Nitrogen: The limiting nutrient for phytoplankton growth in an oligotiophic, tropical lake

The purpose of the present study was to evaluate if the available phosphorus and nitrogen concentrations were limiting the phytoplankton biomass in Lake Alchichica. Here we present a series of nutrient enrichment experiments (nitrogen, phosphorus, nitrogen + phosphorus) conducted in the two characteristic stages of the lake's hydrodynamics: stratification and mixing. We followed up the algal growth potential through in vivo fluorometric analysis of chlorophyll a changes expressed as relative change percentage regarding the control. The nutrient enrichment experiment during the circulation period did not modify significantly the phytoplankton growth or the species. Opposite, during the stratification period, the nutrient addition promoted the phytoplankton growth significantly by modifying its composition and abundance. The results showed that nitrogen is the most likely nutrient limiting phytoplankton growth in Lake Alchichica as has been found in other tropical lakes, opposite to temperate lakes where phosphorus has been found to be the limiting nutrient. The enrichment of one nutrient without the other produced only a brief growth enhancement until depletion of the other nutrient occured. However, when both nitrogen and phosphorus were added, enhanced algal growth were sustained and produced substantial and detectable responses.