Basin-Scale Control on the Phytoplankton Biomass in Lake Victoria, Africa

The relative bio-optical variability within Lake Victoria was analyzed through the spatio-temporal decomposition of a 1997–2004 dataset of remotely-sensed reflectance ratios in the visible spectral range. Results show a regular seasonal pattern with a phase shift (around 2 months) between the south and north parts of the lake. Interannual trends suggested a teleconnection between the lake dynamics and El-Niño phenomena. Both seasonal and interannual patterns were associated to conditions of light limitation for phytoplankton growth and basin-scale hydrodynamics on phytoplankton access to light. Phytoplankton blooms developed during the periods of lake surface warming and water column stability. The temporal shift apparent in the bio-optical seasonal cycles was related to the differential cooling of the lake surface by southeastern monsoon winds. North-south differences in the exposure to trade winds are supported by the orography of the Eastern Great Rift Valley. The result is that surface layer warming begins in the northern part of the lake while the formation of cool and dense water continues in the southern part. The resulting buoyancy field is sufficient to induce a lake-wide convective circulation and the tilting of the isotherms along the north-south axis. Once surface warming spreads over the whole lake, the phytoplankton bloom dynamics are subjected to the internal seiche derived from the relaxation of thermocline tilting. In 1997–98, El-Niño phenomenon weakened the monsoon wind flow which led to an increase in water column stability and a higher phytoplankton optical signal throughout the lake. This suggests that phytoplankton response to expected climate scenarios will be opposite to that proposed for nutrient-limited great lakes. The present analysis of remotely-sensed bio-optical properties in combination with environmental data provides a novel basin-scale framework for research and management strategies in Lake Victoria

Modelling Upwelling Irradiance using Secchi disk depth in lake ecosystems

A simple model for upwelling irradiance has been developed. The model represents the relationship between Photosynthetically Active Radiation diffuse attenuation coefficients and Secchi disk depth described with a physical-mathematical expression. This physical mathematical expression allows the evaluation of the sub surface upwelling irradiance that was generated by the interaction between downwelling irradiance and the water column. The validation of the relation was performed using experimental data collected from five different aquatic ecosystems at different latitudes, solar elevations and irradiance levels. We found a good linear, positive correlation between the theoretical and measured upwelling irradiance (R2 = 0.96). The residues were well distributed, around the null value, according a Gaussian curve (R2 = 0.92). The results confirm the importance and the versatility of the Secchi disk measurements for aquatic optics

Spatial dynamics of chromophoric dissolved organic matter in nearshore waters of Lake Victoria

The underwater light conditions in the African Great Lakes depend on the complex dynamics of ecological and hydrological forces, and are strongly influenced by local environmental conditions and global biogeochemical cycles. Changes in the optical conditions in these lakes have direct impacts on ecosystem productivity, carbon dynamics and nutrient availability. A central role in the underwater light climate is played by dissolved organic matter which is present in all aquatic ecosystems. The chromophoric fraction of these compounds can mediate ecosystem change through its influence on the attenuation of ultraviolet and PAR radiation, microbial carbon cycling and radiative transfer. In the African Great Lakes, little information is available regarding the dynamics of dissolved organic matter and those sources and sinks which control its presence in the water column. We present an extensive spatial analysis of three major bays on the Kenyan and Ugandan shores of Lake Victoria. We use these data to examine the dynamics of chromophoric dissolved organic matter in different bays and we develop a model to estimate its flow from these bays to the Lake, considering both conservative mixing and photodegradation processes. While some bays release chromophoric dissolved organic matter practically unmodified into the Lake, increased residence time and exposure to solar ultraviolet radiation create conditions where chromophores are lost before entering the open lake.</jats:p

Interannual bio-optical variability in Lake Victoria for 1997–2004 and global indices of interannual climate fluctuations.

<p>Trend components extracted from the OC4v4-<i>chl</i> series (bottom). North Region, solid line; Transitional Region, dotted line; South Region, dashed line. Indices of interannual climate variability linked to ENSO (shaded area) and IOD (solid line) phenomenon (top). Linear correlations between trend components and climate indices were statistically significant at the 99% confidence level for all the combinations (R>0.840, p<0.001, n = 79).</p

Temporal variability of water temperature (<i>LST</i>) and net heat flux (<i>NHF</i>) in the surface layer of Lake Victoria.

<p><i>LST</i> time series for North Region (A) and South Region (B) indicated by a solid line. The grey line represents the corresponding monthly variability of OC4v4-<i>chl</i> for each Region. Scaling is the same for both graphs (0–44 mg m⁻³). (C) <i>NHF</i> into the surface layer in the North (solid line) and South (dashed line) Regions. NHF was significantly related with the monthly <i>LST</i> variation (<i>LST_t – LST_t−1</i>) (R = 0.651, p<0.001, n = 158). Black bars in the lower axes of figures A and B indicate periods of bloom collapse during the seasonal warm phase.</p

Bio-optical variability in Lake Victoria for the 1997–2004 period.

<p>OC4v4-<i>chl</i> time series for different lake Regions (bottom) and their corresponding seasonal components (top). North Region, solid line; Transitional Region, dotted line; South Region, dashed line.</p

Bio-optically co-varying regions in Lake Victoria.

<p>(A) Spatial pattern of the Pearson correlation coefficients between the mode 1 and the OC4v4-<i>chl</i> time series of the lake-sections. (B) Spatial pattern of Pearson correlation coefficients between the mode 2 and the OC4v4-<i>chl</i> time series. (C) Contour lines were obtained by kriging interpolation. Spatial distribution of the most representative mode (highest R) for each lake-section. (D) Classification of Lake Victoria in regions sharing similar bio-optical co-variation. The Transitional Region between North and South Regions is delimited by dotted lines. The topography of the study area is also shown through the contour heights (in meters) interpolated from a 10' dataset.</p