Combined effects of food concentration and the herbicide 2,4-dichlorophenoxyacetic acid on the population dynamics of Brachionus patulus (Rotifera)

Herbicides are important in crop protection and management. A number of the m including 2,4-D (2,4-dichlorophenoxyacetic acid), however, may reach wate r bodies and eventually affect the non-target organisms such as rotifers. I n the present work, we studied the influence of 6 concentrations viz. 0 (co ntrol), 100, 200, 300, 400 and 500 mg l(-1) of 2,4-D on the population grow th of the rotifer Brachionus patulus under two algal (Chlorella) food level s (0.5 x 10(6) and 1.5 x 10(6) cells ml(-1)). Regardless of herbicide conce ntration, the population growth of B. patulus was dependent on the algal fo od levels, in that an increase in algal food level supported a better popul ation growth. Similarly at any Chlorella density, the herbicide had a negat ive influence on the population growth of B. patulus. Herbicide level of 50 0 mg l(-1) inhibited population growth of B. patulus beyond 5 days. Rotifer s grown under low food density and high herbicide concentration (300 mg l(- 1) or above) were completely eliminated after day 15. The rate of populatio n increase (r) (mean +/- standard error) in the controls varied from 0.46 /- 0.002 and 0.55 +/- 0.004 under 0.5 X 10(6) and 1.5 X 10(6) cells ml(-1) of Chlorella, respectively. The r values became negative under both, low an d high food levels, at or beyond 300 mg l(-1) of 2,4-D. The maximal populat ion abundance (ind. ml(-1)) in controls varied from 294 +/- 9 to 503 +/- 21 under low and high food levels of Chlorella, respectively. The role of alg ae in mitigating adverse effects of high herbicide concentrations to rotife rs has been discussed.