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 phytoplankton taxonomically or by a rarely used morphological-functional approach (m/f approach) developed by Reynolds (1997). This approach divides phytoplankton into three groups (C-, S- and R-strategists) by morphological characters 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 gradient total phytoplankton biomass ranged from 0.3 to 4.8 mg (wet weight) L-1 and total phosphorus (TP) from 0.19 to 1.07 mu M. 3. The taxonomic groups predominantly represented were cyanobacteria, diatoms, 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 comprised 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 predicted well from physical or chemical variables. This lack of predictability is probably caused, in part, by the problem of rapid seasonal variation in environmental conditions as compared to the replacement time of phytoplankton. Considering the data as a whole, however, several physical and chemical variables were significantly related to taxonomic and m/f groups. Some correlations agreed with expectations from the literature and some did not. 5. For taxonomic groups, as expected, cyanobacteria were positively associated 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 to literature expectations. 6. For m/f groups: R-strategists were correlated with high dissolved nutrients (both N and P) and low light; S-strategists were correlated with low dissolved 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. Opposite to expectations, however, their abundance in the study lakes was negatively correlated to dissolved P. 7. Despite a lack of complete predictive ability, the m/f approach performed better than the taxonomic approach. This suggests that size and shape of phytoplankton may predict their response to physical and chemical conditions better than taxonomy at the division level.