Stable Coexistence of Equivalent Nutrient Competitors Through Niche Differentiation in the Light Spectrum

Abstract

Niche-based theories and the neutral theory of biodiversity differ in their predictions of how the species composition of natural communities will respond to changes in nutrient availability. This is an issue of major environmental relevance, as many ecosystems have experienced changes in nitrogen (N) and phosphorus (P) due to anthropogenic manipulation of nutrient loading. To understand how changes in N and P limitation may impact community structure, we conducted laboratory competition experiments using a multispecies phytoplankton community sampled from the North Sea. Results showed that picocyanobacteria (Cyanobium sp.) won the competition under N limitation, while picocyanobacteria and nonmotile nanophytoplankton (Nannochloropsis sp.) coexisted at equal abundances under P limitation. Additional experiments using isolated monocultures confirmed that Cyanobium sp. depleted N to lower levels than Nannochloropsis sp., but that both species had nearly identical P requirements, suggesting a potential for neutral coexistence under P-limited conditions. Pairwise competition experiments with the two isolates seemed to support the consistency of these results, but P limitation resulted in stable species coexistence irrespective of the initial conditions rather than the random drift of species abundances predicted by neutral theory. Comparison of the light absorption spectra indicates that coexistence of the two species was stabilized through differential use of the underwater light spectrum. Our results provide an interesting experimental example of modern coexistence theory, where species were equal competitors in one niche dimension but their competitive traits differed in other niche dimensions, thus enabling stable species coexistence on a single limiting nutrient through niche differentiation in the light spectrum.

Changes in water color shift competition between phytoplankton species with contrasting light‐harvesting strategies

“…Why not? Previous competition studies showed that species with different photosynthetic pigments can coexist in white light (Stomp et al 2004, 2007, Burson et al 2019. One might therefore expect ample opportunities for species coexistence when cyanobacteria and green algae compete for different colors of light.…”

“…Therefore, the earlier competition models were extended by incorporation of the full spectrum of light (Stomp et al 2004(Stomp et al , 2007. These spectral models predict stable coexistence of species if they absorb different parts of the light spectrum, a theoretical prediction that has been verified by both laboratory competition experiments and field data (Stomp et al 2004, 2007, Burson et al 2019.…”