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

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.

Seasonal variation of microcrustaceans and microbial flora on their surface in the overlying water of a Japanese paddy field

“…and Cypris maculosa (Table 2). Ostracods are known to be adapted to paddy fields, since they can well withstand high temperatures and dryness, and colonize rapidly after flooding (Grant et al 1983). In addition, ostracods are relatively resistant to pesticides (Lim and Wong 1986).…”

Pesticide Effects on Soil Algae and Cyanobacteria

“…Possible explanations for the insecticide-induced enhancement of the growth of the soil flora include stimulation of germination of resting stages by the insecticides and/or their degradation products, and control by the insecticides of predators of the algae and cyanobacteria in the soil (Megharaj et al 1986a(Megharaj et al , 1986c. In support of the latter argument, Grant et al (1983) noted a 3-and 10-fold increase in the biomass and nitrogen-fixing capability, respectively, of rice field cyanobacteria when perthane was used to suppress grazing by members of the Ostracoda.…”

“…The selection of an appropriate treatment level is further complicated by the observation that grazing can be stimulated by very low pesticide concentrations (Grant and Alexander 1981). Grant et al (1983) suggested that the proper chemical control of grazers will result in an increase of cyanobacterial biomass, nitrogen fixation, and rice grain protein. Pesticide use in rice culture has undeniably effected a dramatic increase in yield (Singh and Tiwari 1988).…”

A review of rice field ostracods (Crustacea) with a checklist of species

“…This is attributed to their high fecundity, and generally having a higher resistance to pesticides than their predators (e.g., Odonata larvae), and their subsequent increases can inhibit the development of nitrogen-fixing cyanobacteria blooms (Roger, 1996). The seeds of the neem tree (Azadirachta indica), crushed and distributed in rice fields, has been shown to be an effective alternative or supplement to artificial chemical pesticides in controlling ostracods in rice fields (Roger and Watanabe, 1982;Grant et al, 1983c;1986).…”