Nitrogen Form Impacting Phytoplankton Across Scales in China’s West Lake: from Nitrogen-metabolism to Community Composition

China’s marine and freshwaters are among the most nitrogen (N) polluted in the world. N pollution has been linked with nuisance and toxic algal blooms, oxygen depletion, loss of fisheries, and changes in biodiversity. Although the effect of total nutrient loading has been well explored in terms of promoting algal blooms, the effects of changing nitrogen forms (e.g. NO₃^- and NH₄⁺) and proportions on algal community composition and nitrogen productivity are poorly understood in systems experiencing prolonged nutrient loading. Therefore, we conducted a series of experiments on natural waters collected from West Lake, Hangzhou, China to assess shifts in phytoplankton community composition and nitrogen based metabolism following the addition of changing nitrogen forms. Natural samples were collected during the months of May through August from three sampling sites and were variably enriched with NH₄⁺ and NO₃^-, with and without supplemental additions of phosphate (P) to produce a range of nutrient supply ratios. The samples were incubated under natural light and temperature conditions for several days during which phytoplankton community composition, biomass accumulation, and productivity were tracked. There was no significant change in biomass with the addition of different N forms to the three sampling locations during the spring; however, the addition of P resulted in approximately a three-fold increase in biomass consistently. Overall, the benthic filamentous alga Tribonema spp. dominated the phytoplankton community, until July 24^th when the diatom Nitzschia spp. dominated following the super typhoon Chan-hom. The addition of NO₃^- and P to the mid West Lake station resulted in three times as many Nitzschia spp. cells than the addition of NH₄⁺ and P during late summer. These results will be presented in terms of the spatial conditions that promoted the bloom and how they fit into our observations of dichotomous phytoplankton communities.