Development of cyanobacterial blooms in Valencian rice fields

Seasonal variation of biomass, photosynthetic pigments and C and N contents of the cyanobacterial blooms developed during the crop cycles of 1998, 1999 and 2000 at two locations in Valencian rice fields ( Spain) were studied to find their potential contribution to soil fertility. Blooms of Gloeotrichia sp., Gloeocapsa sp., Microchaete sp., and Nostoc sp. were small and dispersed and appeared only for a few weeks in the experimental fields of the Tancat de Malta location during the crop season of 1998. The biomass of these blooms ranged from 4.4 to 12.8 mg dry wt cm(-2). A bloom of Anabaena sp. was found in the same location during most of the crop cycles of 1998 and 1999, covering up to 80% of the experimental fields. The biomass of this bloom ranged from 2.7 to 11.4 mg dry wt cm(-2). In the moment of its maximal extension ( July 1998) we estimate a total biomass of 2,100 kg dry wt for this bloom, equivalent to 420 kg dry wt ha(-1). In the Sueca location a bloom of Microchaete sp. was observed in August 1999 and during most of the crop cycle of 2000, when it covered a wide area (60-70%) of the experimental field. The biomass of this bloom ranged from 0.9 to 5.6 mg dry wt cm(-2). Carbon contents of the blooms ranged from 150 to 310 mug mg(-1) dry wt and N contents ranged from 11 to 30.2 mug mg(-1) dry wt. Taking into account their N content and their extension we estimate that the incorporation of cyanobacterial blooms could add 4-12 kg N ha(-1) to soil. From an agronomical point of view this means that, in spite of their impressive aspect, cyanobacterial blooms can only fulfill a small proportion of the N requirements of rice plants.