Is the relationship between algal diversity and biomass in North American lakes consistent with biodiversity experiments?

Over the past few 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 ecosystem functions quantified in prior experiments 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, we used a biogeographic dataset to assess how the taxonomic richness of aquatic primary producers relates to community biomass in unmanipulated lake ecosystems in the US, and then compared these findings to prior experiments. We used structural equation modeling to evaluate hypotheses about the effects of algal richness on community biomass while accounting for covariance with environmental parameters measured in the USEPA's National Lakes Assessment (NLA), which sampled 1157 freshwater lakes. These analyses converged on a single best-fit model (χ²= 0.31, p = 0.58) wherein community algal biomass was a function of three explanatory variables - nitrogen, phosphorus, and algal richness. The quantitative magnitude of the algal diversity (x) - biomass (y) relationship in the NLA dataset is statistically greater than that documented in the average biodiversity experiment. It did, however, lie at approximately the 75th percentile of experimental relationships, indicating the diversity-biomass relationship in unmanipulated lakes is within the range that has been characterized experimentally.