Estuaries and farmland run-off

New Zealand has approximately 300 estuaries.

They are historically important cultural sites – Māori settlers valued estuaries as an important source of fish and shellfish, and European settlers valued them as safe harbours, with many of New Zealand’s major towns built around estuaries – but they are not just important from a human perspective.

Estuaries provide critical nursery habitats for fish and other organisms that use their sheltered waters at early stages in their life cycle. Estuaries also play an important role in nutrient cycling and filtering contaminants and nutrients from the land before they flow out to the ocean.

Understanding the impact of land use change

Dr Candida Savage from the University of Otago is interested in understanding more about how changes in land use affect estuaries and the organisms that live in them. Along with colleagues and postgraduate students, she carries out research that aims to inform better natural resource management.

A major focus of her research is looking at run-off from farmland. ‘Run-off’ is a term that describes nutrients and contaminants that are carried from the land, via streams and rivers, into coastal areas. In small amounts, these extra nutrients can improve the growth of a number of organisms, but one of Candida’s most important on-going research areas is looking at threshold levels. In other words, what level of run-off can result in major changes to the function of an ecosystem.

Learn more about life in the estuary and cultural, ecological and economic value of estuaries, in valuing estuaries.

Techniques for measuring the impacts

Trying to measure linkages between changes in land use and changes in the ecology of estuaries is difficult. The research involves a combination of fieldwork and experiments in the lab. The three main methods used to better understand the impact of run-off on estuaries are:

• comparing pristine and impacted areas
• sampling living species
• conducting field and laboratory experiments.

Comparing pristine areas in Stewart Island and the Catlins with impacted areas along the Otago and Southland coast is one way for scientists to quantify human impacts to estuaries. Candida and her students categorise the type of land use and measure and model nutrients and contaminants in the water column in the estuary. They also investigate the growth rates and chemical signatures of a number of species in the estuary, including fish and mussels. This information enables them to measure the impact of land use changes on estuarine and coastal ecosystems.

Sampling living species is also an important tool in learning more about nutrient and contaminant levels. Candida selects a range of species to work with from all levels of the estuarine food web. Detailed studies will then focus on a particular organism.

For example, seaweed and phytoplankton form the base of the food web. Seaweeds act as natural samplers of the environment, absorbing nutrients that they use to grow. By sampling seaweeds and analysing them for nutrient concentrations and chemical make-up, it’s possible to measure levels of nutrients in the water column and how concentrations change across an estuary and during different seasons.

Crabs are also useful indicator organisms of estuarine health. One of Candida’s master’s students, Sam, studied crabs and used a new technique to test crab urine for contaminants. This new method meant that crabs did not have to be killed before they could be sampled.

Estuaries receive run-off from multiple sources, and it is important to confirm that the patterns recorded in nature reflect the different sources of nutrients and contaminants, so Candida and her students run experiments in the field and in the laboratory.

For example, they collect seaweeds from pristine estuaries and transplant them to impacted ones to test how their chemical signatures change. They also do this process in reverse, taking seaweeds from impacted sites and growing them in pristine areas. They have also measured growth rates of mussels and fish in controlled laboratory conditions to see how quickly the organisms grow and to better understand the information they record in the field.

Sediment cores

Taking sediment cores from the bottom of an estuary allows scientists to travel back in time. Analysing these cores is like reading tree rings or ice cores in Antarctica – layers are formed as sediment is deposited on the seabed, allowing scientists to get a glimpse into past ecological communities.

Candida and her colleagues take sections from the cores and interpret chemical markers from different time periods. Sometimes, even tiny plankton skeletons are preserved in the sediment. The preserved plankton skeletons and chemical markers act as records of what was in the water column at the particular point in history. These sediment cores can show changes in estuarine ecology over hundreds of years, giving scientists an idea of the conditions before human impact.

What do the results tell us?

Candida’s estuary research is on-going, but there have been some important results to date, which have important implications for regulating farming practices and managing natural resources (for example, rules for recreational and commercial fishing).

Her studies show that an increase in farm fertiliser use on the land surrounding estuaries results in increased growth in some fish and bivalve species. This finding can be interpreted as a positive impact in many ways, but there is a corresponding reduction in biodiversity in the more impacted estuaries. This is concerning as it changes the food sources available to organisms and affects nutrient cycling

One of her students is now investigating whether this diversity decrease in impacted systems also affects their natural ability to cope with increased levels of nitrogen run-off.

Initial results also suggest that, as estuaries become more degraded they also play a role in climate change by giving off nitrous oxide, a greenhouse gas, so this research highlights the importance of maintaining healthy estuaries.

Learn more about other human impacts on estuaries and the marine environment.