EFFECTS OF HERBICIDES ON 2 SUBMERSED AQUATIC MACROPHYTES, POTAMOGETON-PECTINATUS L AND MYRIOPHYLLUM-SIBIRICUM KOMAROV, IN A PRAIRIE WETLAND

Clopyralid, picloram, 2,4-D and a mixture of 2,4-D plus picloram, (Tor don(R) 202C) were added to the water of 1 m square enclosures in a pra irie wetland in Saskatchewan, Canada to produce concentrations of 0.01 and 0.1 mg active ingredient litre(-1). Effects on the submersed macr ophytes, Potamogeton pectinatus and Myriophyllum sibiricum, were monit ored by taking repeated measurements of plant weight, flower and tuber production and inspecting for injuries at 30 and 60 days after applic ation. Clopyralid did not inhibit weight gain (growth) in either speci es, but stimulated growth and flowering by M. sibiricum at 0.01 mg lit re(-1) and tuber production by P. pectinatus at both rates. The low ra te of 2,4-D stimulated flowering by M. sibiricum and tuber production by P. pectinatus, whereas the high rate inhibited growth of M. sibiric um and injured both species. Picloram did not affect growth of either species, but injured M. sibiricum at both concentrations and inhibited flowering at 0.1 mg litre(-1). Tordon(R) 202C at 0.1 mg litre(-1) cau sed reduced growth and flowering in M. sibiricum and injured both spec ies; 0.01 mg litre(-1) also injured M. sibiricum. Mortality resulted o nly from Tordon(R) 202C and 2,4-D. Field data are lacking to assess th e extent to which submerged macrophytes in prairie ponds are exposed t o harmful concentrations of herbicide from aerial spraying, drift from ground application, runoff or wind erosion of soil. (C) 1997 Elsevier Science Ltd. All rights reserved.