Pesticide Residues in Aquatic Invertebrates - Pesticides: Evaluation of Environmental Pollution - page 323

will result when the chemical becomes more concentrated as it moves up the food chain,
resulting in higher concentrations in the predators than in the prey (Moriarty 1983;
Newman 2001).
11.1.2 Dose Response (Acute and Chronic)
Acute toxicity, as a result of a single pesticide exposure, is most commonly measured using
median lethal dose or lethal concentration (LD 50 or LC 50 , respectively). Lethal dose (LD 50 )
can be calculated as the known amount of toxicant per amount of body weight (milligrams
toxicant/kilogram body weight) or as the amount of toxicant per animal (milligrams toxi-
cant/animal). In contrast, the LC 50 is based on the amount of toxicant in an environmental
medium (milligrams toxicant/liter of media), where the amount of toxicant that enters
the organism is not known. Because of the subtle differences in uncertainties associated
between the two metrics, confusion persists even among toxicologists about the differ-
ences between lethal dose and lethal concentration.
In contrast, chronic toxicity is usually monitored after continuous exposure to sublethal
dose or concentration over a given period of time. This is calculated using no observable
effect concentration (NOEC) or level (NOEL), which is the highest concentration or level
where no effect is observed (Stark and Banks 2003). The end points of interest for chronic
toxicities are life span, weight gain, reproduction, cancer, and birth defects.
11.1.3 Pesticides in Aquatic Ecosystem
Although many pesticides are used to control agricultural pests in farm crops, organ-
isms in aquatic ecosystems can be indirectly affected by the toxicity of pesticides. Aquatic
ecosystems are contaminated by five major routes: (1) surface runoff and sediment trans-
ported from treated soil, (2) industrial waste discharged from factory effluents, (3) direct
application as aerial spray or granules to control pests inhabiting water, (4) spray drift
from normal agricultural operation, and (5) municipal waste discharge (Ray and Ghosh
2006). Many pesticides that contaminate aquatic ecosystems can be hazardous to aquatic
invertebrates. Broad-spectrum pesticides are designed to kill a wide variety of pests.
As a result, broad-spectrum pesticides will not only kill the targeted pest (snail, worm,
and beetle), but might kill nontargeted organisms as well (rotifer, copepods, and benefi-
cial insects). Moreover, some persistent pesticides, such as DDT, can be taken up in the
food web where toxins can be transferred from the aquatic ecosystem to the terrestrial
ecosystem.
11.2 Effect of Pesticides on Aquatic Invertebrates
11.2.1 Effect of Pesticides on the Phylum Rotifera
In the food web, rotifers include predators, competitors, and prey. Due to their small
size and biology, they are often a nontarget organism of pesticide usage. Rotifers are
an important part of the nutrient cycle, and biological activities including decompo-
sition and mineralization in rice fields and aquatic resources. Furthermore, they have
the potential to affect the aquatic ecosystem by increasing soil pH (Kikuchi et al. 1977),