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.
KeywordsLimiting nutrient Monsoon Phytoplankton Nutrient stimulation experiments Nitrogen Phosphorus
We thank Erin Petty for making Figure. 1. We also thank Jennifer Parris, Jennifer Graham, John MacEachern, Robert Bacon, Bruce Perkins, Glenn Wylie, Lance Dorsey, Prem Nepali, and IUCN Nepal for their assistance during the study. We thank Gertrud Nürnberg for advice on internal nutrient loading. This work was supported by the National Science Foundation (INT-9602105).
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