Phytoplankton nutrient deficiencies vary with season in sub-tropical lakes of Nepal

This is one of few studies to comprehensively examine seasonality and phytoplankton nutrient deficiency in sub-tropical lakes over multiple years, to our knowledge. We tested whether phytoplankton communities in two sub-tropical impoundments in the mid-hills of Nepal were nitrogen (N)-, phosphorus (P)-, or co-deficient in N and P across pre-monsoon, monsoon, and post-monsoon seasons spanning a decade. Nutrient limitation to phytoplankton growth was assessed via in situ stoichiometry of N and P (N:P ratios) and nutrient stimulation experiments (NSEs). The experiments indicated co-deficiency of N and P in 97–100% of NSEs in all seasons. N-deficiency was common (> 60% of N:P ratios and ~ 90% of NSEs) during the rainy monsoon, but P-deficiency occurred twice as often as N-deficiency during drier seasons (pre- and post-monsoon). These findings provide perspective for the ongoing debate over N and P as limiting nutrients in freshwater ecosystems by incorporating seasonality and corresponding hydrology. We also re-visit long-standing assumptions regarding the nutrient status of sub-tropical lakes. The warm, stratified season does not fully illustrate lake processes, and more research during dry periods will inform how seasonality affects phytoplankton nutrient deficiencies across aquatic systems.