Some ecological effects of two herbicides, dichlobenil and diquat on pond ecosystems.

Some ecological effects of two herbicides, dichlobenil and diquat on pond ecosystems Two ponds 5.0 metres in diameter were treated with herbicide dichlobenil two were treated with the herbicide diquat, two were left untreated as controls. The effects of herbicide treatment was assessed on the macro-phytes, the algae and zooplankton. Dichlobenil killed all rooted macrophytes within the ponds namely Typha latifolia, Agrostis stolonifera, Ranunculus trichophyllus, Potamogeton gramincus and Chara delicatula. Diquat killed T.latifolia, trichophyllus, P.gramineus and Polygonum amphibium. C.delicatula was partially controlled. The death of the macrophytes lowered the diel amplitude of oxygen in both treatments, but deoxygenation did not occur. This was due primarily to the low macrophyte biomass at the time of spraying and the growth on non susceptible algae which restored the oxygen balance. The production (P) of the algae was related to the biomass or standing crop as g OrgC/mean m2 and differences between the two were due to losses through sedimentation, grazing and parasitism. These losses were determined by sedimentation vessels, an assessment of the grazing pressure by zooplankton on the algae and an assessment of parasitism. The grazing pressure by zooplankton on algae was extremely high and the results indicated that the production of the algae was underestimated by as much as 340%. The ability of zooplankton to control algal production relates not only to the suitability of the algae as a food source but also to the degree of predation on the zooplankton by other secondary producers. Predation on one species of zooplankton Simocephalus vetulus increased when its macrophyte habitat was destroyed. The macrophytes were clearly acting as a refuge. This in turn affected the grazing pressure on the algae. The ponds had a variable recovery time according not only to the type of herbicide but to slight differences in the species composition of the macrophytes in the "replicate" ponds prior to treatment. These differences proved to be of great importance.