Factsheet: Mud content in estuaries

What is mud content?

Mud content refers to the amount of fine silt and clay particles (collectively called ‘mud’) that have washed off the land and are present in the surface layers of estuary sandflats. Mud is defined as sediment particles less than 0.063 mm in diameter, and is characterised by the gloopy, sinky feeling you might feel underfoot when walking through an estuary.

Why monitor mud content?

Mud content is one of the main environmental characteristics that determines where plants and animals can live within an estuary. For example, if mud content is too high plants such as seagrass become smothered and die. Most macrofaunal species also have a limited range of conditions in which they can survive. One large species of marine worm, Travisia olens, prefers sediments with less than 5% mud content, whilst the tubeworm Macroclymenella stewartensis is happiest in sediments with between 10 and 15% mud. This means that the make-up of the macrofauna community changes significantly with changes in mud content, which has consequences for the fish and birds that feed on them as well as the functioning of the estuary.  Monitoring mud content can help us understand which plants and animals are likely to live in an area. Changes in mud content can also alert us to upstream activities that might be generating excess amounts of mud, or the movement of mud around the estuary due to changes in flow dynamics.

The mud that enters estuaries also often carries other pollutants such as nutrients and metal contaminants. This means muddy areas are also more likely to be over-enriched with nutrients, which can lead to eruptions of nuisance algae, and be toxic for macrofauna.

How is mud content monitored?

To sample for sediment mud content in intertidal areas, the top 2 cm of the sediment is collected using a small round corer or scoop at low tide. For very muddy or subtidal sites, samples may be collected from a boat using devices like sediment grabs or large corers that collect a section of sediment from which we can sample the top 2 cm. Multiple samples are often collected from across the site (known as replicates), or multiple sub-samples from across the site might be combined into one sample jar (known as a composite sample) to get an accurate measure of mud content that represents the monitoring site.

Most samples are then analysed in a laboratory using the ‘wet sieving’ method. First, pieces of organic material are removed from the sample by dissolving them in hydrogen peroxide. The sediment sample is then washed over a set of stacked sieves that have gradually decreasing mesh sizes. The final sieve has a very fine mesh with gaps of 0.063 mm that allows only the mud particles to pass through. These particles, as well as the portions of sediment that remain on each of the sieves, are dried in an oven set to 60oC for 48 hours. They are then weighed, and the percentage that each size fraction contributes to the total sediment weight is calculated. Therefore, when we have a result that says sediment mud content at a monitoring site was 50%, this means that half of the weight of the sediment sample (when dried) came from mud particles.

Wet sieving is the most accurate and appropriate method for long-term monitoring and is used by almost all councils currently, however analysis using another method, laser diffraction, has also been used. This method uses a laser to pass light through a sediment sample suspended in water and estimates the particle sizes based on how the light is diffracted. In this case, mud content is presented as a percentage of the sediment sample by volume.

How are mud content results assessed?

It is not easy to define what the mud content in any given estuary should be. This is because estuaries naturally fill up with sediment from land and sea as they age, and the speed of this process varies between estuaries. The proportions of sediment that come from land (in the form of mud) and the sea (usually clean sands) can also be very different. It is also important to consider where in the estuary sediment samples are collected from, as it is expected that the upper reaches where rivers enter will be muddier than near the mouth of the estuary.

There are no national guidelines to assess sediment mud content against. There are, however, many local studies that have investigated how mud content affects macrofaunal communities and ecosystem functions. We have used the knowledge generated by some of these studies to create a set of guidelines for use on LAWA.

How is mud content monitoring data used?

Mud content data are commonly used in State of the Environment (SOE) reporting by regional councils and unitary authorities, and in national environmental reporting by the Ministry for the Environment and Stats New Zealand. These reports are used to assess and inform coastal and catchment management policies and plans.


Read more

Hunt S, Jones HFE 2019. Sediment grain size measurements are affected by site-specific sediment characteristics and analysis methods: implications for environmental monitoring. New Zealand Journal of Marine and Freshwater Research.

Guideline references:   

Rodil IF, Lohrer AM, Hewitt JE, Townsend M, Thrush SF, Carbines M 2013. Tracking environmental stress gradients using three biotic integrity indices: Advantages of a locally-developed traits-based approach. Ecological indicators.

Robertson BM, Stevens L, Robertson B, Zeldis J, Green M, Madarasz-Smith A, Plew D, Storey R, Oliver M 2016. NZ Estuary Trophic Index Screening Tool 2. Determining Monitoring Indicators and Assessing Estuary Trophic State. Prepared for Envirolink Tools Project: Estuarine Trophic Index.

Douglas EJ, Lohrer AM, Pilditch CA 2019. Biodiversity breakpoints along stress gradients in estuaries and associated shifts in ecosystem interactions. Scientific Reports.