AME 57:191-202 (2009)  -  DOI: https://doi.org/10.3354/ame01344

ABSTRACT: The epiphytic cyanobacterium Gloeotrichia pisum forms spherical colonies embedded in a gelatinous matrix that is attached to submerged aquatic vegetation. In surveys of 2 fluvial lakes of the St. Lawrence River conducted in 2006–2007, we examined its growth conditions and diazotrophic ability in conjunction with the biomass and condition (carbon (C), nitrogen (N), phosphorus (P) and chl a content) of its supporting macrophyte, Vallisneria americana. G. pisum occurred in nearshore areas, in sheltered embayments characterized by clear waters with low dissolved inorganic nitrogen (DIN) (NO₂-NO₃ < 35 µg N l^–1, NH₄ < 7 µg N l^–1) and total dissolved phosphorus (<20 µg P l^–1) concentrations. Low water levels, warm temperatures and sunny conditions in 2007 resulted in early colonization, sustained low DIN concentrations and a 4-fold rise in G. pisum biomass compared to 2006. At all sites, the near-zero stable nitrogen isotope (δ¹⁵N) signature of G. pisum revealed its exclusive reliance on atmospheric nitrogen. V. americana leaves that were heavily colonized by G. pisum exhibited significantly lower chl a, %N and %P contents than those at sites where G. pisum was absent. The poor condition of V. americana was more likely related to low DIN concentration than to any direct effect of cyanobacterial colonization. G. pisum thus integrates complex, temporally variable water-quality characteristics that are dependent upon hydrology and water residence time, thus acting as an early warning indicator of incipient habitat degradation that may lead to cyanobacterial proliferation and low biomass of vascular macrophytes.

KEY WORDS: Nitrogen depletion · Phosphorus · Wetlands · Vallisneria americana · Benthic cyanobacteria · Gloeotrichia pisum