Restoration of Lake Geneva: Expected versus observed responses of phytoplankton to decreases in phosphorusAnneville, Orlane; Ginot, Vincent; Angeli, Nadine
doi: 10.1046/j.1440-169X.2002.00179.xpmid: N/A
Long‐term phytoplankton responses in Lake Geneva to a decline in phosphorus (P) loading are examined in terms of summer (July–September) biomass and community structure. With the rapid development of human activity on its banks and within its catchment area in the 1960s, this large subalpine hydrosystem shifted from oligotrophy to eutrophy within approximately one decade. Measures to reduce P loading were initiated successfully in the mid‐1970s, when total P concentrations in the winter overturn altered from 90 μg/L in 1980 to 40 μg/L in 1998. Until the 1990s, algal descriptors improved as expected (biomass decline, reappearance of diatom species, increased contribution of nanoplankton). Then, paradoxically, and in contrast to the reappearance of oligotrophic species, summer algal biomass began to increase. Pre‐summer (period prior to the beginning of the clear water phase) dissolved inorganic phosphorus concentrations and summer phytoplankton composition presented similar interannual trends. However, the succession of phytoplankton structure during the reoligotrophication phase differed greatly from that during the eutrophication period, and a recent abnormal upward trend in algal densities is mainly the result of the development of large species that formerly were only common from late September until November. This community change, mainly triggered by filamentous (Mougeotia gracillima, Tribonema) or motile forms (Dinobryon sociale, Cryptophycea), seems to have been induced by the earlier and greater deepening of the P‐depleted layer. In addition to milder summers, this massive development of larger forms seems to be favoured by four of their biological features: tolerance to warm temperatures, tolerance to low‐light intensity (might exploit deeper layers where P is not yet limiting), shapes not only providing a large surface to volume ratio or motility (adaptation to low‐nutrient concentrations), but increasing resistance to zooplankton grazing. This paradoxical trend, perhaps reinforced by the decline on roach Rutilus rutilus abundance (an opportunistic planktivore), is likely to remain until the P‐depleted zone is extended below the layers that can be frequently resupplied in nutrients by hydrodynamic processes.
Characteristics of the delta formation resulting from sediment deposition in Lake Nasser, Egypt: Approach to tracing lake delta formationEl‐Manadely, M. S.; Abdel‐Bary, R. M.; El‐Sammany, M. S.; Ahmed, T. A.
doi: 10.1046/j.1440-169X.2002.00174.xpmid: N/A
A major problem encountered by managers of large reservoirs, such as Lake Nasser (located in the south of Egypt), is the continuous deposition of sediments, resulting in a bed‐level increase, a decrease in lake capacity, and the creation of a new delta that might eventually hinder navigation. To investigate the sedimentation problem in Lake Nasser, a series of surveyed cross‐sections were reviewed and samples of suspended sediment and bed material collected. These sets of data were analysed to investigate the progress of the delta formation, the change in its properties in all directions and the corresponding patterns of suspended and deposited sediment particles. Results showed that deposited sediments, characterized by a variable pattern of particle composition, are building up at the lake entrance. Although the expansion of the delta is multidirectional, its accelerated advance towards the High Aswan Dam would depend on the frequent occurrence of high floods following extended drought periods. Based on the calculated deposited sediment volume in Lake Nasser during the period 1964–1998, the lifetime of the dead storage zone allocated for sediment deposition (31 × 109 m3) is estimated at a minimum of 310 years.
Limnological features of Funil Reservoir (R.J., Brazil) and indicator properties of rotifers and cladocerans of the zooplankton communityBranco, Christina W. C.; Rocha, Maria‐Isabel A.; Pinto, Gláucia F. S.; Gômara, Gisele A.; Filippo, Rodrigo De
doi: 10.1046/j.1440-169X.2002.00177.xpmid: N/A
Funil Reservoir receives inflow from a highly industrialized region and acts as a natural sink to pollutants. Among the consequences of the uncontrolled nutrient loading is the constant presence and periodic heavy blooms of Microcystis aeruginosa.
This study verified limnological features and zooplankton assemblage of the Funil
Reservoir focusing on the environmental‐indicator properties of rotifers and cladocerans.
The summer bloom of M. aeruginosa caused reductions in water transparency,
nitrate and orthophosphate concentrations and raises in chlorophyll a, pH,
dissolved oxygen and chemical oxygen demand values. All zooplankton species presented
spatial and temporal variations with the exception of the copepods, which were present
in all samples. According to canonical correspondence analysis, ammonium was the
variable most related to zooplankton variability and different Rotifera and Cladocera
assemblages indicated distinct environment conditions. Rotifer taxa associated with
increases of water temperature and chlorophyll a concentration were found
with high densities during M. aeruginosa blooms. Species of rotifer and cladocerans are suggested as indicators that can be used to identify different physical and chemical gradients or eutrophic increases in Funil Reservoir.
Seasonal evaporative concentration of an extremely turbid water‐body in the semiarid tropics of AustraliaTownsend, Simon A.
doi: 10.1046/j.1440-169X.2002.00173.xpmid: N/A
Many lakes and other standing water‐bodies in arid and semiarid Australia are extremely turbid, although the nature of this turbidity and its seasonal and interannual variation are not widely known. The water quality of the Longreach Waterhole was investigated over the dry season (April–November). Turbidity exceeded 100 Nephelometric Turbidity Units, comprising mainly colloidal clay and silica minerals. The high total solids’ concentrations of the Waterhole (maximum 1150 mg/L) resulted in euphotic depths as low as 19 cm. Evaporative concentration, inferred from increased conductivity and observed reductions in water level, increased the concentration of total solids, phosphorus, nitrogen and reduced euphotic depth. Elevated nutrient concentrations, however, were not accompanied by increased phytoplankton biomass, measured as chlorophyll a. The water‐body was well oxygenated, although concentrations tended to decline towards the end of the dry season. The influence of evaporative concentration on water quality and the need to relate water quality monitoring and investigations to the hydrologic history of the Longreach Waterhole, and other arid water‐bodies, is emphasized.
Effects of nutrient enrichment on channel catfish growth and consumption in Mount Storm Lake, West VirginiaBlanc, Todd J.; Margraf, F. Joseph
doi: 10.1046/j.1440-169X.2002.00184.xpmid: N/A
With the objective of augmenting fish production in Mount Storm Lake, Virginia Electric and Power Company initiated a programme of phosphorus addition to increase primary production, and ultimately, channel catfish (Ictaturus punctatus) growth in the 486 ha cooling reservoir. We simulated channel catfish growth dynamics using two bioenergetics modelling scenarios: (i) effects of average reservoir temperature on growth, conversion efficiency and consumption; and (ii) effects of reservoir enrichment on growth, which is simulated by increasing feeding rates. During 1991–1993, fish were sampled monthly, but sampling was increased to every 2 weeks during the peak growing season (June–September). Most of the channel catfish collected were aged 0 year and aged 1 year with rapid annual growth rates ranging from 9.0 to 13.7 J/g. We found many age 1 250–300 mm catfish, but few beyond this size. Conversion efficiency (joules gained/joules consumed) was low at approximately 18–19%. High algae consumption (40%) was evident, but consumption of zooplankton and Asiatic clam (Corbicula sp.) increased over the study. Simulated increased feeding rates showed that channel catfish were food limited in summer and fall (July–December). Weight gains with 5 and 10% feeding increases were 6–13% and 18–38%, respectively, from the baseline. Catfish of all sizes should benefit from phosphorus additions.
Seasonal stratification and mixing in a recently flooded mining void in tropical AustraliaBoland, K. T.; Padovan, A. V.
doi: 10.1046/j.1440-169X.2002.00176.xpmid: N/A
Enterprise Pit Lake is a water‐filled mining void located in Australia’s wet/dry tropics south of Darwin in the Northern Territory (13°49.6′S, 131°49.8′E). In the mid‐1990s, the void was rapidly filled by the artificial diversion of the nearby Pine Creek watercourse. Profile data and surface water quality data were collected in the lake on a monthly basis from November 1998 to October 1999. Analysis of this data showed that Enterprise Pit Lake was strongly stratified for most of the year with deep mixing occurring once during the middle of the cool, dry season (August, 1999). The relatively small volume of its epilimnion and its low productivity allowed significant oxygen concentrations to remain in the hypolimnion of the lake when it was strongly stratified. Low pH values in a layer of water with a depth of approximately 35 m and located in the lake’s hypolimnion might be associated with several factors, including enhanced sulphide oxidation on the walls of the pit in this region.