Seasonal succession of functional traits in phytoplankton communities and their interaction with trophic state

1. Understanding and explaining the structure of communities in response
to environmental gradients is a central goal in ecology. Trait-based
approaches are promising but yet rarely applied to understand community
dynamics in response to changing environmental conditions. 2. Here we
investigate seasonal succession patterns of functional traits in
phytoplankton communities and how nutrient reductions (oligotrophication)
alter these patterns. We used phytoplankton data from 40 years of
observation from the Rappbode Reservoir (Germany), which underwent a
strong shift in trophic conditions, and translated taxonomic composition
into functional traits by assigning trait values compiled from the
literature. 3. All studied traits (morphological, behavioral and
physiological traits) responded to changing environmental conditions and
showed consistent, reoccurring seasonal developments. The seasonal
succession of phytoplankton communities was shaped by a trade-off between
small celled, fast growing species that are able to rapidly incorporate
existing resources (r-strategists) and large celled species with more
complex and efficient mechanisms to exploit scarce mineral nutrients or
acquire previously unexploited nutrient pools (k-strategists). In summer,
when nutrients were scarce, the k-strategy was prevailing (important
traits: phosphate affinity, nitrogen fixation, motility and mixotrophy).
During the rest of the year, nutrients and turbulence were high and
r-strategists dominated (important traits: maximum growth rate, light
affinity). 4. A comparison between eutrophic and oligotrophic years
revealed that the main features of functional trait succession were
largely preserved, but intra-annual fluctuations from spring to summer
were stronger during eutrophic years. Nutrient reductions mainly affected
functional traits and biomass in spring, while in summer the functional
community composition changed little. 5. Synthesis. This study provides
for the first time a quantitatively supported functional template for
trait-based succession patterns in lakes under different nutrient
conditions. By translating taxonomic composition into trait information,
we demonstrate that the quantification of functional characteristics
enables ecological interpretation of observed community dynamics and
provides not only a testable template but also a powerful tool towards a
more mechanistic understanding. The quantification of functional traits
further improves the predictability of community shifts in response to
changing environmental conditions and thus opens new perspectives for
predictive limnology using lake-ecosystem models.