Phytoplankton biomass and water chemistry in two high-mountain tropical lakes in central Mexico

In this work we report the vertical phytoplankton biomass distribution (chlorophyll a depth-profiles) and the integrated phytoplankton biomass (chlorophyll a per unit area) of two high-mountain Mexican Lakes: Et Sol and La Luna (19degrees06'N, 99degrees45'W, 4200 m a.s.l.). Et Sol and La Luna are transparent, nonglacier-fed lakes. Both lakes are continuous warm polymictic with a uniform vertical distribution of phytoplankton biomass. Slight heterogeneities were observed in El Sol with a deep-water chlorophyll maximum above the sediments (maximum difference 0.71 mug L(-1)) associated to phytoplankton sunken cells and the presence of filamentous benthic algae. Phytoplankton biomass per unit area was low in both takes (El Sol 1.91-8.36 and La Luna 1.13-4.80 mg Chla m(-2)), and similar to that of temperate high-mountain lakes. The temporal variations in the phytoplankton biomass were attributed mainly to a combined effect of nutrient input and zooplankton grazing pressure; the effect of low pH is also considered in La Luna.