A shallow water ecosystem : rice - fields . The relevance of cyanobacteria in the ecosystem

In this paper we review the knowledge of the ecology of the largest freshwater ecosystem on Earth: the rice-fields, and in particular the rice-fields from Valencia (Spain) making a special consideration to the cyanobacteria present in this ecosystem. Rice-fields are artificial shallow aquatic ecosystems in which the land management and the agricultural practices together with the rice plant growth govern the major environmental variables affecting the aquatic biota and its relationships. Primary producers are dominated typically by macrophytic algae as Chara and cyanobacteria, both planktonic and benthic (beside the rice plants). Most rice-fields can be considered nutrient replete, since the fertilization inputs and the low ratio volume/surface make that main nutrients are typically available. Under these circumstances other environmental variables as photosynthetically active radiation availability or filtration rates and predation may explain the growth limitation of primary producers. Irradiance availability identify two periods within the cultivation cycle: when plants are short, irradiance is not limiting and some water chemistry variables (as pH, oxygen and dissolved inorganic C concentrations) change drastically as a function of the primary production; when plants are large and the canopy is intense, then irradiance is limiting and the water chemistry changes only slightly along the day. N2-fixation is a main activity in the N cycle in rice-fields, since N2-fixing cyanobacteria represent an important fraction of the aquatic biota of this ecosystem. We will discuss in detail the relevance of this process from the ecological point of view.