Abstract
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.
Highlights
The world’s great lakes constitute vital ecosystems with complex conservation challenges.While high spatio-temporal variability is often shown in these massive ecosystems [1,2,3], specific basin-scale mechanisms controlling biological and biogeochemical variability have yet to be thoroughly explored [4]
Standardised calibration algorithms are still problematic for turbid waters, the use of statistical methods based on the relative optical variability within the lake provides valuable information on central processes operating in the lakes
The robust patterns documented for Lake Victoria show that basin-scale inaccuracies in the atmospheric correction of the water surface reflected radiance may be offset by the optical variability of an extensive data series with high spatio-temporal resolution
Summary
The world’s great lakes constitute vital ecosystems with complex conservation challenges.While high spatio-temporal variability is often shown in these massive ecosystems [1,2,3], specific basin-scale mechanisms controlling biological and biogeochemical variability have yet to be thoroughly explored [4]. Understanding the spatiotemporal dynamics and their relationship with the environmental drivers is a basic step in establishing effective strategies for the management of the great lakes. This task often lacks suitable data at basin and long-term scales. General calibration algorithms have yet to be developed for inland waters, where nonalgal water components often play an important role in the optical properties [7]. Phytoplankton dynamics play a major role in the optical properties of these extensive lakes because of the relatively limited influence of water components of terrestrial origin over much of the lake surface
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