Agriculture feeds the world, but the chemicals it relies on don't stay on the farm. Pesticides, herbicides, and fertilisers travel through soil, into rivers, and eventually reach coastal waters — where they can devastate marine ecosystems.
The Pathway
When it rains, water flowing across treated fields carries pesticide residues into ditches, streams, and rivers. This agricultural runoff is the primary route by which pesticides enter aquatic environments. In coastal regions, these chemicals reach estuaries and the sea.
The problem is exacerbated by the timing of application. Pesticides are often applied in spring and early summer — precisely when rainfall is frequent and many marine species are in critical reproductive phases.
Neonicotinoids: A Case Study
Neonicotinoid insecticides, widely used on crops, have drawn attention for their devastating effects on pollinators. But their impact on aquatic life is equally concerning. Studies show they are highly toxic to aquatic invertebrates at environmental concentrations — the insects, crustaceans, and worms that form the base of aquatic food chains.
Research in the Netherlands found that neonicotinoid concentrations in surface water correlated directly with declining populations of aquatic insects, with cascading effects on the fish and birds that depend on them.
Herbicides and Seagrass
Herbicides from agricultural and urban runoff are a major threat to seagrass meadows — vital marine habitats that store carbon, nursery fish, and stabilise sediments. The herbicide diuron, used in both agriculture and antifouling paints, inhibits photosynthesis in seagrass at concentrations commonly found in coastal waters.
Australia's Great Barrier Reef has suffered significant seagrass loss linked to herbicide-laden runoff from sugarcane and grazing land.
Solutions
Precision agriculture — applying chemicals only where and when needed — can dramatically reduce runoff. Buffer strips of vegetation along waterways filter agricultural chemicals before they reach water. Integrated pest management reduces reliance on chemical pesticides.
At a policy level, the EU's Farm to Fork Strategy aims to reduce pesticide use by 50% by 2030. Stronger water quality monitoring and enforcement are essential to protect coastal ecosystems.
This article is part of the CONTRAST project, funded by the European Union under Horizon Europe. Views expressed are those of the author(s) only.