Nitrogen fixation in seagrass communities during summer in the Gulf of Carpentaria, Australia

Abstract

Rates of acetylene reduction in seagrass communities in the Gulf of Carpentaria were determined in intact cores of sediment and seagrass and in slurries. Short-term incubations and three different methods were used to ensure that results could be reliably converted to rates of nitrogen fixation. At Groote Eylandt, values ranged from 16 to 47 mg N m^-2 day^-1 in a Syringodium isoetifolium community and from 13 to 19 mg N m^-2 day^-1 on a reef flat with Thalassia hernprichii. At Weipa, mean rates of nitrogen fixation were 25 mg N m^-2 day^-1 in an Enhalus acoroides community and 20 mg N m^-2 day^-1 on a mud bank below mangroves. About 5% of fixation was due to epiphytes on the seagrass leaves, and 8% of fixation associated with the Syringodium isoetifolium community occurred in washed roots and rhizomes; the remainder was due mostly to bacteria in the sediment. Nitrogen fixation supplied 8 to 16% of the nitrogen requirements of the plants.