Diversity predicts stability and resource use efficiency in natural phytoplankton communities

Abstract

The relationship between species diversity and ecosystem functioning has been debated for decades, especially in relation to the "macroscopic" realm (higher plants and metazoans). Although there is emerging consensus that diversity enhances productivity and stability in communities of higher organisms; however, we still do not know whether these relationships apply also for communities of unicellular organisms, such as phytoplankton, which contribute approximately 50% to the global primary production. We show here that phytoplankton resource use, and thus carbon fixation, is directly linked to the diversity of phytoplankton communities. Datasets from freshwater and brackish habitats show that diversity is the best predictor for resource use efficiency of phytoplankton communities across considerable environmental gradients. Furthermore, we show that the diversity requirement for stable ecosystem functioning scales with the nutrient level (total phosphorus), as evidenced by the opposing effects of diversity (negative) and resource level (positive) on the variability of both resource use and community composition. Our analyses of large-scale observational data are consistent with experimental and model studies demonstrating causal effects of microbial diversity on functional properties at the system level. Our findings point at potential linkages between eutrophication and pollution-mediated loss of phytoplankton diversity. Factors reducing phytoplankton diversity may have direct detrimental effects on the amount and predictability of aquatic primary production.

Fig. 1.

Resource use efficiency (RUE) as a function of diversity [genus richness (G)]. (Top) RUE in terms of chlorophyll-a per unit phosphorus (RUE_Chl). (Bottom) The same for algal carbon per unit phosphorus (RUE_Carb). (Left) Raw data with a fit for all observations. (Right) Fits for the individual datasets, corresponding to coefficients given in Table 1. The horizontal box plots show the diversity distribution for each dataset. Color codes refer to the single datasets. FI, Finland; NO, Norway; SE, Sweden; BS, Baltic Sea.

Fig. 2.

RUE_chl (A) and chlorophyll-a (B) as functions of total phosphorus (TP, x axis) and diversity (G, y axis). Contour lines in A indicate the fitted model for RUE_chl given in Table 1. The contour lines in B correspond to the same model with RUE_chl being replaced by chlorophyll-a (see Diversity as a Predictor of RUE).

Fig. 3.

Results from time-series analysis. Partial residuals (R_p) for linear regressions predicting community turnover σ_s (A and B) and standard deviation (SD) of RUE_chl (C and D) as functions of mean annual diversity (G) and total phosphorus (TP) (see Table 3). Each dot represents 1 lake year. Lines indicate the mean effect of a predictor together with confidence intervals.