Phytoplankton-Specific Response to Enrichment of Phosphorus-Rich Surface Waters with Ammonium, Nitrate, and Urea

<div><p>Supply of anthropogenic nitrogen (N) to the biosphere has tripled since 1960; however, little is known of how <em>in situ</em> response to N fertilisation differs among phytoplankton, whether species response varies with the chemical form of N, or how interpretation of N effects is influenced by the method of analysis (microscopy, pigment biomarkers). To address these issues, we conducted two 21-day <em>in situ</em> mesocosm (3140 L) experiments to quantify the species- and genus-specific responses of phytoplankton to fertilisation of P-rich lake waters with ammonium (NH₄⁺), nitrate (NO₃⁻), and urea ([NH₂]₂CO). Phytoplankton abundance was estimated using both microscopic enumeration of cell densities and high performance liquid chromatographic (HPLC) analysis of algal pigments. We found that total algal biomass increased 200% and 350% following fertilisation with NO₃⁻ and chemically-reduced N (NH₄⁺, urea), respectively, although 144 individual taxa exhibited distinctive responses to N, including compound-specific stimulation (<em>Planktothrix agardhii</em> and NH₄⁺), increased biomass with chemically-reduced N alone (<em>Scenedesmus</em> spp., <em>Coelastrum astroideum</em>) and no response (<em>Aphanizomenon flos-aquae</em>, <em>Ceratium hirundinella</em>). Principle components analyses (PCA) captured 53.2–69.9% of variation in experimental assemblages irrespective of the degree of taxonomic resolution of analysis. PCA of species-level data revealed that congeneric taxa exhibited common responses to fertilisation regimes (e.g., <em>Microcystis aeruginosa</em>, <em>M</em>. <em>flos-aquae</em>, <em>M</em>. <em>botrys</em>), whereas genera within the same division had widely divergent responses to added N (e.g., <em>Anabaena</em>, <em>Planktothrix</em>, <em>Microcystis</em>). Least-squares regression analysis demonstrated that changes in phytoplankton biomass determined by microscopy were correlated significantly (<em>p<</em>0.005) with variations in HPLC-derived concentrations of biomarker pigments (<em>r</em>² = 0.13–0.64) from all major algal groups, although HPLC tended to underestimate the relative abundance of cyanobacteria. Together, these findings show that while fertilisation of P-rich lakes with N can increase algal biomass, there is substantial variation in responses of genera and divisions to specific chemical forms of added N.</p> </div