Riparian planting and the effect of chemicals on waterways

Planting next to streams and river banks (called riparian planting) has many benefits, especially on farms. It can decrease erosion and run-off while improving or maintaining the water quality, as well as health of plants, fish and animals living in and around the water.

By Marieke Soeter – Senior Advisor Hazardous Substances – Ecotoxicology

19 August 2020

Read the author's biography

In this piece we hear from one of our ecotoxicologists on the benefits of riparian planting, and how you can take part in community planting bees.

Ecologists are a funny bunch of people. Why? After work we generally don’t “clock out” but enjoy doing ecology-related stuff. If you have two ecologists in the household (Marieke = eco-toxicologist and the husband = aquatic ecologist) there is little separation between work and home life. Like most people we enjoy “just” being out in our beautiful environment. Our free time is often spent working with community groups to riparian plant or sample waterways, to help keep them healthy.

The good news is, you don’t have to be an ecologist to partake in 'ecology-related stuff'. There will most likely be a stream care group organising restorations activities in your area. You can take a look at the Nature Space: Ecological Restoration in Aotearoa to find out what's happening locally.

Check out Nature Space: Ecological Restoration in Aotearoa - Nature Space website

One of the community projects my family is involved in is the South Wairarapa Biodiversity Group. The group focuses on restoring and enhancing biodiversity, as well as community education, communication and participation. One of the main projects is the restoration of the riparian vegetation around the Okorewa Lagoon (Wairarapa behind the Lake Ferry Hotel).

Okorewa lagoon in the Wairarapa

Looking across Okorewa lagoon and surrounding wetlands in Wairarapa

Riparian planting means less erosion and run off

Riparian planting is often associated with farming, but similar benefits can be achieved in urban zones, for example, and even in conservation areas. One of the benefits of healthy riparian zones is that they reduce erosion and runoff of soil particles. This means that fewer soil particles end up in the water. Besides the fact that soil particles can smother organisms, they can also transport chemicals, for example pesticides, which stick to them (a process called adsorption). When the soil particles enter the water they release some of those chemicals again (desorption), which can cause all kinds of effects on the inhabitants.

The plants in a riparian zone can act as a filter, trapping those soil particles and allowing the microorganisms to degrade the substances before the particles enter the water. Lots of the pesticides used in agricultural settings are sprayed on the land, and part of that spray will “blow away” via a process called spray drift. The plants in the riparian zone will stop the pesticide in its tracks (how efficiently they do this depends on height, shape and foliage), preventing it from entering the waterways. Less chemicals enter waterways, which is better for their general health.

Greater biodiversity makes the ecosystem more stable

A nicely diverse planted riparian zone is more pleasant to look at for us humans, and this higher quality environment also brings greater biodiversity. The riparian zone provides a great variety of habitats for different species (different types of places have different type of organisms that like to live there). They also provide shade, and the roots can be hiding and spawning spots, which many aquatic organisms appreciate. In ecosystems, all species have their own role to play and (in general) the more different species there are, the more stable this system is. When a system is stable, it is better able to deal with stress and disruption. Chemicals, like pesticides or road run-off, are such a stress. In essence, the higher the biodiversity, the higher the chances that if one species is struggling or disappears, another one is available to do the same “job” so the “work” gets done.

Worst-case scenarios and promoting best practice

Planting riparian zones is not just a good way to decrease nutrient influx, erosion, or keep cattle out of the river. By changing the environment, you can change environmental pathways of chemicals - benefitting the receiving waterways. Unfortunately, not all waterways in New Zealand have a healthy riparian zone. In the risk assessments EPA staff do for the aquatic environment, we assume this beneficial planting is not present so we capture the realistic worst-case scenario in our assessments. We do this to ensure we do not underestimate the risks, and any chemicals that are approved are subject to appropriate rules around how they can be used.

As a proactive regulator we have a role to play in promoting mitigation measures, even if they go above and beyond the legal framework our decisions are made under. Riparian planting has proven benefits to the health of our waterways, and there are many other positive outcomes that come from community led restoration projects. As an EPA employee, and ecologist, I enjoy putting on my gumboots and grabbing my tools to enhance our landscapes with my local community biodiversity group. Why not join the next community riparian planting in your area? You don’t need an ecology degree to take part.

Two people standing in the middle of a waterway in South Wairarapa.

Marieke out at her local waterway, doing some monitoring with the South Wairarapa Biodiversity Group.

Marieke Soeter’s biography

Marieke did a Bachelor in Biology and a Master in Biological Sciences at the University of Amsterdam focussing on Ecotoxicology. She collaborated with several institutes (IBED, Alterra, NIOO) with a key interest in the effects of biotic interactions on the distribution of pollutants. After gaining regulatory experience in a research lab in the Netherlands as a Study Director – Ecotoxicology she moved to New Zealand to work for the EPA as an Ecotoxicology Advisor (2016). In 2017 Marieke was appointed as a Senior Advisor in Ecotoxicology.