Journal Article

Nitrate, ammonium, and phosphorus drive seasonal nutrient limitation of chlorophytes, cyanobacteria, and diatoms in a hyper-eutrophic reservoir

Williamson, Tanner J., González, Maria J., Andersen, Isabelle M., Vanni, Michael J.

2020

Overview

Nitrogen (N) and phosphorus (P) inputs influence algal community structure and function. The rates and ratios of N and P supply, and different N forms (e.g., NO₃ and NH₄), from external loading and internal cycling can be highly seasonal. However, the interaction between seasonality in nutrient supply and algal nutrient limitation remains poorly understood. We examined seasonal variation in nutrient limitation and response to N form in a hypereutrophic reservoir that experiences elevated, but seasonal, nutrient inputs and ratios. External N and P loading is high in spring and declines in summer, when internal loading because more important, reducing loading N:P ratios. Watershed NO₃ dominates spring N supply, but internal NH₄ supply becomes important during summer.We quantified how phytoplankton groups (diatoms, chlorophytes, and cyanobacteria) are limited by N or P, and their N form preference (NH₄ vs. NO₃), with weekly experiments (May–October). Phytoplankton were P-limited in spring, transitioned to N limitation or colimitation (primary N) in summer, and returned to P limitation following fall turnover. UnderNlimitation (or colimitation), chlorophytes and cyanobacteriaweremore strongly stimulated by NH₄ whereas diatoms were often equally, or more strongly, stimulated by NO₃ addition. Cyanobacteria heterocyte development followed the onset of N-limiting conditions,with a severalweek lag time, but heterocyte production did not fully alleviate N-limitation. We show that phytoplankton groups vary seasonally in limiting nutrient and N formpreference, suggesting that dual nutrient management strategies incorporating both N and P, and N formare needed to manage eutrophication.

• Publisher: John Wiley and Sons, Inc,John Wiley & Sons, Inc