Watershed, lake and food web factors influence diazotrophic cyanobacteria in mountain lakes

Cyanobacterial blooms can occur in freshwater ecosystems largely isolated
from development and not experiencing extensive cultural eutrophication.
For example, remote mountain lakes can experience intense blooms of
diazotrophic (nitrogen-fixing) cyanobacteria caused by factors acting at
different spatial and temporal scales. In this study, we examined how
cross-scale interactions among watershed, lake, and food web
characteristics influence diazotrophic cyanobacteria biovolume in mountain
lakes. We quantified diazotrophic cyanobacteria biovolume, zooplankton
abundance, and physico-chemical variables for 29 lakes in the Cascade
Mountains of Oregon, USA, in summer 2019. Watershed characteristics were
compiled from historical data sets available for the region. Diazotrophic
cyanobacteria biovolume ranged across the lakes from 0 to 1,930,000 µm3
mL-1; Dolichospermum was the most common genus. Random forest models
showed that 11 watershed, lake, and food web characteristics explained 76%
of the variance in diazotrophic cyanobacteria biovolume among the sampled
lakes. Structural equation models suggested that the drainage ratio (i.e.,
relative area of the lake to the watershed) correlated positively with
phosphorus concentrations and in turn, with diazotroph biovolume. Among
lakes, hypolimnetic dissolved oxygen was negatively correlated with
diazotroph biovolume, possibly due to the release of nutrients, like
phosphate and iron, bound to sediments. In addition, zooplankton grazers
were negatively related to diazotrophic cyanobacteria biovolume,
potentially reflecting the influence of stocked fish.  Thus, lake
management must account for bottom-up factors, such as nutrient loading,
which is influenced by lake morphometry and watershed size, as well as
top-down factors, such as fish stocking, to effectively mitigate
diazotrophic cyanobacterial blooms.