^–3 h^–1, and was confined to the upper 0.5 m of thewater column. Daily areal productivity, P _d,varied between 1.2 and 4.3 g C m^–2 d^–1, and that ofthe maximum chlorophyll-a specific photosynthetic rate,P _{B max}, between 1.6 and 7.9 mg C (mgChl-a)^–1 h^–1. Primary production was limited bywater temperature and the attenuation of PAR. The highfrequency of wind-induced mixing resulted in regular mixingof the phytoplankton through the euphotic zone, and reducedthe overall importance of P _max at a single layer inthe depth profile. Similarly, the regularity of mixing wasrecognized as a limitation of the incubation of bottle chainsto determine primary production levels.     

Control of phosphorus loading and flushing as restoration methods for Lake Veluwe,The Netherlands

As a result of high nutrient loading Lake Veluwe suffered from an almost permanent bloom of the blue-green algaOscillatoria agardhii Gomont. In 1979, the phosphorus loading of the lake was reduced from approx. 3 to 1 g P.m^–2.a^–1. Moreover, since then the lake has been flushed during winter periods with water low in phosphorus. This measure aimed primarily at interrupting the continuous algal bloom. The results of these measures show a sharp decline of total-phosphorus values from 0.40–0.60 mg P.l^–1 (before 1980) to 0.10–0.20 mg P.l^–1 (after 1980). Summer values for chlorophylla dropped from 200–400 mg.m^–3 to 50–150 mg.m^–3.The increase in transparency of the lake water was relatively small, from summer values of 15–25 cm before the implementation of the measures to 25–45 cm afterwards. The disappointing transparency values may be explained by the decreasing chlorophylla and phosphorus content of the algae per unit biovolume. Blue-green algae are gradually loosing ground. In the summer of 1985 green algae and diatoms dominated the phytoplankton for the first time since almost 20 years. To achieve the ultimate water quality objectives (transparency values of more than 100 cm in summer), the phosphorus loading has to be reduced further.     

Nutrient additions by waterfowl to lakes and reservoirs: predicting their effects on productivity and water quality

Lakes and reservoirs provide water for human needs and habitat for aquatic birds. Managers of such waters may ask whether nutrients added by waterfowl degrade water quality. For lakes and reservoirs where primary productivity is limited by phosphorus (P), we developed a procedure that integrates annual P loads from waterfowl and other external sources, applies a nutrient load-response model, and determines whether waterfowl that used the lake or reservoir degraded water quality. Annual P loading by waterfowl can be derived from a figure in this report, using the days per year that each kind spent on any lake or reservoir. In our example, over 6500 Canada geese (Branta canadensis) and 4200 ducks (mostly mallards, Anas platyrhynchos) added 4462 kg of carbon (C), 280 kg of nitrogen (N), and 88 kg of P y^–1 to Wintergreen Lake in southwestern Michigan, mostly during their migration. These amounts were 69% of all C, 27% of all N, and 70% of all P that entered the lake from external sources. Loads from all external sources totaled 840 mg P m^–2 y^–1. Application of a nutrient load-response model to this concentration, the hydraulic load (0.25 m y^–1), and the water residence time (9.7 y) of Wintergreen Lake yielded an average annual concentration of total P in the lake of 818 mg m^–3 that classified the lake as hypertrophic. This trophic classification agreed with independent measures of primary productivity, chlorophyll-a, total P, total N, and Secchi disk transparency made in Wintergreen Lake. Our procedure showed that waterfowl caused low water quality in Wintergreen Lake.Contribution 824 of the National Fisheries Research Center-Great Lakes, 1451 Green Road, Ann Arbor, Michigan 48105, U.S.A. and 722 of the Kellogg Biological Station, Michigan State University.     

Primary productivity and related parameters in three different types of inland waters in Sri Lanka

Gross and net primary production together with chlorophyll-a biomass were investigated with respect to depth and diurnal changes in three categories of inland waters (reservoirs, temporary ponds, brackish water lagoons) in Sri Lanka. Ten field sites, in both the dry and wet zones of the island, were investigated. Bimodal productivity profiles were recorded in two of the three reservoirs studied. The diel pattern of net photosynthetic rate varied between sites although peak photosynthetic efficiency occurred at solar noon. Surface photoinhibition was characteristic of the reservoirs and brackish water lagoons but not of the temporary ponds. Mean gross primary production was 3.02 g C m^–2 d^–1 but was higher in the temporary ponds than in the reservoirs. The gross primary production in the brackish water Koggala Lagoon at 0.08 g C m^–2 d^–1 is a record low for tropical lagoons and was 2.5 times less than the two other lagoons investigated. Variability in net primary production between sites was similar to the variation in gross production with a relatively low mean value for tropical inland waters of 0.495 C m^–2 d^–1. Mean maximum photosynthetic rate was 0.30 mg C m^–3 h^–1 but was lower in the reservoirs than in the temporary ponds and lagoons.     

The relation of biotic and abiotic interactions to eutrophication in Tjeukemeer,The Netherlands

During the summer months of 1974–1985 chlorophyll-a and total P concentration, biomass of Daphnia hyalina, smelt Osmerus eperlanus, bream Abramis brema and pikeperch Stizostedion lucioperca, water temperature and water intake from lake IJsselmeer were monitored in Tjeukemeer. During this period there were manipulations with the bream and pikeperch stocks as a consequence of the termination of a gill-net fishery in 1977, and larval smelt immigrated each year from the large lake IJsselmeer and contributed largely to the yearly smelt recruitment.The correlation matrix of the nine variables mentioned above showed a positive correlation between bream and chlorophyll-a, but surprisingly a negative one between smelt and chlorophyll-a. The latter can only be explained when smelt is the dependent variable. In a multi-linear regression there was a negative effect of temperature, chlorophyll a and pikeperch on smelt and a positive effect of water intake. Daphnia hyalina was negatively influenced by the biomass of smelt and the water intake of lake IJsselmeer. The positive relation of Daphnia hyalina and chlorophyll-a was probably related to better survival chances of D. hyalina in an Oscillatoria-rich environment when smelt is the most important predator. An increasing biomass of bream coincided with higher total-P levels and probably contributed to higher chlorophyll-a levels.     

Analysis of the deep chlorophyll maximum across the Azores Front

Physical, chemical and biological observations made in late July and August 1997 across the Azores Front (37° N, 32°W to 32° N, 29°W) are presented. The objectives of the study were: (1) to analyse horizontal and vertical profiles of temperature, salinity, density, nutrients and chlorophyll-a (Chl a) of the top 350 m; (2) to identify the main differences in the deep Chl a Maximum (DCM) and hydrographic structure between the water masses that pass north and south of the Azores Front; and (3) to estimate phytoplankton primary production in these water masses. Horizontal and vertical profiles of salinity, temperature, density, nutrients and phytoplankton pigments in the top 350 m were analysed. The Front separates two distinct water types: the 18°C Mode Water (18 MW) of sub-tropical origin, and the 15°C Mode Water (15 MW) of sub-polar origin. Differences in the DCM and hydrographic structure between 18 MW and 15 MW were observed in the contour plots of each section. The average Chl a concentration between 5 and 200 m depth decreased significantly from 15 MW to 18 MW. The same pattern was observed for the Chl a concentration at the DCM depth. A vertical one-dimensional model was used to estimate the phytoplankton primary production in the 15 MW and 18 MW and led to an estimated water column average gross primary productivity (GPP) between 1.08 and 2.71 mg C m^–3 d^–1 for the 15 MW and about half of these values for the 18 MW. These results indicate that the typical south–north positive slope on DCM depth parallels a latitudinal increase on GPP, suggesting that the location of the Azores Front may have a significant regional impact on GPP.     

Phytoplankton pigments and adenosine triphosphate (ATP) in Lake Peipsi-Pihkva

The material for pigment analysis was collected 1–3 times a year from Lake Peipsi-Pihkva in 1983, 1987, 1988, 1991 and 1992–1995. Concentrations of chlorophyll a, b and c (Chla, Chlb, Chlc), pheopigment (Pheo) and adenosine triphosphate (ATP) were measured biweekly in 1985–1986. The mean of all Chla values was 20.2 mg m^–1 (median 13.3 mg m^–1) indicating the eutrophic state of the lake. Average Chlb, Chlc, Pheo and carotenoid (Car) contents were 3.7 mg m^–3, 4.1 mg m^–3, 3.0 mg m^–3 and 4.8 mg m^–3, respectively. The average Chlb/Chla ratio was 22.9%, Chlc/Chla 23.4%, Pheo/Chla 38%, Car/Chla 37% and ATP/Chla 3%, the medians being 14.3, 13.6, 17.5, 39.4 and 1.9%, respectively. The proportion of Chla in phytoplankton biomass was 0.41%, median 0.32%. There were no significant differences in temperature, oxygen concentration, Chla, and ATP between the surface and bottom water; the lake was polymictic during the vegetation period. The Chla concentration had its first peak in May followed by a decrease in June and July. In late summer Chla increased again achieving its seasonal maximum in late autumn. The ATP concentration was the highest during spring and early summer, decreasing drastically in autumn together with the decline of primary production. ATP/Chla was the highest during the clear water period in June and early July, which coincided also with the high proportion of Chla in phytoplankton biomass. The highest Chla occurred in November (average 37.2 mg m^–3) when Secchi transparency was the lowest (1.05 m). Concentrations of Chlb, Chlc and carotenoids were the highest in August, that of Pheo in June. Concentrations of Chla and other pigments were the lowest in the northern part of Lake Peipsi (mean 14.7 mg m^–3, median 12.5 mg m^–3) and the highest in the southern part of Lake Pihkva (mean 47.9 mg m^–3, median 16.3 mg m^–3). An increase of Chla and decrease of Secchi depth could be noticed in 1983–1988, while in 1988–1994 the tendency was opposite.     

Predicting concentration of total phosphorus and chlorophyll a in a lake with short hydraulic residence time

The relationship between total phosphorus and chlorophyll a concentration was determined for Skinner Lake, Indiana over an annual cycle in 1978–79. Total nitrogen:total phosphorus ratios in the epilimnion ranged from 19 to 220 suggesting a phosphorus-dependent algal yield in the epilimnion. Approximately 90% of annual TP loading reached the lake via streamflow, and 93% of this entered during snowmelt and spring-overturn periods. At that time incoming water flushed the lake 2.4 times. Atmospheric loading accounted for 1.4% of annual TP load. Internal hypolimnetic TP loading occurred during summer stratification. Mean [chl a] for the ice-free period was 15.15 mg m^–3, within the range expected for eutrophic lakes.The 1978–79 data were used in conjuction with the Vollenweider & Kerekes (1980) model to produce a model specific for the Skinner Lake system. The model predicted mean epilimnetic total phosphorus and chlorophyll a concentrations from mean total phosphorus concentration in inlet streams and from lake water residence time during the period of spring overturn and summer stratification. The Skinner-specific model was tested in 1982 and it closely predicted observed mean epilimnetic [TP] and [chl a] during the ice-free period. This study shows that variability in lake models which average data over an annual period can be reduced by considering lake-specific seasonal variation in hydrology and external TP loading.     

楚科奇海及其海台区粒度分级叶绿素a与初级生产力

2003年夏季中国第二次北极科学考察期间,在楚科奇海及其海台区进行了叶绿素a浓度与初级生产力的现场观测。结果表明,观测海区叶绿素a浓度范围为0.009～30.390μg/dm3。表层浓度为0.050～4.644μg/dm3,平均值为(0.875±0.981)μg/dm3;陆架区次表层和底层的浓度高于表层,海台区深层水的浓度较低,200m层的浓度为(0.015±0.007)μg/dm3。水柱平均叶绿素a浓度区域性特征明显,陆架区高于海台区。R断面进行3趟重复观测,平均叶绿素a浓度分别为(2.564±1.496)μg/dm3,(1.329±0.882)μg/dm3和(0.965±0.623)μg/dm3,浓度呈下降趋势。观测站潜在初级生产力为0.263～4.186mgC/(m.3h),陆架区平均潜在初级生产力((2.305±1.493)mgC/(m.3h))比海台区((0.527±0.374)mgC/(m.3h))高近4倍。平均同化数为(1.22±1.14)mgC/(mgChla.h)。观测区细胞粒径>20μm的小型浮游生物对总叶绿素a浓度和初级生产力的贡献率分别为63.13%和65.16%,细胞粒径2.0～20μm的微型浮游生物和细胞粒径<2.0μm的微微型浮游生物对总叶绿素a和初级生产力的贡献率相差甚小,其对总叶绿素a浓度的贡献率分别为19.18%和17.69%,对总初级生产力的贡献率分别为20.11%和14.73%。     

Succession of phytoplankton in a deep stratifying lake: Mondsee,Austria

Phytoplankton numbers, biovolume, chlorophyll-a and various physico-chemical characteristics were followed at weekly intervals in Mondsee, Austria during the year 1982. Secchi-disk transparency varied from 10 m in winter to 2 m in September. Prior to the onset of stratification phosphate-phosphorus concentration was 4 µg 1^–1 decreasing to undetectable values thereafter. Nitrate-nitrogen dropped from 590 µg 1^–1 to about 100 µg 1^–1 during the same time. The vernal bloom was dominated by Asterionella formosa Hass. which abruptly declined after silicon depletion. Spring growth ceased in early June, when Tabellaria flocculosa (Lyngb.) Kütz var. asterionelloides Grun. dominated. Oscillatoria rubescens D.C. and Microcystis aeruginosa Kütz. dominated summer and early autumn followed by the chrysophyte Dinobryon divergens Imh. and D. sociale Ehr. which formed up to 69% of total biovolume in October. Thereafter diatoms and Cryptophyceae (Rhodomonas lacustris Pascher and Ruttner, Cryptomonas pusilla Bach.) became abundant again.Maximum chlorophyll-a concentration in the epilimnion (16 µg 1^–1) was reached during spring growth of the diatoms. During summer higher chlorophyll-a levels were always associated with the metalimnetic layer of Oscillatoria.Compared with earlier studies, both the total biovolume and the share of Oscillatoria rubescens significantly decreased because of reduced nutrient loading of the lake and wash-out of Oscillatoria (theor. renewal time of the lake: 1.7 years).     

Phytoplankton biomass and production in shelf waters off NW Spain: spatial and seasonal variability in relation to upwelling

Summary Chlorophyll-a and primary production on the euphotic zone of the N-NW Spanish shelf were studied at 125 stations between 1984 and 1992. Three geographic areas (Cantabrian Sea, Rías Altas and Was Baixas), three bathymetric ranges (20 to 60 m, 60 to 150 m and stations deeper than 200 m), and four oceanographic stages (spring and autumn blooms, summer upwelling, summer stratification and winter mixing) were considered. One of the major sources of variability of chlorophyll and production data was season. Bloom and summer upwelling stages have equivalent mean and maximum values. Average chlorophyll-a concentrations approximately doubled in every step of the increasing productivity sequence: winter mixing — summer stratification — high productivity (upwelling and bloom) stages. Average primary production rates increased only 60% in the described sequence. Mean (± sd) values of chlorophyll-a and primary production rates during the high productivity stages were 59.7 ± 39.5 mg Chl-a m^–2 and 86.9 ± 44.0 mg C m^–2 h^–1, respectively. Significant differences in both chlorophyll and primary production resulted between geographic areas in most stages. Only 27 stations showed the effects of the summer upwelling that affected coastal areas in the Cantabrian Sea and Rías Baixas shelf, but also shelf-break stations in the Rías Altas area. The Rías Baixas area had lower chlorophyll than both the Rías Altas and the Cantabrian Sea areas during spring and autumn blooms, but higher during summer upwelling events. On the contrary, primary production rates were higher in the Rías Baixas area during blooms in spring and autumn. Mid-shelf areas showed the highest chlorophyll concentrations during high productivity stages, probably due to the existence of frontal zones in all geographic areas considered. The estimated phytoplankton growth rates were comparable to those of other coastal upwelling systems, with average values lower than the maximum potential growth rates. Doubling rates for upwelling and stratification stages in the northern and Rías Altas shelf areas were equivalent, despite larger biomass accumulations during upwelling events. Low turnover rates of the existing biomass in the Rías Baixas shelf in upwelling stages suggests that the accumulation of phytoplankton was due mainly to the export from the highly productive rías, while the contribution of in situ production to these accumulations was relatively lower.     

Primary Production of Phytoplankton in a Strongly Stratified Temperate Lake

Lake Verevi (12.6 ha, maximum depth 11.0 m, mean depth 3.6 m) is a strongly eutrophic and stratified lake. Planktothrix agardhii is the most characteristic phytoplankton species in summer and autumn, while photosynthesizing sulphur bacteria can occur massively in the metalimnion. Primary production (PP) and chlorophyll a concentration (Chl a) were seasonally studied in 1991, 1993, 2000, and 2001. Vertical distribution of PP was rather complex, having usually two peaks, one at or near the surface (0–1 m), and another deeper (at 3–7 m) in the metalimnion. The values of dark fixation of CO₂ in the metalimnion were in most cases higher than those in the upper water layer. Considering the average daily PP 896 mg C m⁻² and yearly PP 162 mg C m⁻², Secchi depth 2.34 m, and epilimnetic concentrations of chlorophyll a (19.6 mg m⁻³), total nitrogen and total phosphorus (TP, 52 mg m⁻³) in 2000, L. Verevi is a eutrophic lake of a ‘good’ status. Considering the total amounts of nutrients stored in the hypolimnion, the average potential concentrations in the whole water column could achieve 1885 mg m⁻³ of TN and 170 mg m⁻³ of TP reflecting hypertrophic conditions and a ‚bad’ status. Improvement of the epilimnetic water quality from the 1990s to the 2000s may have resulted from incomplete spring mixing and might not reflect the real improvement. A decreased nutrient concentration in the epilimnion has supported the establishment of a ‘clear epilimnion state’ allowing light to penetrate into the nutrient-rich metalimnion and sustaining a high production of cyanobacteria and phototrophic sulphur bacteria.     

Annual productivity of Spirulina (Arthrospira) and nutrient removal in a pig wastewater recycling process under tropical conditions

An evaluation was made of the annual productivity of Spirulina (Arthrospira) and its ability to remove nutrients in outdoor raceways treating anaerobic effluents from pig wastewater under tropical conditions. The study was based at a pilot plant at La Mancha beach, State of Veracruz, Mexico. Batch or semi-continuous cultures were established at different seasons during four consecutive years. The protein content of the harvested biomass and the N and P removal from the ponds were also evaluated. Anaerobic effluents from digested pig waste were added in a proportion of 2% (v/v) to untreated sea-water diluted 1:4 with fresh water supplemented with 2 g L^–1 sodium bicarbonate, at days 0, 3 and 5. A straight filament strain of Spirulina adapted to grow in this complex medium was utilized. A pH value 9.5 ± 0.2 was maintained. The productivity of batch cultures during summer 1998 was significantly more with a pond depth of 0.10 m than with a depth 0.065 m. The average productivity of semi-continuous cultures during summer 1999 was 14.4 g m^–2 d^–1 with a pond depth of 0.15 m and 15.1 g m^–2 d^–1 with a depth of 0.20 m. The average annual productivity for semi-continuous cultures operating with depths of 0.10 m for winter and 0.15 and 0.25 m for the rest of the year, was 11.8 g m^–2 d^–1. This is the highest value reported for a Spirulina cultivation system utilising sea-water. The average protein content of the semi-continuous cultures was 48.9% ash-free dry weight. NH₄-N removal was in the range 84–96% and P removal in the range of 72–87%, depending on the depth of the culture and the season.     

Comparison of water level and eutrophication indicators during the wet and dry period in a eutrophic urban lake

Eutrophication modifies lakes' ecological balances and threatens its viability. To date, eutrophication management strategies have been related to nutrient reduction in the lakes' water column. However, nutrient reduction strategies are complicated by the variations of the lake's water level, nutrient concentration, and eutrophication symptom, which are primarily known to be influenced by the local rainfall patterns. Therefore, this study aimed to compare the variability of water level, total phosphorus, and total chlorophyll-a concentrations in Slim River Lake during wet and dry seasons. In this study, water sampling and depth measurements were carried out from six sampling points for 1 year. Water samples were used to quantify total phosphorus and total chlorophyll-a. Our results showed that mean water levels in the studied lake ranged from 1.36 m to 5.46 m in the wet season and from 1.31 m to 5.41 m in the dry season, which implicated no significant difference (p > .05) between seasons in most sampling points. Total phosphorus present at concentrations exceeding 10 mg/L and showed small variations between wet and dry seasons. Mean total phosphorus concentrations varied from 10.55 mg/L to 26.66 mg/L in the wet season and 10.77 mg/L to 21.76 mg/L in the dry season and showed no significant difference between seasons. In addition, mean chlorophyll-a concentrations ranged from 14.35 mg/m³ to 180.13 mg/m³ and from 14.15 mg/m³ to 39.27 mg/m³ in wet and dry seasons, respectively. Chlorophyll-a concentrations showed significant differences (p < .05) between seasons in the deepest sampling points in the lake. The observed seasonal variations in total chlorophyll-a suggest the importance of algae monitoring during the wet season even when no apparent surge of phosphorus concentration is detected.     

Microbial population dynamics in an extreme environment: controlling factors in talus soils at 3750 m in the Colorado Rocky Mountains

Variation of CH₄ emissions over a three-year period was studied in a reed-dominated (Phragmites australis) littoral transect of a boreal lake undergoing shoreline displacement due to postglacial rebound. The seasonal variation in plant-mediated CH₄ emissions during open-water periods was significantly correlated with sediment temperature. The highest plant-mediated emission rates (up to 2050 mg CH₄ m^–2 d^–1) were found in the outermost reed zone, where culms of the previous growing seasons had accumulated and free-floating plants grew on the decomposing culms. In reed zones closer to the shoreline as well as in mixed stands of reed and cattail, the maximum daily rates were usually > 500 mg CH₄ m^–2 d^–1. The total plant-mediated CH₄ emission during the open-water period was significantly correlated with the seasonal maximum of green shoot biomass. This relationship was strongest in the continuously flooded (water depth > 25 cm) outermost zones. In this area, emissions through ebullition were of greatest importance and could exceed plant-mediated emissions. In general, total emissions of the open-water periods varied from ca. 20 to 50 g CH₄ m^–2 a^–1, but in the outermost reed zone, the plant-mediated emissions could be as high as 123 g CH₄ m^–2 a^–1; ebullition emissions from this zone reached > 100 g CH₄ m^–2 a^–1. The proportion of CH₄ released in winter was usually < 10% of annual emissions. Emissions of CH₄ were higher in this flooded transgression shore the than those measured in boreal peatlands, but the role of ancient carbon stores as a substrate supply compared with recent anthropogenic eutrophication is unknown.     

Underwater light penetration,phytoplankton biomass and photosynthetic activity in Lake Xolotlán (Managua)

We measured underwater light penetration, phytoplankton biomass and photosynthetic activity during three years (1987–1990) in Lake Xolotlán (L. Managua), Nicaragua. Phytoplankton biomass governed the light climate of the photic zone, but as biomass also was composed of a varying proportion of dead algae, light availability for the potential biomass of actively photosynthesizing algae (170 mg Chl-a.m^–2) was reduced. The concentration of chlorophyll-a within the photic zone was thus lower and ranged between 58 and 141 mg Chl-a.m^–2. Still, photosynthetic activity was high (2,162 mg 02.m-2.h^–1) due to an extremely high specific rate of photosynthesis; light was the only factor that limited growth. As also other conditions in Lake Xolotlán, beside light limitation, met with the requirements of the models that have been used to analyse production and photosynthetic characteristics in tropical lakes there was a striking agreement between observed and predicted values.     

Implications of rapid changes in chlorophyll-<Emphasis Type="Italic">a</Emphasis> of plankton,epipelon, and epiphyton in a Pampean shallow lake: an interpretation in terms of a conceptual model

Biomass assessments of algae in wetlands usually include only the phytoplankton community without considering the contribution of other algal associations to total algal biomass. This omission prevents an accurate evaluation of the phytoplankton community as an integral part of the total ecosystem. In the present work, the biomass contributions (expressed as chlorophyll-a content per m² of lake) of phytoplankton, epiphyton on both submerged and emergent macrophytes, and epipelon were measured in Lacombe Lake, Argentina, for the purpose of (1) establishing the relative importance of the phytoplankton and (2) evaluating the entire contribution of algal biomass within the context of the Goldsborough & Robinson conceptual model. Our sampling was carried out monthly for a year in sites representative of different conditions with respect to water depth and type of macrophytes. Physicochemical analyses of water were performed following standard methods. Plankton was collected in a five-level profile at deeper stations and in subsurface samples at the shallow one. Samples of sediment obtained with corers were collected for epipelon sampling and segments of plants were cut at different levels, so as to obtain the epiphytes by scraping. Pigment was extracted with aqueous acetone and calculations were made by means of the Lorenzen equation. According to the Goldsborough & Robinson model, a Lake State developed here during the winter (phytoplankton maxima: 150 mg chlorophyll-a per m²). Then, through the subsequent growth of the submerged macrophytes, an Open State was observed, characterized by a maximum epiphyton biomass (at 3,502 mg chlorophyll-a per m²) along with lower levels of phytoplankton biomass. The epiphytic algae on the emergent macrophytes were always present but attained only relatively low biomass values (maximum: 120 mg of chlorophyll-a per m² in February). The epipelon biomass varied between 50 and 252 mg chlorophyll-a per m², registering a considerable contribution of settled algae from the water column (phytoplankton). This study contributes to our knowledge of wetland dynamics through its assessment of the rapid changes in the relative contributions of both planktonic and attached algae to the total algal biomass within the context of specific environmental factors. Guest editors: U. M. Azeiteiro, I. Jenkinson & M. J. Pereira Plankton Studies