Print PageA truly predictive understanding of the often positive effects of biodiversity on ecosystem function requires the a priori identification of traits conferring specific functions to individual species. While planktonic organisms are responsible for half of the world’s primary production, few studies have reported on the relationship between phytoplankton diversity and planktonic primary production. Compared with the physical complexity of most terrestrial environments, the simple structure of the pelagic habitat seems to strongly limit opportunities for niche complementarity among planktonic primary producers. In contrast to terrestrial environments, however, where a single photosynthetic pigment (chlorophyll a) accounts for essentially all primary production, the phylogentetic diversity of photosynthetic pigments within the phytoplankton could allow for spectral partitioning of available light energy. We therefore hypothesize that taxon-specific differential equipment with photosynthetically active pigments provides a biochemical mechanism of resource use complementarity among phototrophic microorganisms. This mechanism could enable more diverse communities to more completely harvest the light spectrum, thus producing a positive relationship between phytoplankton diversity and pelagic primary production. We tested this hypothesis in a series of controlled laboratory experiments and field studies.
Results/Conclusions
In both the field and the laboratory we found a positive relationship between phytoplankton diversity and primary production and biomass accrual. Statistically, this relationship was entirely attributed to a complementarity effect. In line with our hypothesis, more diverse phytoplankton communities showed higher pigment diversity and higher biomass-specific light absorbance, suggesting that more efficient light use was responsible for higher production and biomass in more diverse communities. This is consistent with the observation that higher production in more diverse assemblages was primarily driven by higher rates of light-dependent carbon fixation, as evidenced by higher lipid content and higher carbon:nutrient ratios of the resulting biomass. The spectral niche complementarity hypothesis was further corroborated in an experiment in which we independently manipulated species vs. pigment diversity. Species richness had only a moderate, and inconsistent, positive effect on primary production when communities were assembled from phylogenetically related taxa sharing similar pigments. In contrast, when communities were assembled from phylogenetically distant taxa with greater pigment diversity, a much stronger diversity-productivity relationship was found. We conclude that spectral niche partitioning enables more diverse phytoplankton communities to more efficiently use the light spectrum. Intriguingly, this same mechanism could also promote the very persistence of such diverse phytoplankton communities, providing a possible mechanistic and predictive link between species traits, community composition, and ecosystem function.
