Cladoceran Moina branchiata can not reduce suspended solids in Lemna System macrophyte wastewater treatment plant

Mass development of filtering Cladocera in wastewater stabilization ponds is believed to facilitate the removal of particulate organic matter from suspension, thus enhancing the purification efficiency. To evaluate this premise, we monitored the seasonal changes of wastewater characteristics such as suspended solids and chlorophyll a concentration, pH, oxygen concentration and biological oxygen demand (BOD), and population density of the cladoceran Moina branchiata in the 3.5 m deep ponds of two Lemna System plants, differing in the degree of surface cover by Lemna minor. We found no detectable effect of M. branchiata on any of the above parameters, despite its occasionally very high population densities (over 2000 ind. L−1). We conclude that cladoceran grazing is negligible in organic particle removal from wastewater passing deep Lemna ponds that are heavily loaded with dissolved and particulate organic matter.