Macrosystem community change in lake phytoplankton and its implications for diversity and function

Aim: We use lake phytoplankton community data to quantify the
spatio-temporal and scale-dependent impacts of eutrophication, land-use,
and climate change on species niches and community assembly
processes while accounting for species traits and phylogenetic
constraints. Location: Finland Time period: 1977-2017 Major taxa:
Phytoplankton Methods: We use Hierarchical Modelling of Species
Communities (HMSC) to model meta-community trajectories at 853
lakes over four decades of environmental change, including a hierarchical
spatial structure to account for scale-dependent processes. Using a
‘region of common profile’ approach, we evaluate compositional changes of
species communities and trait profiles and investigate their temporal
development.  Results: We demonstrate the emergence of novel and
widespread community composition clusters in previously more compositional
homogeneous communities, with cluster-specific community trait profiles,
indicating functional differences. A strong phylogenetic signal of
species' responses to the environment implies similar responses among
closely related taxa. Community cluster-specific species prevalence point
to lower taxonomic dispersion within the current dominant clusters
compared to the historically dominant cluster and overall higher
prevalence of smaller species sizes within communities. Our findings
denote profound spatio-temporal structuring of species co-occurrence
patterns and highlight functional differences of lake
phytoplankton communities. Main conclusions: Diverging community
trajectories have led to a nationwide reshuffling of lake phytoplankton
communities. At regional and national scales, lakes are not single
entities but metacommunity hubs in an interconnected waterscape. The
assembly mechanisms of phytoplankton communities are strongly structured
by spatio-temporal dynamics, which have led to novel community types but
only a minor part of this reshuffling could be linked to temporal
environmental change.