Episodic nutrient enrichments stabilise protists coexistence in planktonic oligotrophic conditions

Abstract

1. Seasonal compositional changes in plankton communities are usually considered as species replacements. Provided the enormous number of individuals integrating the communities and our limited capacity for counting and determining a substantial proportion of them, it may happen that we would only be observing alternative population peaks of a large number of coexisting species. The contemporary coexistence theory addresses coexistence in communities of competing species considering simultaneously relative fitness inequalities and stabilising niche differences as components of average long-term growth rates. Here, we experimentally show that response patterns predicted by the theory occur when varying nutrient pulses fertilise the planktonic community. 2. We used gently self-filling 100 L enclosures to minimise disturbance of the initial community and different pulse P and N additions to manipulate the apparently species-poor epilimnetic community of an ultraoligotrophic P-limited lake. We measured and compared protist species growth response to a gradient of P enrichment and another of N stoichiometric imbalance. The P and N levels selected were within the oligotrophic seasonal and interannual variation of the lake, plus some extreme treatments providing mesotrophic conditions of remote regions affected by N atmospheric contamination. All treatments were replicated using ammonium or nitrate alternatively as N source. 3. Most protist species ¿ recorded across seasons in previous studies in this lake ¿ were recovered, indicating a persistent assemblage of species that is seasonally-hidden to observation. Recovery included some rare species only observed in the slush layers of the seasonal snow and ice cover. Coexistence stabilising mechanisms were indicated by treatment response features such as frequency-dependent growth, inverse relationship between fitness inequality and niche differentiation proxies, high-rank taxonomic levels clustering across the limiting nutrient gradient but segregation at the species level according to the type of nitrogen supply, and resting stage development depending on nutrient conditions. Response similarities between autotrophic and heterotrophic organisms indicate a network of interactions that may reinforce coexistence. 4. Synthesis. The results indicate that many planktonic protist species in oligotrophic waters can show stable long-term non-equilibrium coexistence by alternately recovering from very low densities when episodic nutrient enrichments ¿ of varying P and N amounts and composition ¿ occur.