Stoichiometric and growth responses of a freshwater filamentous green alga to varying nutrient supplies: slow and steady wins the race

Abstract

Summary We examined how the nutrient stoichiometry of the filamentous green alga, Mougeotia sp., is affected by the relative availability of dissolved nitrogen (N) and phosphorus (P). Using a bioassay experiment, we grew Mougeotia in medium having a range of dissolved N and P concentrations and N:P ratios (0.62 to 411 by mol). After growing Mougeotia in an environmental chamber for 17 days, we measured its carbon (C), N and P content (% of dry mass), molar C:N:P ratios, mass‐specific growth rate (MSGR, day−1), chlorophyll a concentration (Chl, μg L−1) and carbon:chlorophyll a (C:Chl) ratios. Due largely to variation in cellular %N and %P, Mougeotia C:N:P ratios varied considerably at the end of the experiment, with C:N ratios ranging from 6 to 41, C:P ratios ranging from 85 to 999 and N:P ratios ranging from 4 to 118. We found Mougeotia to be weakly homeostatic with respect to N:P content, with non‐homeostatic tendencies found for N content and P content. Despite the wide range of N and P concentrations in the medium at the start of the experiment, Mougeotia MSGR was relatively invariant. Nevertheless, high C:Chl ratios observed for N‐ and P‐limited Mougeotia largely matched those expected for algae experiencing nutrient‐limited growth. We further compared our experimental results with stoichiometric data for Mougeotia collected from the Kawartha Lakes in southern Ontario. The high C:P and N:P ratios found in field samples matched those of laboratory Mougeotia experiencing intense and sustained P limitation and are suggestive of severe P limitation of Mougeotia in Kawartha Lakes throughout the summer. The slow growth rates and flexible stoichiometry of this alga would enable it to continue growing even during periods of acute nutrient stress. Hence, future management and control of this nuisance alga may prove difficult if it is unlikely to respond strongly to reductions in external P loading and lower water column P concentrations.