The specific inherent optical properties of three sub-tropical and tropical water reservoirs in Queensland, Australia

The underwater light climate, which is a major influence on the ecology of aquatic systems, is affected by the absorption and scattering processes that take place within the water column. Knowledge of the specific inherent optical properties (SIOPs) of water quality parameters and their spatial variation is essential for the modelling of underwater light fields and remote sensing applications. We measured the SIOPs and water quality parameter concentrations of three large inland water impoundments in Queensland, Australia. The measurements ranged from 0.9 to 42.7 μg l⁻¹ for chlorophyll a concentration, 0.9–170.4 mg l⁻¹ for tripton concentration, 0.36–1.59 m⁻¹ for a _CDOM(440) and 0.15–2.5 m for Secchi depth. The SIOP measurements showed that there is sufficient intra-impoundment variation in the specific absorption and specific scattering of phytoplankton and tripton to require a well distributed network of measurement stations to fully characterise the SIOPs of the optical water quality parameters. While significantly different SIOP sets were measured for each of the study sites the measurements were consistent with published values in other inland waters. The multiple measurement stations were allocated into optical domains as a necessary step to parameterise a semi-analytical inversion remote sensing algorithm. This article also addresses the paucity of published global inland water SIOP sets by contributing Australian SIOP sets to allow international and national comparison.     

Testing relationships between macroalgal cover and Secchi depth in the Baltic Sea

The present study tests whether relationships between macroalgal cover and water quality, recently developed for Danish coastal waters, are more universal and also applies at the other extreme of the Baltic Sea in Finnish coastal waters. We found that algal cover increases as a function of Secchi depth according to the same logarithmic function in Danish and Finnish coastal waters. Algal cover at a given depth (here modelled for 4 m) increases with increasing Secchi depth and approaches a maximum at the high Secchi depths found in the clearest areas of the Danish coastal waters. For a given Secchi depth the combined Danish/Finnish algal model thus predicts a similar cover of the algal community at a given water depth at both extremes of the Baltic Sea which represent quite different algal habitats. These results suggest that light limitation, and thus shading effects of eutrophication may cause similar reductions of macroalgal cover across ecosystems.     

The spectral distribution and attenuation of underwater irradiance in Tasmanian inland waters

SUMMARY. 1. Measurements were made of the attenuation and spectral distribution of downwelling and upwelling photosynthetically-available radiation (PAR) in all the principal types of natural waters found in Tasmania. 2. Most lakes in the State are clear and non-turbid, with water itself and the low concentrations of gilvin being the principal determinants of the green underwater light climate. Many others are deeply coloured by dissolved and colloidal organic material (gilvin, gelbstoff) which rapidly attenuates short wavelengths, specifying a shallow, predominantly red euphotic zone. 3. A spectrophotometric measure of colour, the absorption coefficient at 440 nm, is statistically related to measurements on the platinum scale with good precision. 4. Few Tasmanian lakes are turbid but in those that are the underwater light climate is almost identical to that of non-turbid, humic lakes. 5. Reflectance, R, varied with depth but not in the asymptotic way previously encountered. A linear relationship existed between the scattering coefficient, b, and nephelometric turbidity, but not at the approximate 1:1 ratio reported elsewhere. 6. Most Tasmanian lakes are oligotrophic or dystrophic and phytoplankton rarely influenced the underwater light field. 7. Seasonal variation in optical character is not great in natural lakes and their optical properties and light fields can be used typologically. 8. Simple and multiple regression analysis showed that Secchi depth was a poor predictor of euphotic depth but the optical properties and the underwater light field of inaccessible lakes could be reasonably predicted from laboratory measurements made on small water samples, using regressions developed for a wide range of lake types and by reference to the quantaradiometric scans of lakes with comparable optical properties. 9. An optical classification of Tasmanian lakes made by cluster analysis agreed reasonably well with one based on edaphic, vegetational and chemical criteria.     

Light availability as a possible regulator of cyanobacteria species composition in a shallow subtropical lake

1. Variations in the relative biovolumes of dominant cyanobacterial taxa were evaluated in the context of environmental conditions using canonical correlation analysis (CCorrA) and Redundancy Analysis (RDA). The objective was to test a conceptual model in which underwater irradiance determines dominance by bloom-forming (high light adapted) or non-blooming (low light adapted) taxa. 2. The data set consisted of 404 contiguous observations, collected over a 3-year period at eight pelagic sites, in shallow Lake Okeechobee, Florida, U.S.A. Data included species biovolumes, total phosphorus (TP), total nitrogen (TN), dissolved oxygen (DO) and chlorophyll a concentrations, as well as two indices: underwater irradiance (Secchi depth) and the ratio of Secchi:total depth. 3. The first environmental canonical variable was strongly correlated with the two light-related indices, and negatively correlated with TP. This reflects the predominant role of resuspended P-rich lake sediments in controlling underwater irradiance in the shallow lake. The first species canonical variable displayed a strong negative correlation with Lyngbya limnetica and L. contorta, and positive correlations with Anabaena circinalis, Aphanizomenon flos aquae and Microcystis spp. The results support the conceptual model; the first pair of canonical variables explained 55% of the variation in the species–environmental data set. RDA results provided further support for the hypothesis that irradiance was the major force controlling community structure. 4. One unexpected result was a positive association between Oscillatoria spp. dominance and indicators of high irradiance. This conflicts with past research indicating that Oscillatoria is a low light adapted taxon, and the finding that it is the most abundant taxon in Lake Okeechobee. This may reflect the fact that the two Lyngbya taxa were more strongly associated with low light conditions than Oscillatoria. CCorrA results indicated that Oscillatoria densities are strongly controlled by water temperature. There is a need for more detailed studies of cyanobacteria ecophysiology in order to explain fully the seasonality of phytoplankton in this and other shallow subtropical lakes.     

New distribution record for the Asian tapeworm Bothriocephalus acheilognathi Yamaguti, 1934 in the Eastern Cape province,South Africa

Assuming that the inshore and offshore waters of Lake Victoria are impacted differently by human activities in its catchment, this study investigated the water quality dynamics of the lake. A total of 29 stations were sampled in 2005–2008 for dissolved oxygen (DO), pH, Secchi transparencies, temperature, turbidity, chlorophyll a, NO₃, SRSi, TN and TP. There was a decreasing trend of the measured parameters towards offshore sites, except for Secchi transparency and NO₃, which increased towards the offshore waters. DO concentrations (mean ± SD) varied between 6.97 ± 0.57 mg O₂ l^–1 and 5.80 ± 0.72 mg O₂ l^–1 in the inshore and offshore waters, respectively. Turbidity values were comparatively higher in the inshore (3.73 ± 2.21 NTU) than the offshore waters (2.19 ± 1.81 NTU). Chlorophyll a concentrations varied between 17.36 ± 6.13 µg l^–1 and 8.09 ± 4.38 µg l^–1 in the inshore and offshore waters, respectively. Increases of unsustainable human activities in the lake and its catchments, plus increased degradation of wetlands, are among the causes of the observed water quality changes. In order to be fruitful and sustainable, the management of Lake Victoria and its catchment needs to take an ecosystem approach, and to involve all key stakeholders.     

The impact of phytoplankton on spectral water transparency in the Southern Ocean: implications for primary productivity

Spectral water transparency in the Northern Weddell Sea was studied during Austral spring. The depth of the 1-% surface irradiance level (euphotic depth) varied between 35 and 109 m and was strongly influenced by phytoplankton biomass. Secchi depths were non-linearly related to euphotic depth. In phytoplankton-poor water, the most penetrating spectral region was restricted to a relatively narrow waveband in the blue (488 nm), but the range was broader, between 488 and 525 nm when phytoplankton were abundant. Water transparency in the red spectral range was always low and only to a small extent affected by phytoplankton. Two independent procedures were used to quantify the impact of phytoplankton on spectral water transparency: (1) Regression analysis of spectral in situ vertical light attenuation coefficients in the sea, against coincident chlorophyll concentrations. This method gave chlorophyll-specific light attenuation coefficients; the y-intercept could be interpreted as a measure of light attenuation by pure water plus non-algal material. (2) Spectra of in vivo light absorption derived by spectroscopy, using phytoplankton enriched to varying degrees onto filters. Thus chlorophyll-specific absorption cross-sections were determined. Estimates obtained by both procedures were in close agreement. By integrating over the spectrum of underwater irradiance, in situ chlorophyll-specific absorption cross sections of phytoplankton suspensions, related to all photosynthetically active radiation, were calculated. Light absorption by phytoplankton for photosynthesis is accomplished mainly in the blue spectral range. Also dissolved and particulate organic matter contributed to the attenuation of blue light. Because in water poor in phytoplankton, underwater irradiance was progressively restricted to blue light, chlorophyll-specific absorption cross-sections of phytoplankton, averaged over the spectrum of photosynthetically active irradiance, increased with water depth. In water with elevated phytoplankton biomass, overall light attenuation was generally enhanced. However, because the spectral composition of underwater light changed relatively little with depth, except immediately below the water surface, light absorption cross-sections of phytoplankton changed little below 10 m depth. Vertical differences in the proportions of underwater light absorbed by the phytoplankton community here were mainly dependent on biomass variations. Because of the comparatively small attenuation of blue light by non-algal matter, the efficiency of light harvesting by phytoplankton at any given concentration of chlorophyll in Antractic waters is greater than in other marine regions. At the highest phytoplankton biomass observed by us, as much as 70% of underwater light was available for phytoplankton photosynthesis. When phytoplankton were scarce, <10% of underwater light was harvested by phytoplankton.Contribution within the European Polarstern Study (EPOS), supported by the Deutsche Forschungsgemeinschaft, Grant Ti 115/16-1 to MMT, the European Science Foundation, and by the Alfred Wegener Institut für Polar-und Meeresforschung, Bremerhaven     

Seasonal light characteristics for a newly formed reservoir in Southwestern USA

Seasonal transparency and extinction coefficients are presented for two major habitats in the Lake Powell system, a large newly formed reservoir in the arid southwestern USA. These habitats include a shallow side canyon station subjected to periodic influx of dissolved and suspended materials and an open bay system. Maximum Secchi disk levels in the open bay station were at 7.7 meters with the Secchi disk estimated to disappear at a level of penetration approximating 10% of the solar radiation entering the lake surface. Mean compensation depth was calculated at 11.7 meters. Seasonal extinction coefficient for unfiltered light in the deeper bay station was 0.321 while extinction coefficients for blue, green and red filtered light were 0.323, 0.309 and 0.474 respectively. The effect of periodic influx of dissolved and suspended substances on the underwater light climate and phytoplankton populations are discussed for desert lakes.This study was supported in part by funds provided by the Navajo and Kaiparowits participants.This study was supported in part by funds provided by the Navajo and Kaiparowits participants.     

The light-climate of Loch Leven, a shallow Scottish lake, in relation to primary production by phytoplankton

Seasonal changes in incident irradiance and underwater light penetration at Loch Leven from 1968 to 1971 are discussed in relation to the photosynthetic behaviour and crop density of phytoplankton. Light extinction was highest in the blue and lowest in the orange spectral regions, a pattern typical of other turbid waters. Euphotic depth varied between 1·2 and 7·4 m and was on average c. three times the Secchi disc transparency. Underwater light extinction depended chiefly on phytoplankton crop density (estimated as chlorophyll a). Despite the shallowness and wind-exposed situation of the loch there was no evidence of appreciable light extinction due to sediment disturbance. Possible causes of variability in the relationship between the minimum vertical extinction coefficient (k _min) and the concentration of chlorophyll a are discussed. The value of k_s, the increment in k_min per unit increment in algal concentration, was estimated from field data as 0·0086 In units per mg chl a/m² and from laboratory spectroradiometer data as 0·0079 In units per mg chl a/m². These k_s values imply theoretical upper limits for the amount of chlorophyll a in the euphotic zone (Σn _max) of 430 and 468 mg chl a/m², respectively. Observed euphotic chlorophyll a contents (Σn) were sometimes close to these upper limits. Typical photosynthesis/depth profiles are described. Profile area is shown to be related to the logarithm of the ratio between surface-penetrating irradiance (I_o') and the irradiance (I_k) defining the onset of light-saturation of photosynthesis. Standardized profiles, plotted on a common scale of ‘optical depth’, are used to illustrate the relatively minor influence of variations in I_o' and I_k on hourly rates of photosynthesis per unit area. The saturation parameter (I_k) generally increased as photosynthetic capacity (P_max) increased; the temperature-dependence of I_k is explained by the temperature-dependence of the enzyme-controlled (dark) reactions of photosynthesis, which control P_max. A spring peak in the ratio between surface penetrating irradiance (I_o') and I_k is interpreted as a result of a lag in the seasonal increase in water temperature with increase in surface irradiance. The gradient (K') of the linear light-limited region of the photosynthesis-irradiance curve showed little variation and had an average value of 0·31 mg O₂/mg chl a.h per 1 W/m² (PAR). Interactions between mixed depth, underwater light extinction and phytoplankton productivity are discussed; comparisons are made with other shallow, optically deep lakes.     

Reference conditions for phytoplankton at Danish Water Framework Directive intercalibration sites

Phytoplankton is one of the biological quality elements included in the EU Water Framework Directive (WFD). Classification of water quality according to the WFD is based on the deviation of the present conditions from reference conditions. Given the lack of data from pristine conditions, this study used approximately 100-year-old measurements of Secchi depths from Danish waters in combination with relationships between Secchi depth and chlorophyll a (as a proxy for phytoplankton biomass) obtained from recent monitoring to calculate ‘historical’ or reference chlorophyll a (Chl-a) concentrations. Historical Secchi depth data were available for 9 out of the 11 Danish WFD intercalibration sites. At eight of the sites, reference summer (May–September) Chl-a concentrations were in the range 0.7–1.2 μg l⁻¹. At one site, west of Bornholm in the western Baltic Sea, historical Secchi depth measurements date back to only the late 1950s corresponding to a calculated Chl-a concentration of 1.3 μg l⁻¹. This value cannot be considered representative of reference conditions. Guest editors: J. H. Andersen & D. J. Conley Eutrophication in Coastal Ecosystems: Selected papers from the Second International Symposium on Research and Management of Eutrophication in Coastal Ecosystems, 20–23 June 2006, Nyborg, Denmark     

Relationships between Fish Species Abundances and Water Transparency in Hypertrophic Turbid Waters of Temperate Shallow Lakes

We explored the relationships between Secchi disc depth and the abundance of fish species in very shallow, hypertrophic, turbid waters of Pampa Plain lakes, Argentine. We tested whether the abundance of any of the species present was associated with water transparency for lakes where water transparency, as measured by Secchi disc depth, ranged from 0.1 to 0.4 m. Overall, the abundance of five species (Cnesterodon decemmaculatus, Jenynsia multidentata, Corydoras paleatus, Pimelodella laticeps and Odontesthes bonariensis) seemed to be affected by this narrow gradient in water transparency. These findings represent an interesting result for turbid hypertrophic environments where narrow ranges in water transparency are traditionally neglected as important factors for fishes. We show, however, how water transparency patterns may be still important for some species in highly turbid waters with extremely narrow gradients in Secchi disc depth. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Light penetration in different types of lakes in Central Finland

Light penetration measurements of blue (Schott filter BG 12), green (VG 9), and red (RG 630) light was made in 30 Finnish lakes with different degree of humus content and eutrophy for the comparison of the structure of phytoplankton communities and the light environment in the waters.
The relations between Secchi disc visibility and the penetrations of different spectral regions and relations between the light conditions and the colour of the water was examined.
The increase of ɛ_min m₋₁ caused by humus material is 0.008 In units per colour unit (mg Pt l⁻¹). The optical depth was especially in brown waters smaller than calculated by Formula after Talling (1965).     

Modeling underwater light climate in relation to sedimentation,resuspension, water quality and autotrophic growth

The underwater light climate ultimately determines the depth distribution, abundance and primary production of autotrophs suspended within and rooted beneath the water column. This paper addresses the underwater light climate, with reference to effects of suspended solids and growth responses of autotrophs with emphasis on phytoplankton.Effects of the most important factors contributing to the absorption and scattering of light in surface waters were described. A comparison between spectral and scalar approaches to underwater light climate modeling was made and examples of linear approximations to light attenuation equations were presented. It was demonstrated that spectral and scalar photosynthesis models may converge to similar values in spectral-flat, high photon flux environments, but that scalar PAR models may overestimate biomass-specific production by 70%. Such differences can lead to serious overestimates of habitat suitability for the growth and survival of submersed macrophytes, particularly in relatively turbid, coastal waters.Relationships between physical and optical properties of suspended sediments were described theoretically, and illustrated with modeling examples and measurements. It was found that the slowly settling particulate fraction contributed substantially to the suspended solids concentration, and greatly to light attenuation within the water column. It was concluded that distinguishing particles by fall velocity and concomitant light attenuation properties in the modeling of underwater light conditions allowed the establishment of useful, although not simply linear, relationships.In eutrophic, shallow lakes, the largest contribution to light attenuation often originates from phytoplankton on a seasonal basis (months–years), but from suspended solids behavior on a shorter time scale (days–weeks), particularly when water bodies are wind-exposed. Temporal and spatial variabilities in wave height, suspended solids concentrations, and light attenuation within the water column, and their importance for autotrophic growth were described, and illustrated with a case study pertaining to Markermeer, The Netherlands. The influence of underwater light conditions on phytoplankton succession was briefly discussed and illustrated with a case study pertaining to Lake Veluwe, The Netherlands. It was concluded that modeling the underwater light climate in a water body on a few sites only can indicate how important various components are for the attenuation of light, but based on the current state of the art, it can not be expected that this will provide accurate predictions of the underwater light climate, and of phytoplankton and submersed macrophyte growth.     

Prediction of Secchi disc depths in Florida lakes: Impact of algal biomass and organic color

A model for the prediction of Secchi disc depths in Florida lakes was developed and tested using data from 205 lakes. A statistical analysis showed that the best estimate of lake Secchi disc depths could be obtained by In (SD) = 2.01 ? 0.370 In (Chla) ? 0.278 In (C) where SD is Secchi disc depth (m), Chla is the chlorophyll a concentration (mg/m³) and C is the organic color concentration (mg/l as Pt). The model yields unbiased estimates of lake Secchi disc depths over a wide range of algal and organic color concentrations and has a 95% confidence interval of 47 to 224% of the calculated Secchi disc depth. Other published Secchi-Chlorophyll models are less precise but can be used almost equally well. This indicates organic color concentrations do not affect lake Secchi disc depths as much as algal levels. Further reductions in the remaining error term, however, might be accomplished by including a variable for suspended inorganic sediment.     

Sediment resuspension and light attenuation in Peoria Lake: can macrophytes improve water quality in this shallow system?

We examined sediment resuspension and light attenuation in relation to the potential for macrophytes to improve water quality conditions in Peoria Lake, Illinois (U.S.A.). The lake exhibited high total suspended solids (TSS) loading and retention of predominantly fine-grained particles in 2000. Large fetches along prevailing wind rose, coupled with shallow morphometry and sediment particles composed of >90% silt and clay resulted in frequent periods of sediment resuspension. As calculated (wave theory) shear stress increased above the critical shear stress (measured experimentally), turbidity increased substantially at a resuspension monitoring station. Resuspension model explorations suggested that establishment of submersed aquatic macrophytes could substantially reduce sediment resuspension in Peoria Lake. However, K _d is currently very high, while Secchi transparency low, at in-lake stations. Thus, in order to establish a persistent macrophyte population in the lake to control resuspension, the underwater light regime will have to improve quite dramatically.     

EARLY OBSERVATIONS ON SOME LIMNOLOGICAL CHARACTERISTICS OF THREE ORANGE FREE STATE IMPOUNDMENTS

SUMMARY

The first limnological investigations on three impoundments in the Orange Free State are described. No thermal stratification was found and water temperatures below 8,6°C were not measured. The waters were turbid and the unfavourable underwater light climate was the major factor limiting algal growth. The catchment areas influenced the ionic composition of the waters and large quantities of ions were adsorbed onto the suspended silt.     

Sources,concentrations and characteristics of organic matter in softwater lakes and streams of the Swedish forest region

18 Swedish forest lakes covering a wide range of dystrophy were studied in order to quantify and characterize the organic matter in the water with respect to origin (allochthonous or autochthonous), physical state (particulate or dissolved) and phosphorus content. Samples were collected repeatedly during a two-year period with unusually variable hydrological conditions. Water from three different depths and from tributaries was analysed with standard monitoring methods, including water colour, Secchi disk transparency, total organic carbon (TOC), COD_Cr, COD_Mn, total phosphorus and molybdate reactive phosphorus. Interrelationships were used to compare different methods and to assess the concentration and composition of organic matter. It is estimated that in remote softwater lakes of the Swedish forest region, autochthonous carbon is typically < 5 g m⁻³. Most lakes in this region receive significant amounts of humic matter originating from coniferous forest soils or peatland in the catchment area. In most humic lakes with a water colour of ≥ 50 g Pt m⁻³, more than half of the organic carbon in the surface water is of allochthonous origin, and in polyhumic lakes (> 200 g Pt m⁻³) the proportion can exceed 90%. Secchi depth readings were related similarly to organic matter from both sources and provided good estimates of TOC with a single optical measurement. Water colour was used to distinguish allochthonous and autochthonous matter. High concentrations of phosphorus were found in humic waters, most of it being molybdate reactive, and probably associated with humic matter rather than as dissolved free inorganic forms. COD_Mn yielded only 25–60% of TOC and appears to include mainly truly dissolved substances of low molecular weight.     

The effect of non-algal turbidity on the relationship of Secchi depth to chlorophyll a

Data from four reservoirs representative of different trophic states and with different apparent optical properties were analyzed to determine the relationship of Secchi depth to algal biomass as measured by chlorophyll a. In the eutrophic reservoir Secchi depth was determined partially by the chlorophyll a content (r² = 0.31) but only when chlorophyll a data from bloom conditions are included. In the two mesotrophic reservoirs, Secchi depth was entirely determined by non-algal turbidity. In the oligotrophic reservoir, Secchi depth was determined neither by chlorophyll a nor non-algal turbidity and was probably determined by dissolved color. When data from the four reservoirs were pooled (N = 205), 53% of the variation in Secchi depth was explained by: SD = 2.55–0.52 ln (Turbidity) + 0.005 (Chlorophyll a). It is apparent that attempts to estimate algal biomass for trophic state classification or other management practices from Secchi depth data are inappropriate even where moderate amounts of non-algal turbidity are present.     

The influence of the spectral composition of irradiance on primary production in the Eastern Scheldt (The Netherlands)

The error inin vivo ¹⁴C incubator measurements of primary production in the Eastern Scheldt when neutral density filters were used and the error obtained when no account was taken of the spectral changes in submarine irradiance that occur with increasing depth, were evaluated theoretically. By multiplying the photosynthetic action spectra of two marine algae by calculated irradiance in the euphotic layer using Kd and Kd() respectively, the gross primary production P[Ed(400–700)] and P[Ed()] was computed. In the green-brown waters of the Eastern Scheldt estuary the use of neutral density filters was sufficient to simulate the underwater light conditions. In clear waters it can cause an overestimation of the gross production.     

Remote sensing as a tool for assessing water quality in Loosdrecht lakes

The underwater light field in 7 lakes in the Loosdrecht lake area was measured in situ. Subsurface upwelling irradiance and irradiance reflectance, together with estimations of scattering and laboratory measurements of absorption by aquatic humus and particulate matter, enabled an analysis of the spectral signature of these waters. Aircraft imaging spectrometer measurements of upwelling radiance at 1 km altitude were used to simulate the PMI Chlorophyll #1, the CAESAR Inland Water Mode spectral bandsets and the Thematic Mapper bands 1 to 4. This made it possible to compare the effects of spectral band width and selection on the estimation of water quality parameters. Correlations increased to r > 0.94, at a significance level of 1% for the simulated C-IWM data with the 6 water quality parameters. Images of the PMI Chlorophyll #1 and of the TM were analysed and found to be in accordance with the statistical modelling results.A significant increase in correlation of remote sensing data with water quality parameters can be achieved through the selective use of 10 to 20 nm wide bands in the spectral range of 500 to 720 nm in these eutrophic waters. Sum of chlorophyll a and phaeopigments, seston dry weight, Secchi disc transparency, and coefficients for vertical attenuation of light, absorption and scattering can be estimated accurately. TM image data for water quality assessment is of limited use due to the relatively low spectral and radiometric resolution. However, the revisit capability and relatively low price per area are positive aspects of these satellite images.Abbreviations CAESAR = CCD Airborne Experimental Scanner for Applications in Remote sensing - C-IWM = CAESAR Inland Water Mode - CCD = charge coupled device - EOS-A = Earth Observation System Platform A - PAR = photosynthetically active radiation from 400–700 nm. - PMI = Programmable Multispectral Imager - RSLL = Remote Sensing Loosdrecht Lakes Project - SPOT = Systeme Pour l'Observation de la Terre - SPOT-HRV = Sensor on board of the SPOT satellite - TM = Thematic Mapper instrument aboard the Landsat 5 satellite     

The Environment of Lake Victoria (East Africa): Current Status and Historical Changes

Beginning in the mid‐1980s Lake Victoria experienced severe eutrophication and it was suggested that deteriorating water quality might lead to a collapse of its fisheries. A series of lake‐wide surveys carried out 1999–2001 and 2005–2009 revealed that the temperature of the lake had risen by > 1 °C since 1927, with more rapid warming of the deeper waters reducing the thermal gradient in the water column and thus weakening stratification and the extent and severity of deoxygenation. The chlorophyll a concentrations in open water decreased since the 1980s, while Secchi disc visibility increased, indicating a reduced severity of algal blooms. Chlorophyll a was higher and Secchi disc visibility lower in inshore waters but there has been no deterioration in these areas since the 1980s. The conductivity remained unchanged, although it was about 50% greater in the semi‐enclosed Nyanza Gulf than in the open lake. The water quality of the lake has therefore improved considerably despite the fact than concentrations of plant nutrients have not decreased and the reasons why this may be the case are discussed. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)