Macroalgal (Spirogyra) nutrient and chlorophyll stoichiometric response to nutrient concentrations in a tropical river

A tenet of ecological stoichiometric theory is that autotroph stoichiometry is normally responsive to the variable supply of environmental resources of nutrients and light. The extent of autotroph stoichiometric plasticity is likely to vary with environmental resource variability, autotroph species, growth form and physiological needs. A field study was conducted in the Daly River catchment in the Australian wet/dry tropics to test the hypothesis that the carbon (C), nitrogen (N), phosphorus (P) and chlorophyll a (Chl a ) stoichiometry of the riverine, filamentous macroalga Spirogyra fluviatilis is responsive to concentrations of dissolved inorganic nitrogen (DIN), dissolved phosphorus and light, as well as temperature which can influence autotroph stoichiometry indirectly. Linear regressions were undertaken between stoichiometric dependent variables and independent environmental variables. With increased DIN concentrations, S. fluviatilis percentage content of N, C and Chl a and molar N:P ratios increased, whilst C:Chl a ratios decreased. No effect of light or temperature on algal stoichiometry was detected. The regressions, however, only explained one-third or less of S. fluviatilis stoichiometric variability, due most probably to methodological constraints, cell nutrient storage and uptake dynamics, and possibly undetected light and temperature effects. S. fluviatilis demonstrated N plasticity in response to DIN concentrations, to enrich the alga with N under relatively high DIN concentrations, enhancing its nutritional quality for herbivores and omnivores. This study complements others which have demonstrated macroalgal P plasticity. Stoichiometric variables can be used to monitor the impact of nutrient pollution and identify the stoichiometric basis for ecosystem-wide implications of nutrient pollution. • The macroalga Spirogyra fluviatilis biomass exhibited nitrogen plasticity. • Increased dissolved inorganic nitrogen concentrations enriched the alga’s nitrogen content . • The stoichiometry of the alga overlaps with freshwater and marine macroalgae.