Phytoplankton geographic spatialization in two ponds in Limousin (France)

Phytoplankton distribution was not homogeneous among Limousin ponds and within each pond, as shown by chlorophyll a and diatoms in two representative eutrophic ponds. On a small cartographic scale, differences depended on geographic location and water temperature. Spring bloom occurred with a time-lag of several weeks in the highland pond. On a large-scale geographic distribution depended on hydroclimatic factors, especially wind direction and inflow of the rivers. The high concentrations of chlorophyll a near the dam of the water bodies roughly coincided with the maximum fetch and the direction of the dominant winds during the investigations. In both ponds the heaviest algae seemed not to be influenced by wind currents and the greater number of diatoms upstream in the highland pond was probably due to inflow of silica from the river. Phytoplankton mapping is more explanatory and rigorous than Secchi disk transparency mapping, it is useful to the regional authorities for zoning activities: bathing should be avoided in bays and along longitudinal shores, fishing location may be less restrictive. In plateau regions in general phytoplankton is not randomly distributed and spatial heterogeneity of ponds is not only vertical but also horizontal. Deep and embanked ponds are complex organisms analogous to great lakes for some features and to shallow water bodies for other ones.