Stable coexistence of equivalent nutrient competitors through niche differentiation in the light spectrum.

Amanda Burson,Jef Huisman,Lisette Mekkes,Maayke Stomp

doi:10.1002/ecy.2873

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.

Highlights

Many ecosystems experience major changes in nutrient loads due to anthropogenic eutrophication and the impacts of an expanding human population on land use and climate (Smith 2003, Pen~uelas et al 2012, Glibert et al 2014)
The phytoplankton communities sampled from the North Sea were grown in laboratory experiments under either N-limited or P-limited conditions to investigate which species would become dominant under which nutrient limitation
The phytoplankton mixture sampled from the North Sea and used as inoculum for the multispecies experiments consisted of a species-rich community of nanoflagellates (31% of total biovolume), picoeukaryotes (30%), and diatoms (21%), with smaller contributions by dinoflagellates, nonmotile nanoeukaryotes, and picocyanobacteria

Summary

Introduction

Many ecosystems experience major changes in nutrient loads due to anthropogenic eutrophication and the impacts of an expanding human population on land use and climate (Smith 2003, Pen~uelas et al 2012, Glibert et al 2014). To understand and predict how changes in nutrient loads will affect the species composition of aquatic and terrestrial communities, we may turn to niche-based theories such as resource competition theory (Tilman 1982, Grover 1997, Brauer et al 2012). Resource competition theory assumes that species differ in their competitive abilities. The R* value is defined as the lowest possible environmental resource concentration at which a species can still thrive. Manuscript received 9 April 2019; revised 3 July 2019; accepted 15 July 2019.

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