Are benthic cyanobacteria indicators of nutrient enrichment? Relationshipsbetween 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. Cyanobacteria l blooms, like macroalgal blooms, may result from a combination of coastal eutrophication and reduced herbivory. Because benthic cyanobacteria are unp alatable 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 abu ndance may be an indicator of high nutrient availability. In addition, cyan obacteria may compete with macroalgae for light and nutrients. Thus, we als o hypothesized that cyanobacterial abundance may be negatively associated w ith macroalgal abundance. Since many strains of cyanobacteria wash ashore d uring periods of high wave action, we hypothesized that cyanobacterial abun dance is negatively associated with wave height. We monitored cyanobacteria l abundance, macroalgal abundance, nitrogen availability, phosphorus availa bility, 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 u sed to determine which variable or combination of variables best explained variation in cyanobacterial abundance. Although nutrient availability was n ot significantly associated with cyanobacterial abundance, a positive assoc iation 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 varia tion in the abundance of the cyanobacterium Oscillatoria margaritifera. Var iation in cyanobacterial abundance can be best explained by examining indiv idual strains of cyanobacteria, rather than by treating all cyanobacteria a s a single ecological unit. Physical disturbance can be a more important in fluence on cyanobacterial abundance and distribution than either nutrient a vailability or interactions with macroalgae.