Understanding the relationship between nutrient availability and freshwater cyanobacterial growth and abundance

Abstract

Anthropogenic eutrophication is a well-established cause of cyanobacterial blooms in freshwaters. Early studies proposed eutrophication control focused on phosphorus (P) as the key limiting nutrient. The rationale was that nitrogen (N) limitation is alleviated by diazotrophic cyanobacteria that fix atmospheric N. However, more recently, studies suggest that the importance of N fixation may have been overstated, and some call for anthropogenic N control as well as P. Reducing nutrient concentrations below critical levels clearly reduces phytoplankton biomass, but increasing evidence shows that cyanobacterial species can adapt to low and variable nutrient conditions, outcompeting eukaryotic phytoplankton. Strategies supporting cyanobacterial dominance include (1) high affinity P uptake, (2) preferencing cellular storage of P over immediate metabolic utilization for growth, (3) alternation between N fixation and inorganic N utilization as needed, and (4) use of organic nutrients when inorganic nutrient supplies are limited. Predicting the ecological responses of cyanobacteria to nutrients therefore requires more complex models than the simple resource models using inorganic nutrients widely used for contemporary studies. Additionally, many ecological studies are not conducted on timescales relevant to understanding how nutrient fluxes drive physiological responses. We advocate for greater use of high-frequency automated measurements of nutrients and cyanobacteria to address the timescale issue. Additionally, field-based experimental studies focused on nutrient cycling, fluxes, and pools are needed to validate laboratory findings and extrapolate them to in situ conditions. The challenge is to undertake these studies at sufficient spatial and temporal scales to predict large-scale responses of cyanobacteria to nutrients.