Pesticides are non-biodegradable substances. When they enter water bodies, they are absorbed by aquatic plants, then pass to herbivores, then to carnivores. At each trophic level, their concentration increases — a process called biological magnification. By the time they reach top consumers (including humans), the concentration becomes dangerously high, harming health and disrupting the ecosystem.
The key concept here is biological magnification — examiners expect you to name it and explain that non-biodegradable pesticides accumulate progressively at each trophic level. Two marks = name the phenomenon + explain the consequence (harm to top consumers/ecosystem). Avoid writing vague answers like "it is harmful" without explaining why (i.e., the magnification mechanism).