When is spring green-up? Interaction of species phenology and community composition

Background/Question/Methods

Global change is leading to shifts in the timing of growth and maturation in primary producers, having potentially critical effects on the timing and duration of the growing season in temperate and polar ecosystems. Remote sensing is increasingly used to measure the timing of primary production in both aquatic and terrestrial ecosystems but there is often poor correlation between those results and direct observations of the life-history responses of individual species. One explanation may be that in addition to phenological shifts, environmental drivers are also causing shifts in community composition among species with different seasonal timing of growth and maturation. We quantified how shifts in species timing and in community composition translated into phenological change in a diverse phytoplankton community (104 species) from 1962-2000 in Lake Washington.

Results/Conclusions

Over 38 years, the spring/summer phytoplankton peak has shifted earlier by 63 days, but this is partly driven by nutrient dynamics. When eutrophication-associated species are removed from the analysis, the total phytoplankton peak shift is 31 days earlier. Individual phytoplankton species showed highly variable responses; 58% of species shifted earlier with a mean shift of 3 days earlier, but the range was 106 days earlier to 126 days later. Shifts in timing of individual species explained 62% of the observed community phenological shift. The remaining community shift was attributed to dominant early season species increasing in abundance. In diverse communities experiencing multiple stressors, changes in species composition must be considered to fully understand and predict shifts in the timing of primary production.