IMIS

We hypothesize that diazotrophs increase community P use, and decrease N use efficiencies, as new N is brought into the system, relaxing N, and concomitantly aggravating P limitation. We test this by analyzing an extensive dataset from the Baltic Sea (> 3700 quantitative phytoplankton samples), known to harbor conspicuous and recurrent blooms of Nodularia spumigena and Aphanizomenon sp. 2. System-level phosphorus use efficiency (RUEP) was positively related with high proportion of diazotrophic cyanobacteria, suggesting aggravation of phosphorus limitation. However, concomitant decrease of nitrogen use efficiency (RUEN) was not observed. Nodularia spumigena, a dominant diazotroph and a notorious toxin producer, had a significantly stronger relationship with RUEP, compared to the competing non-toxic Aphanizomenon sp., confirming niche differentiation in P acquisition strategies between the major bloom-forming cyanobacterial species in the Baltic Sea. Nodularia occurrences were associated with stronger temperature stratification in more offshore environments, indicating higher reliance on in situ P regeneration. 3. By using constrained and unconstrained ordination, permutational multivariate analysis of variance, and local similarity analysis, we show that diazotrophic cyanobacteria explained no more than a few percent of the ambient phytoplankton community variation. The analyses furthermore yielded rather evenly distributed negative and positive effects on individual co-occurring phytoplankton taxa, with no obvious phylogenetic or functional trait-based patterns. 4. Synthesis. Our study reveals that despite the widely acknowledged noxious impacts of cyanobacterial blooms, the overall effect on phytoplankton community structure is minor. There are no predominantly positive or negative associations with ambient phytoplankton species. Species-specific niche differences in cyanobacterial resource acquisition affect important ecosystem functions, like biomass production per unit limiting resource.