Are benthic cyanobacteria indicators of nutrient enrichment? Relationships between cyanobacterial abundance and environmental factors on the reef flats of Guam

While benthic, filamentous cyanobacteria can be common on coral reefs, the factors influencing their distribution are poorly understood. Cyanobacterial blooms, like macroalgal blooms, may result from a combination of coastal eutrophication and reduced herbivory. Because benthic cyanobacteria are unpalatable to generalist herbivores, there may be little top-down control of cyanobacterial abundance. Since bottom-up factors may exert more influence on cyanobacterial populations, we hypothesized that high cyanobacterial abundance may be an indicator of high nutrient availability. In addition, cyanobacteria may compete with macroalgae for light and nutrients. Thus, we also hypothesized that cyanobacterial abundance may be negatively associated with macroalgal abundance. Since many strains of cyanobacteria wash ashore during periods of high wave action, we hypothesized that cyanobacterial abundance is negatively associated with wave height. We monitored cyanobacterial abundance, macroalgal abundance, nitrogen availability, phosphorus availability, salinity, and water temperature at nine reef flat locations around Guam. Average wave height for eastern and western shores, rainfall, minutes of sunshine, and wind speed were also monitored. Stepwise regression was used to determine which variable or combination of variables best explained variation in cyanobacterial abundance. Although nutrient availability was not significantly associated with cyanobacterial abundance, a positive association with macroalgal abundance explained 11.5% of the observed variation in total cyanobacterial abundance. At one site, negative relationships with macroalgal abundance and wave height explained 79.4% of the observed variation in the abundance of the cyanobacterium Oscillatoria margaritifera. Variation in cyanobacterial abundance can be best explained by examining individual strains of cyanobacteria, rather than by treating all cyanobacteria as a single ecological unit. Physical disturbance can be a more important influence on cyanobacterial abundance and distribution than either nutrient availability or interactions with macroalgae.