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

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.

Nile perch distribution in south-east Lake Victoria is more strongly driven by abiotic factors, than by prey densities

“…The rise of the water temperature in Lake Victoria also resulted in higher stability of the water column (Hecky, 1993;Cózar et al, 2012) and may also have affected Nile perch distribution. For instance, Getabu et al (2003) observed high Nile perch densities in warmer, shallow waters.…”

Flood regime and water table determines tree distribution in a forest-savanna gradient in the Brazilian Pantanal

“…Nonetheless, the expected raise in climate fl uctua tions will occur with unprecedented levels by human impacts (Úbeda et al 2013). Typical examples are (1) eutrophication of rivers resulting in cyanobacteria blooms (Cózar et al 2012), (2) conversion of natural vegetation into pastures for cattle grazing, (3) conversion of Cerrado vegetation into crop fields in the high plateau headwaters, (4) environmental contamination (eg. agricultural chemicals and mercury contamination by gold mining), (5) unregulated tourism, (6) introduction of exotic species and (7) infrastructure and unplanned human occupation (Alho 2011, Nunes da Cunha and Junk 2015).…”

Coupled human and natural system dynamics as key to the sustainability of Lake Victoria’s ecosystem services

“…Total nitrogen concentrations have increased since the 1960s, primarily through cyanobacteria fixation, and nitrogen generally follows a decreasing concentration pattern from the coast to offshore waters . While nutrient loading has created the light-limited conditions of the lake, seasonal and interannual variations in primary productivity are now driven primarily by climate (Silsbe et al 2006, Cózar et al 2012. This is of particular relevance since it highlights an important characteristic of qualitative models such as the one we use: they do not account for time or effect lags (Justus 2005).…”

“…Indeed, the whole ecosystem is influenced by climate change , Cózar et al 2012) and has been degraded through eutrophication (Dobiesz et al 2010), which impoverishes the system by driving an irreversible decline in biodiversity (Seehausen et al 1997a). Beyond a certain threshold, eutrophication threatens to cause a more general collapse of stocks, not only of Nile perch (Kolding et al 2008).…”