Phytoplankton contribution to sestonic mass and elemental ratios in lakes: Implications for zooplankton nutrition

Phytoplankton carbon and particulate organic carbon (POC), nitrogen (PON), and phosphorus (POP) (POC :PON: POP) were analyzed in 109 temperate lakes covering a wide span in productivity and other key parameters. Seasonal means of total POC (four samples) ranged from 206 to 7160 µg C L −1 , with a grand mean of 960 µg C L −1 , whereas estimated phytoplankton C ranged 12 to 1,770 µg C L −1 , with a mean of 217 mg C L −1 . Sestonic C: P ratios ranged from 59 to 553 (atom : atom), with a mean of 207. The elemental contributions from phytoplankton and other sestonic compartments (mainly detritus) were analyzed with a simple regression model, in which autochthonous and allochthonous components were separated. Model‐derived estimates for N: P ratios of phytoplankton and allochthonous seston compartments were nearly equal (15.4 ± 2.5 and 16.0 ± 2.0) and were not significantly different from the Redfield N: P ratio (16). The estimated C: P ratio of allochthonous detritus was 2.7 times higher than that for phytoplankton (123 ± 15), which again was not significantly different from the Redfield C: P ratio (106). Altogether, this indicates that sestonic components of autochthonous origin should be closer to Redfield proportions in eutrophic than in oligotrophic lakes. It also indicates that major contributions of allochthonous detrital C in oligotrophic lake seston may explain deviations from the Redfield ratio and calls for caution when interpreting elemental ratios in algae versus total seston. The regression model indicates that live phytoplankton cells rarely exceed 40% of total POC, yet it suggests that a major fraction of detritus is derived from autotrophs. This close link between live and dead cells could explain why total seston apparently carries the stoichiometric and biochemical footprints from the phytoplankton. Judged from algal biomass alone, Daphnia would face severe food limitation in a majority of lakes, while if we were to include total seston, Daphnia would be above threshold food levels in all lakes. Likewise, the effect of food quality limitation related to C: P ratios will turn out differently if total seston or only the phytoplankton fraction is considered.