Abstract
Supply of anthropogenic nitrogen (N) to the biosphere has tripled since 1960; however, little is known of how in situ 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 in situ 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 (Planktothrix agardhii and NH4 +), increased biomass with chemically-reduced N alone (Scenedesmus spp., Coelastrum astroideum) and no response (Aphanizomenon flos-aquae, Ceratium hirundinella). 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., Microcystis aeruginosa, M. flos-aquae, M. botrys), whereas genera within the same division had widely divergent responses to added N (e.g., Anabaena, Planktothrix, Microcystis). Least-squares regression analysis demonstrated that changes in phytoplankton biomass determined by microscopy were correlated significantly (p<0.005) with variations in HPLC-derived concentrations of biomarker pigments (r 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.
Highlights
Human activities such as farming and industrial fixation of atmospheric nitrogen (N) have tripled the supply of N to the biosphere since 1960 and are expected to double present levels of N influx by 2050 to meet future demands for food production [1,2]
Instead the unique objectives of the present study are four-fold: 1) to use microscopic analysis to quantify interspecific variation among algae in the response to N amendments; 2) to determine how phytoplankton-specific responses vary with the chemical form of added N; 3) to evaluate the influence of the taxonomic resolution of microscopic analysis on interpretation of N effects on phytoplankton, and; 4) to compare changes in phytoplankton assemblages derived from microscopic enumeration of cell densities and chromatographic analysis of algal pigments
Lake and Mesocosm Conditions As presented in [25], nutrient concentrations in Wascana Lake were elevated (,125–175 mg P L-1,1.2–1.6 mg N L-1), total dissolved N (TDN) : total dissolved P (TDP) mass ratios were low (,10–15), and bottle bioassays revealed that phytoplankton growth exhibited instantaneous limitation by N supply during both August and September
Summary
Human activities such as farming and industrial fixation of atmospheric nitrogen (N) have tripled the supply of N to the biosphere since 1960 and are expected to double present levels of N influx by 2050 to meet future demands for food production [1,2]. Application of N fertilisers will exceed 275 Tg N year and will be concentrated in regions where centuries of farming may have saturated soils with phosphorus (P) [3,4], increased P export to lakes [5], and overloaded surface waters with P [6] In these regions, lakes already exhibit poor predictive relationships between P influx and algal abundance [7], continuously high concentrations of dissolved P despite abundant phytoplankton [3,8], insufficient biological fixation of N to support primary production [9,10], and strong positive correlations between N influx and total algal or cyanobacterial abundance [6,11]. Lack of reconciliation between these two robust data sets has resulted in vigorous and occasionally acrimonious debate over the unique and interactive roles of N in regulating baseline lake productivity [26,27] and cultural eutrophication [13,28,29]
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