The relationship between phytoplankton composition and physical-chemical variables: a comparison of taxonomic and morphological-functional descriptors in six temperate lakes

1. The relationship between phytoplankton composition and physical-chemical variables was studied in six temperate lakes, characterizing the phytoplan kton taxonomically or by a rarely used morphological-functional approach (m /f approach) developed by Reynolds (1997). This approach divides phytoplank ton into three groups (C-, S- and R-strategists) by morphological character s thought to be related to light and nutrient acquisition. 2. The lakes ranged from oligotrophic to moderately eutrophic and exhibited both polymictic and dimictic patterns of mixing. Across the trophic gradie nt total phytoplankton biomass ranged from 0.3 to 4.8 mg (wet weight) L-1 a nd total phosphorus (TP) from 0.19 to 1.07 mu M. 3. The taxonomic groups predominantly represented were cyanobacteria, diato ms, dinoflagellates, chrysophytes and chlorophytes. The m/f group that was dominant (>50% of total biomass) in five of the six lakes was composed of S -strategists. In four of the six lakes the second most abundant group compr ised R-strategists. In only one lake was biomass dominated by C-strategists . 4. Within-lake seasonal variations in taxonomic and m/f groups were not pre dicted well from physical or chemical variables. This lack of predictabilit y is probably caused, in part, by the problem of rapid seasonal variation i n environmental conditions as compared to the replacement time of phytoplan kton. Considering the data as a whole, however, several physical and chemic al variables were significantly related to taxonomic and m/f groups. Some c orrelations agreed with expectations from the literature and some did not. 5. For taxonomic groups, as expected, cyanobacteria were positively associa ted with temperature, pH and TP and negatively with light and NO3:TP ratios . Chrysophytes were negatively related to temperature, pH, soluble reactive phosphorus and TP. No other division showed a substantial correspondence t o literature expectations. 6. For m/f groups: R-strategists were correlated with high dissolved nutrie nts (both N and P) and low light; S-strategists were correlated with low di ssolved N. C-strategists were generally in low abundance in the study lakes , as might be expected from the generally low nutrients in the six lakes. O pposite to expectations, however, their abundance in the study lakes was ne gatively correlated to dissolved P. 7. Despite a lack of complete predictive ability, the m/f approach performe d better than the taxonomic approach. This suggests that size and shape of phytoplankton may predict their response to physical and chemical condition s better than taxonomy at the division level.