Abstract
A combination of large tidal range, high temperatures and low rainfall in NW Western Australia markedly restricts mangrove development to a narrow belt which fringes the shores and short tidal creeks. Nonheterocystous cyanobacterial mats cover a large proportion of the extensive saline flats behind the most landward mangroves (Avicennia marina). These mats are predominantly composed of Microcoleus chthonoplastes and Phormidium sp. and show significant nitrogen fixation rates. From 1-128 kg of nitrogen could be fixed per hectare annually. The nitrogen contained within these mats may be leached by tidal inundation and by the sporadic, but intense, rainfall. Leaching rates were 3-7 mg N m-2 h-1 in seawater. Enclosure experiments estimated that 68 kg N ha-1 could be exported annually to the adjacent mangrove systems.