Contrasting effects of low‐level phosphorus and nitrogen enrichment on growth of the mat‐forming alga Didymosphenia geminata in an oligotrophic river

Abstract

Summary The stalked diatom Didymosphenia geminata forms proliferations of stalk‐dominated mats in rivers. Over its range in New Zealand, mats rarely occur where average dissolved reactive phosphorus (DRP) concentrations exceed ~2 mg m−3, but do occur in rivers with dissolved inorganic nitrogen (DIN) concentrations up to ~250 mg m−3. Previous studies indicated that low DRP concentrations limit D. geminata cell division and stimulate stalk production and mat development, even at very low ambient DIN concentrations (~10 mg m−3). We ran two nutrient‐enrichment experiments in instream channels in an oligotrophic river with a persistent D. geminata proliferation to (i) test the hypothesis that elevated DRP concentrations reduce or prevent mat development by reducing stalk production; and (ii) investigate the effect of DIN enrichment on D. geminata growth and mat development. In Experiment 1, treatments comprised low‐level enrichment with DRP (to 1.6 mg m−3, from superphosphate fertiliser), DIN (to 13.5 mg m−3, from ammonium sulphate), and both DRP and DIN. Compared to unenriched controls, DIN and DRP enrichment enhanced D. geminata cell densities, biomass, cell division (frequency of dividing cells), stalk biovolume and percentage cover of mats, indicating both N‐ and P limitation for all growth metrics. In the DRP‐enriched treatments, biomass accrual may have been stimulated rather than reduced as predicted because DRP concentrations did not exceed the putative 2 mg m−3 threshold. In Experiment 2, we expanded the range of DRP enrichment using additions of orthophosphate as well as superphosphate, and added DIN to all treatments. Higher DRP concentrations (up to 4 mg m−3) initially enhanced D. geminata growth. By the end of the experiment, D. geminata‐dominated biomass in the orthophosphate treatments was lower than in the superphosphate or DIN‐only treatments. Outflows from the experimental channels caused slight nutrient enrichment in the river downstream. During Experiment 1, this DRP and DIN enrichment had no detectable effect on D. geminata standing crop on the riverbed. During Experiment 2, D. geminata standing crop downstream of the experiment declined relative to the unenriched area upstream. The specific mechanism linking P enrichment to D. geminata biomass decline is unclear. In the experimental channels there was evidence for a predominance of cell division over stalk production in orthophosphate treatments, as hypothesised. In the river, we observed higher rates of sloughing in the river downstream of the experiment than upstream. There was no evidence that competition from other algal taxa reduced D. geminata standing crop. The results supported the hypothesis that DRP enrichment at concentrations over ~2 mg m−3 leads to reduced D. geminata biomass and % cover in this river. However, biomass reduction occurred only when DRP was added as orthophosphate, not as superphosphate. N limitation of D. geminata growth in river water with ambient mean DIN concentrations below ~6 mg m−3 suggests that D. geminata proliferations may be stimulated by increases in DIN concentration in rivers where both DIN and DRP are naturally low. The threshold for N limitation was ~10 mg m−3.