The impacts of nutrient supply and imbalance on subcontinental co-occurrence networks and metacommunity composition of stream algae

The amounts and ratios of nutrients (nitrogen and phosphorus) are
important determinants of producer community biodiversity and composition
and their responses to climate and dispersal. However, the nutrient
effects on co-occurrence network topology, particularly in freshwaters,
are understudied. Here, we investigate 1) whether nutrient supply and
ratio constrain topological properties of algal co-occurrence networks in
streams and 2) to what extent climate and space (a surrogate for
dispersal) affect co-occurrence network topology vs. metacommunity
composition across nutrient supply and ratio contexts. We used a
subcontinental dataset of benthic algae from 840 stream localities in the
conterminous US. We constructed co-occurrence networks representing
nutrient supply contexts (oligotrophic vs. eutrophic) and nutrient ratio
contexts (N-limited vs. P-limited) and statistically assessed topological
variability within each pair via randomization. We then used a null model
framework and direct gradient analysis to ascertain the importance of
climate and space in driving, respectively, network topology and
metacommunity composition. Nutrient supply was only positively related to
network size (node counts), which was driven by motile species, while
other topological differences were non-significant. Climatic and spatial
variables had pronounced and for the most part comparable effects on
network topology that further depended on nutrient context. A comparative
assessment of topological vs. compositional responses to climate and space
across nutrient contexts identified both similarities and differences.
While climate and space contributed to both network topology and
metacommunity composition, space was a stronger predictor of compositional
variability than climate, regardless of nutrient context. Our findings
highlight the need for developing integrative multi-level approaches (from
metacommunities to co-occurrence networks) to fully understand biological
responses to complex and interactive abiotic forces.