Regulation of Invertebrate Grazers as a Means to Enhance Biomass and Nitrogen Fixation of Cyanophyceae in Wetland Rice Fields

Abstract

Abstract The effect of invertebrate grazing on the growth and N fixation of blue‐green algae (Cyanophyceae) in flooded rice soils was investigated by depressing grazer populations. Grazers were controlled with commercial pesticides and seeds of neem ( Azadirachta indica ). Algal N fixation and standing biomass were estimated by acetylene‐reduction activity and chlorophyll a measurements. Suppression of ostracod (Ostracoda) grazing by Pertbane or neem seeds tripled blue‐green algal biomass and increased N fixation rates 10‐fold. In the absence of ostracods, free living blue‐green algae multiplied rapidly early in the rice cultivation cycle to be succeeded by chlorophytes. Carbofuran was not an effective control measure. Suppression of molluscan grazing had little effect. The population of tubificids (Tubificidae) was higher in the plots where algal growth was stimulated than in other plots. Total rice grain N was increased up to 37% when grazing was arrested.