Does Algal Biodiversity Affect the Production of Biomass in Natural Lake Systems

Abstract

Over the past several decades, a large body of research has examined how biodiversity loss influences the functioning of ecosystems, as well as the cascading impacts on the goods and services ecosystems provide to humanity. The relationship between biodiversity and various ecosystem-level functions quantified in experiments to date suggests that initial losses of biodiversity have relatively small impacts on properties like community biomass production; however, beyond some threshold, increasing losses lead to accelerating declines in function. Some have questioned whether a saturating relationship between diversity and community biomass production is an artifact of overly simplified experiments that manipulate diversity in homogeneous conditions over short time-scales in which niche differences may not be realized. Others have questioned whether even the modest effects of biodiversity observed in experiments would be discernible in natural systems where they could be over-ridden by the stronger influence of abiotic factors. Here, I used a biogeographic dataset to assess how the taxonomic richness of aquatic primary producers relates to community biomass in unmanipulated lake ecosystems, and then compared these findings to prior experiments. I used Structural Equations Modeling (SEM) to quantify statistical relationships between algal richness and community biomass while accounting for covariance with environmental parameters measured in the USEPA's National Lakes Assessment (NLA), which sampled 1157 freshwater lakes across the U.S. These analyses converged on a single best-fit model (Χ2 = 0.31, P = 0.58) wherein total community algal biomass was a function of three explanatory variables – nitrogen, phosphorus, and algal taxa richness. The quantitative magnitude of the algal diversity (x) - biomass (y) relationship in the NLA dataset suggests that experiments to date have, if anything, underestimated the relationship between diversity and biomass production in more natural, unmanipulated systems. I discuss possible implications of this finding to future experimental manipulations and conservation strategies.