Mineral Turbidity Research Articles

Influence of fisheries and shoreline management on limnological characteristics of three Missouri reservoirs

ABSTRACT Landscape-level analyses based on land cover, morphology, and hydrology account for most of the cross-system variation in pelagic nutrients and suspended solids in Missouri reservoirs. They are based on geometric means, which reduce the influence of extreme temporal variation measured in individual reservoirs. This analysis of 3 conservation reservoirs, managed to benefit recreational fisheries, details how internal processes can alter nutrients, chlorophyll, mineral turbidity, and transparency in long-term (21–42 year) datasets, which contribute to temporal variation. Management practices include the addition of grass carp and herbicides to control nuisance macrophytes and shoreline stabilization with rock and water willow. Among these reservoirs, there is strong evidence that macrophyte removal can increase pelagic nutrients by >90%, resulting in a switch to plankton-dominated conditions (alternative states). In one case, eradication of aquatic vegetation increased mineral turbidity by >60%, which was reversed by reestablishing macrophytes and stabilizing the shoreline. This temporal series supports the modifications of phytoplankton–nutrient relations by mineral turbidity shown in state-wide analyses. Collectively, the long-term data show a significant increase in cyanobacteria biovolume and cyanotoxins, with maximum microcystin concentrations increasing as much as 20 times. Actively flipping lakes to plankton-dominated systems via fisheries management and shoreline stabilization practices has negative impacts on overall water quality, with implications for human and wildlife health.

Optimized coagulation of humic acid and mineral turbidity at alkaline pH using high basicity PACl

Abstract In this study, the simultaneous removal of inorganic turbidity and organics was investigated at alkaline pH to avert the need for pH adjustment and overdosing. The aim was to compare the doses for conventional and enhanced coagulation and, consequently, arrive at optimized coagulation where both had synergistic maximum removal. High basicity PACl was used to coagulate simulated waters prepared by humic acid (HA) and kaolin. The removal of turbidity, total organic carbon (TOC), dissolved organic carbon (DOC) and UV254 was evaluated. The impact of varying input concentrations of HA and turbidity on doses was studied. The enhanced coagulation doses were higher than conventional ones. However, with an increase in input TOC, the difference between enhanced and conventional doses narrowed. The doses for optimized coagulation ranged from 2 to 9 mg Al/L. At optimized coagulation, the removal of TOC, DOC and UV254 varied from 30–85%, 30–89% and 73–91% respectively. Fourier transform infrared (FTIR) spectroscopy revealed the presence of Si-O-C bond. The interactions of unsaturated bonds of hydrophobic organics to inorganic clay were possibly favoured over hydrophilics. HA agglomeration reduced coagulant consumption as the input TOC increased. It was concluded that instead of a stoichiometric approach, a modified dosing approach can be applied for limiting underdosing and overdosing while ensuring maximum removal of impurities.

The Effect of Mineral Sediments on Satellite Chlorophyll-a Retrievals from Line-Height Algorithms Using Red and Near-Infrared Bands

Red and near-infrared line-height algorithms such as the maximum chlorophyll index (MCI) are often considered optimal for remote sensing of chlorophyll-a (Chl-a) in turbid eutrophic waters, under the assumption of minimal influence from mineral sediments. This study investigated the impact of mineral turbidity on line-height algorithms using MCI as a primary example. Inherent optical properties from two turbid eutrophic lakes were used to simulate reflectance spectra. The simulated results: (1) confirmed a non-linear relationship between Chl-a and MCI; (2) suggested optimal use of the MCI at Chl-a < ~100 mg/m3 and saturation of the index at Chl-a ~300 mg/m3; (3) suggested significant variability in the MCI:Chl-a relationship due to mineral scattering, resulting in an RMSE in predicted Chl-a of ~23 mg/m3; and (4) revealed elevated Chl a retrievals and potential false positive algal bloom reports for sediment concentrations > 20 g/m3. A novel approach combining both MCI and its baseline slope, MCIslope reduced the RMSE to ~5 mg/m3. A quality flag based on MCIslope was proposed to mask erroneously high Chl-a retrievals and reduce the risk of false positive bloom reports in highly turbid waters. Observations suggest the approach may be valuable for all line-height-based Chl-a algorithms.

An analysis of MODIS-derived algal and mineral turbidity in Lake Erie

Satellite-derived estimates of chlorophyll concentrations based on colour ratio algorithms traditionally fail in turbid waters such as those found in Lake Erie, resulting in chlorophyll concentrations often orders of magnitude in error and spatial distributions mirroring that of known suspended sediment distributions. Methods are presented here that were used to simultaneously extract algal and mineral suspended particulate matter for Lake Erie from the red and near-infrared bands of NASA's MODIS-Aqua sensor. Results produced spatially and temporally distinct seasonal cycles in agreement with bio-geo-physical processes on the lake. Derived imagery was used to monitor seasonal cycles of both algal and mineral particulate matter on the lake and determine areas of persistently elevated concentrations that may highlight regions of potential water quality concern.

Variation of water quality and phytoplankton along different zones of Aswan High Dam Reservoir

his study was carried out at Aswan High Dam Reservoir and focused on the spatial variation of water quality parameters and phytoplankton composition. Water samples were collected from eighteen sampling stations, where six stations for each of the different zones were selected to represent the lacustrine, transition and riverine zones of the reservoir during 2007 and 2008. The results of the environmental factors showed wide variations in their concentrations along the different zones of the reservoir. In this study, chlorophyll a concentration was lower in the lacustrine zone than in the riverine, although turbidity and secchi depth values were all optimal for light availability. This result was influenced by higher abundances in the reservoir phytoplankton assemblages where riverine conditions predominate, by species tolerant to turbulent conditions and typical high mineral turbidity. Calculation of the trophic state index showed that riverine and transition zones are classified as eutrophic, while lacustrine zone is mesotrophic. Phytoplankton composition recorded thirty nine species belonging to Chlorophyceae (19 species), Cyanophyceae (10 species), Bacillariophyceae (7 species), and rare groups including Dinophyceae (2 species) and Eugelnophceae (1 species). Cyanophyceae were the most encountered group in the lacustrine and transition zones, while Bacillariophyceae were most encountered group in the riverine zone. This is due to that Cyanophyceae prefer the steady, transparent and low nutrients water (lacustrine), while Bacillariophyceae prefer flowing, turbid and nutrient-rich water (riverine). Statistical analysis showed that certain environmental factors do affect the phytoplankton growth. The results of the one-way ANOVA revealed that the different environmental factors (DO, water temperature, secchi depth, water velocity, turbidity, nitrate, phosphorus, magnesium, total dissolved solids, and trophic state index) and biotic factors (such as chlorophyll a and phytoplankton groups) were significantly different at the three zones of Aswan High Dam Reservoir (p<0.05). This result supports the speculation of variation in water quality and phytoplankton along the different zones of the reservoir.

NOTE: Empirical estimation of non-chlorophyll light attenuation in Missouri reservoirs using deviation from the maximum observed value in the Secchi-Chlorophyll relationship

A quadratic reference line delineates the maximum observed Secchi transparency across the measured range of algal chlorophyll (0.7–297 μg/L) in Missouri reservoirs. Deviations below maximum Secchi transparency were common (median = 45% of maximum, interquartile range between 35 and 57%) and are characteristic of water bodies with mineral turbidity. Subtracting the reciprocal of maximum Secchi from the reciprocal of measured Secchi provides an estimate of non-chlorophyll light attenuation (NCLA, m−1= [1/Secchi] – [1/maximum Secchi]). Analyses of several datasets show most cross-system variation in this empirical estimate of NCLA is accounted for by non-algal seston measured gravimetrically and as clay-sized particles. Residual variation is attributed to characteristics of the algal community and color. This simple approach can be applied directly, or with modification, to estimate NCLA in lakes and reservoirs when measurements of mineral particulates are not available. The degree to which response variables, such as Secchi transparency, are less than the maximum is fundamental to our understanding of lake process and their management.

Phytoplankton assemblages in a reservoir cascade of a large tropical - subtropical river (SE, Brazil)

The phytoplankton assemblages from eight reservoirs of the Paranapanema River were studied during two consecutive years. Chlorophyceae and Bacillaryophyceae dominated in richness. The observed high number of taxa, 234, reflects the extensive sampling programme and evidences the necessity of considering the whole hydrograph basin to assess the biodiversity status of inland water ecosystems. The dams had a negative effect on phytoplankton richness, with higher number of taxa associate to riverine (non-regulated) stretches. The tributary rivers also exhibited high species richness, showing the importance of considering the lateral dimension, in addition to the longitudinal one, for aquatic biota inventories in large river basins/reservoirs. Richness and diversity were also positively influenced by the connectivity with lateral wetlands (macrophyte-dominated lakes) due to the periphyton influence. The phytoplankton abundance/biomass was not influenced by higher water retention time. Higher values occurred in the middle basin stretches (river-passage reservoirs) due to the increase in the trophic conditions. There was a positive correlation with phosphorus. Poorer light conditions in the cascade do not limit the phytoplankton biomass, with assemblages dominated by species tolerant to turbulent conditions and high mineral turbidity. Bacillariophyceae and Cryptophyceae dominated numerically. The first group (unicellular forms) was prominent in the large and oligotrophic upstream reservoirs. The second was highly abundant in the river-passage (low retention time), and more eutrophic, reservoirs. Cyanophyceae growth is probably controlled by advection processes (wash-out effect). The zooplankton does not control the phytoplankton biomass and the diversity of both groups is positively associated in the cascade. The structure of the phytoplankton assemblages showed to be a good indicator of the operationally distinct reservoirs of the Paranapanema cascade and also reflected the changes in the trophic conditions along the basin.

Effects of clay mineral turbidity on dissolved organic carbon and bacterial production

Effects of inorganic turbidity from clay minerals on allochthonous dissolved organic matter (DOM), its consumption by bacteria and photodegradation by solar radiation were investigated. In controlled experiments of adsorption of DOM of six specific leaf leachates onto montmorillonite, kaolinite, and clay extracted from drainage basin soil, up to 31 mg C of leaf leachate was adsorbed g clay−1. A suspension of clay adsorbed preferentially the chromophoric portion of dissolved organic carbon (DOC) and enhanced photochemical decomposition of CDOM and DOC under experimental conditions representative of the surface of the water column. Significant amounts of DOM were desorbed from the clay-organic aggregates into water of low ionic strength and low concentrations of DOM. Solar radiation decomposed chromophoric organic matter adsorbed onto suspended clay in part to low molecular organic products and to CO2. Montmorillonite clay significantly decreased (>90%) bacterial production, unless the clay had appreciable adsorbed DOM. With organic matter adsorbed, the addition of montmorillonite or of natural clay minerals resulted in significant increases (+100%) in bacterial production. This increase indicated that clay-organic matter aggregates can play an important role in the transport of terrestrial organic matter to aquatic environments and that this organic matter adsorbed to clay particles is available to bacteria through desorption.

Nutrient concentrations in Nyanza Gulf, Lake Victoria, Kenya: light limits algal demand and abundance

Nyanza Gulf is a large shallow embayment connected to Lake Victoria by the deep narrow Rusinga Channel. Between December 2000 and May 2002 the gradient of nutrients along the axis of Nyanza gulf into the main lake was determined. Nutrient concentrations in the gulf were found to be different from those in the main lake with phosphorus fractions, SRP and TP being significantly higher in the main lake than in the gulf. Well oxygenated conditions in the gulf keep the PO4–P strongly bound to mineral particles whereas in the main lake, where deeper depths allows for development of anoxia, it is released into solution. The low TN:TP molar ratio in the gulf and in the main lake indicates many algae may be N-limited and heterocystous N-fixing cyanobacteria may be favored. However, the high mineral turbidity in the gulf reduces light availability and hence limits algal abundance resulting in easily measure concentrations of mineral nutrients and in particular high levels of dissolved reactive silica. The gulf is a net source of dissolved silica and total nitrogen to the main lake while the main lake is potentially a source of P to the gulf depending on interchange through Rusinga Channel.

Limnology of southern African coastal lakes — new vistas from Mozambique

Fourteen mostly small, isolated, and relatively deep coastal inundation lakes were surveyed during the cool season (August) of 1996. No thermal or chemical stratification existed, but near-bottom hypoxia was evident in several systems. Ionic conductivity varied quite widely, but was highest in isolated systems, suggesting evaporative dominance. Low water transparency, attributable to biogenic turbidity, was reflected in high chlorophyll levels (mean = 28.3μg l −1) in most lakes. However, maximal penetration of red light in six lakes implies the influence of mineral turbidity — dystrophy being excluded by the alkaline character (pH > 8) of all 14 lakes. Unusually high standing stocks of zooplankton (mean = 604mg m−2DW), mostly small-bodied freshwater species (rotifers, cyclopoids, cladocerans) as well as larger calanoid and cyclopoid copepods were present, consistent with algal indications of eutrophy. Characteristically relict estuarine/marine fauna (a calanoid copepod, crown crab, amphipod and fish) were present in two oligotrophic lakes, including the largest — putatively a drowned-valley lake. Significant contrasts between these Chidenguele lakes and other southern African coastal lakes are evaluated and discussed. Artisanal fisheries, targetting two indigenous freshwater cichlids and a clariid, as well as an estuarine ambassid and gobiid, were active. Survey findings were used to assess the suitability of these lakes as prospective habitats for exotic grass and silver carp. As these lakes support artisanal fisheries based on indigenous taxa the need for and wisdom of alien introductions is challenged, and we call for parallel summer surveys to precede any such decision. Evidence at hand indicates the potential suitability of more lakes for silver than for grass carp, but four lakes were identified as meriting special conservation status, in view of certain unique attributes and/or values.

Limnology of Lakes in Gates of the Arctic National Park and Preserve, Alaska

ABSTRACT Limnological reconnaissance data were collected during summers 1992–93 and 1995 from 16 major lakes within Gates of the Arctic National Park and Preserve, Alaska (GAAR) located above the Arctic Circle. In GAAR the southern lakes (~67°N) and those in the Brooks Range foothills are in watersheds with taiga and tundra vegetation. The northern lakes (~68°N) are at higher elevations in the Brooks Range in watersheds that lie wholly beyond the treeline. Average sum of the cations for all lakes matches the world average for fresh waters and the average for drainage from tundra and taiga landscapes. Local lithography explains the measured decrease in calcium equivalents and silica, and an increase in magnesium with altitude because of more calcareous rock in the southern basins at low altitude and shale/conglomerate in northern mountain catchments. In most low altitude lakes Secchi depth was located in sub-surface algal peaks at temperatures averaging ~8.5 C cooler than the surface, typically with double the surface chlorophyll value. Mineral turbidity, and less so color, controlled transparency in several high altitude lakes where turbid inflows were a factor. Lakes were oligotrophic based on nutrients and algal chlorophylli, but a doubling of TP and concurrent halving of TN was measured in GAAR lakes with altitude. Both patterns were correlated with the decreasing density of terrestrial vegetation with altitude, resulting in a sharp decline in the TN:TP ratio. This pattern suggests sources of these nutrients change across the landscape continuum within GAAR described by altitude and vegetation zones. Nitrogen fixation associated with terrestrial vegetation most likely accounts for greater TN in lakes within the taiga and moist or wet tundra, whereas TN levels approximated the N content of regional rainfall in high elevation lakes with predominately barren land and prostrate shrub in the catchments. Ratios of TN:TP and Nutrient Stimulation Bioassays suggest P limitation was likely among low altitude lakes and N limitation increased in importance in lakes at high altitude. The particulate composition ratio (as C:N:P molar ratio) of these lakes averaged ~200:20:1.

N:P ratios, light limitation, and cyanobacterial dominance in a subtropical lake impacted by non-point source nutrient pollution

A long-term (28-year) data set was used to investigate historical changes in concentrations of phosphorus (P), nitrogen (N), N:P ratios, and Secchi disk transparency in a shallow subtropical lake (Lake Okeechobee, Florida, USA). The aim was to evaluate changes in the risk of N 2-fixing cyanobacterial blooms, which have infrequently occurred in the lake's pelagic zone. Predictions regarding bloom risk were based on previously published N:P ratio models. Temporal trends in the biomass of cyanobacteria were evaluated using phytoplankton data collected in 1974, 1989–1992, and 1997–2000. Concentrations of pelagic total P increased from near 50 μg l −1 in the mid-1970s to over 100 μg l −1 in the late 1990s. Coincidentally, the total N:P (mass) ratio decreased from 30:1 to below 15:1, and soluble N:P ratio decreased from 15:1 to near 6:1, in the lake water. Published empirical models predict that current conditions favor cyanobacteria. The observations confirm this prediction: cyanobacteria presently account for 50–80% of total phytoplankton biovolume. The historical decrease in TN:TP ratio in the lake can be attributed to a decreased TN:TP ratio in the inflow water and to a decline in the lake's assimilation of P, relative to N. Coincident with these declines in total and soluble N:P ratios, Secchi disk transparency declined from 0.6 m to near 0.3 m, possibly due to increased mineral turbidity in the lake water. Empirical models predict that under the turbid, low irradiance conditions that prevail in this lake, non-heterocystous cyanobacteria should dominate the phytoplankton. Our observations confirmed this prediction: non-N 2-fixing taxa (primarily Oscillatoria and Lyngbya spp.) typically dominated the cyanobacteria community during the last decade. The only exception was a year with very low water levels, when heterocystous N 2-fixing Anabaena became dominant. In the near-shore regions of this shallow lake, low N:P ratios potentially favor blooms of N 2-fixing cyanobacteria, but their occurrence in the pelagic zone is restricted by low irradiance and lack of stable stratification.

A baseline limnological study of Wagendrift Dam (Thukela basin, KwaZulu-Natal)

Basic features of the physical and biological limnology of Wagendrift Dam, a moderately large impoundment on the Boesmans River, were studied between July 1989 and May 1990. This man-made lake showed a typical monomictic pattern of summer stratification (November through April) and holomictic winter circulation (May to October). Water quality was chemically good (Department of Water Affairs and Forestry records), with little evidence of nutrient enrichment. Water clarity during the study was moderately low (mean SD and K d PAR values of 0.45 m and 2.55 m-1), with little evidence of persistent mineral turbidity, suggesting reasonably satisfactory catchment conditions. In keeping with the low nutrient status, surface chlorophyll content remained below 5 ?g ·l -1 throughout the study. In terms of ecological groupings, the phytoplankton was numerically dominated mostly by 'Competitive' green algae for most of the year. 'Stress-tolerant' cyanophytes were continuously sparse, but were marginally more abundant during summer stratification. Diatoms and other disturbance-tolerant 'ruderal' algae persisted throughout the year. No spring peak in ruderal algae was evident, but it may have been masked by high zooplankton grazing pressure. An autumnal peak in ruderal algae coincided broadly with flood-related inflows and annual de-stratification. Zooplankton was dominated (numerically and/or gravimetrically) by typical clear-water taxa. The copepod Tropodiaptomus spectabilis and cladoceran Daphnia pulex along with various unidentified cyclopoids and rotifers were seasonally persistent, while other cladoceran taxa (D. laevis and Diaphanosoma excisum) showed considerable seasonal periodicity. Zooplankton standing stock was collectively substantial, annually averaging around 0.65 g·m -2 DM, and approaching 1 g·m -2 DM in spring/early summer, implying potentially significant natural grazer-control of phytoplankton, aided perhaps by large numbers of benthic bivalve molluscs (Unio caffer), observed stranded in littoral margins following reservoir draw-down. WaterSA Vol.27(4) 2001: 507-516

Limnology Of Missouri Reservoirs: An Analysis of Regional Patterns

ABSTRACT Data from 94 Missouri reservoirs demonstrated a large interregional variation in total phosphorus (TP), total nitrogen (TN), algal chlorophyll (Chl), and suspended solids among the four physiographic provinces with numerous reservoirs. The interprovincial pattern for nutrients, algal biomass, and mineral turbidity was Ozark Highlands < Ozark Border ≃ Glacial Plains < Osage Plains corresponding to an interregional gradient between forest and agriculture as the dominant land cover. On the basis of TP most Missouri reservoirs were eutrophic (61%) or mesotrophic (21%) with few hypereutrophic (7%) or oligotrophic (11%). Statewide, water clarity was low (median Secchi depth 1.0 m) and more strongly related to non-volatile suspended solids (NVSS, R2 = 0.80) than algal Chl (R2 = 0.30). Summer mean values showed a curvilinear relation between Chl and TP (log10 transformed) reaching an asymptote above ≃ 50 μg/L TP/L — a range where most reservoirs have high NVSS and low TN:TP. Nitrogen limitation, light limitation, or binding of phosphorus by non-algal suspended material may explain reduced Chl:TP ratios in turbid reservoirs. Survey data, however, do not distinguish the roles of differing limiting nutrients and light limitation in reservoirs with > 10 mg NVSS/L. Conventional lake management scenarios based on phosphorus control seem appropriate for Missouri reservoirs of low to moderate turbidity.

Type of suspended clay influences zooplankton response to phosphorus loading

The effects on zooplankton community structure of P loading and two different kinds of suspended sediments were tested in a small lake. Limnocorrals were used in a complete, triplicated, six‐treatment‐block design. Treatments were loading with P, kaolinitic clay (K), K+P, montmorillonitic clay (M), and M+P plus a control without additions. P fertilization caused blooms of blue‐green bacteria and resulted in reductions of the dominant copepod (Diaptomus mississippiensis) and cladocerans (Diaphanosoma brachyurum, Bosmina longirostris, and Ceriodaphnia reticulata), while causing a 5‐fold increase in rotifers (mostly Keratella cochlearis). Zooplankton in the K and K+P treatments were similar to the control, but cladoceran species other than Diaphanosoma were reduced. M was more detrimental to zooplankton than K, causing a 10‐fold decrease for all crustaceans. We attribute this in part to lower algal densities in the M treatment. Simultaneous P loading mitigated the effects of M on Diaphanosoma, rotifers, and copepods and may involve a transfer loop in which dissolved organic C adsorbed on clay becomes available to filter‐feeding zooplankton. The effects of mineral turbidity apparently cascaded up the food chain, with both kinds of clays reducing densities of Chaoborus larvae.

Effects of mineral turbidity on freshwater plankton communities: three exploratory tank experiments of factorial design

We conducted three exploratory experiments of factorial design on the effects of mineral turbidity in 18 large (7 000 1) fiberglass tanks containing plankton communities derived from Lake Texoma (Oklahoma-Texas), a large reservoir often subject to turbid inflows. Replicate plankton communities developed for 30–45 d in response to 4–9 treatments of planktivorous fish, dead fish, nutrient additions and artificial removal of zooplankton with plankton netting (or combinations of these manipulations). In one experiment we then added three concentrations of kaolin and in the other two experiments we added bentonite or powdered silica to the replicate tank communities. We measured the responses of the diverse plankton communities to the addition of mineral turbidity for an additional period of 30–45 d. Effects of introduced minerals (quality or concentration effects) and food web treatments after mineral addition (fish, nutrients, etc.) were determined by analysis of covariance after observations were adjusted for the value of each response variable at the time of mineral addition. Covariance analysis resulted in a statistical model explaining > 80% of the variance for most response variables. The values of response variables at the time of mineral addition were often the most important source of variation, suggesting the importance of biotic community resiliency to the effects of mineral turbidity. There were few effects of mineral particles on physical or chemical (temperature, conductivity, oxygen, pH), nutrient (nitrate, phosphate, alkalinity) or biotic (algal or zooplankton populations) components of the tank communities. Mineral effects were found for several measures of water clarity (Secchi depth, turbidity, and the concentration of small sestonic particles).

Effects of mineral turbidity on freshwater plankton communities: Three exploratory tank experiments of factorial design

We conducted three exploratory experiments of factorial design on the effects of mineral turbidity in 18 large (7 000 1) fiberglass tanks containing plankton communities derived from Lake Texoma (Oklahoma-Texas), a large reservoir often subject to turbid inflows. Replicate plankton communities developed for 30–45 d in response to 4–9 treatments of planktivorous fish, dead fish, nutrient additions and artificial removal of zooplankton with plankton netting (or combinations of these manipulations). In one experiment we then added three concentrations of kaolin and in the other two experiments we added bentonite or powdered silica to the replicate tank communities. We measured the responses of the diverse plankton communities to the addition of mineral turbidity for an additional period of 30–45 d. Effects of introduced minerals (quality or concentration effects) and food web treatments after mineral addition (fish, nutrients, etc.) were determined by analysis of covariance after observations were adjusted for the value of each response variable at the time of mineral addition. Covariance analysis resulted in a statistical model explaining > 80% of the variance for most response variables. The values of response variables at the time of mineral addition were often the most important source of variation, suggesting the importance of biotic community resiliency to the effects of mineral turbidity. There were few effects of mineral particles on physical or chemical (temperature, conductivity, oxygen, pH), nutrient (nitrate, phosphate, alkalinity) or biotic (algal or zooplankton populations) components of the tank communities. Mineral effects were found for several measures of water clarity (Secchi depth, turbidity, and the concentration of small sestonic particles).

Zooplankton feeding rates in relation to suspended sediment content: potential influences on community structure in a turbid reservoir

SUMMARY. 1. Changes in zoopiankton composition and abundance in Lake le Roux, a turbid subtropical reservoir on the Orange River in South Africa, were correlated with changes in water transparency (related to suspended sediment levels) during a 7 year field study. Results of radiotracer studies of the effect of mineral turbidity on zooplankton feeding rates which potentially influence competitive ability, and thus community structure, are reported here.2. Feeding rates of five zooplankters were very variable, but consistently declined with rising turbidity: rates of decline differed between species. A regression estimate of the critical turbidity threshold at which food intake matched the estimated respiratory need was derived for each species. This yielded the following “turbidity‐tolerance” ranking: Moina brachiata Jurine &gt; Metadiaptomus meridianus (van Douwe) ×Daphnia gibba Methuen &gt; D. barbata Weltner &gt; D. longispina O. F. Muller. The consistency between this ranking and one based upon abundance‐transparency relationships in the field study suggests that community structure is related to differential feeding capabilities, although other influences are not excluded.3. Tests on D. gibba and M. meridianus failed to reveal any detectable feeding rate saturation (incipient limiting food level) below 1.2mg 1−1 C. The relative reduction in feeding rates at elevated turbidity was nearly 3 times greater for the daphnid than the copepod over a range of food concentrations, and considerably reduces the competitive ability of this (and other) daphnids. The turbidity tolerance disparity between Moina and the daphnids demonstrates a more complex situation than a simple copepod/cladoceran dichotomy. These findings and their implieations are discussed in relation to wider features of zooplankton ecology.

Assessment of the trophic state of a water body

The difference in point of view between the limnologist and the public on the level of a water body is discussed in relation to actions promoted by administrators. Numerical and probabilistic classification, models, trophic indices and biological indicators for assessing the degree of lakes are briefly illustrated. The contribution of the OECD's Eutrophication (1971-1979) to the assessment of the degree of lakes is relevant. In agreement with Vollenweider's model, this programme adopted as a guide-line the relationship between phosphorus load from the watershed and the phosphorus concentration in the lake water and between this concentration and that of phytoplankton, expressed as chlorophyll concentration. These relationships are statistically significant but a certain number of lakes deviate from them. These deviations and their causes are considered ; for example, nutrient release from sediments, short hydrological relention time, high mineral turbidity, excessive development of macrophytes, non available forms of phosphorus, high predation pressure by zooplankton. The opportunity to take into consideration the influence of toxic pollutants in eutrophicated water bodies is emphasized. To compare the degree of lakes laying at different latitudes, the pattern of solar radiation and temperature should be considered, in addition to nutrient loading. As a case study to show the difficulties involved in assessing the degree of a lake and identifying the causes of its evolution the results from a study carried out from 1969 to 1980 on the small alpine lake Monate (Northern Italy) are discussed.