What are estuary macrofauna?
Estuary macrofauna are small, invertebrate animals in estuaries that can be seen with the naked eye (greater than 0.5 mm). There are thousands of species of macrofauna including worms, snails, crustaceans and bivalve shellfish like pipi and cockles.
In this LAWA topic, we refer specifically to benthic, soft sediment macrofauna. These are the macrofauna that live on and in the sand and mudflats.
Why monitor estuary macrofauna?
It is important to monitor estuary macrofauna for several reasons. Firstly, they form a significant component of Aotearoa New Zealand’s biodiversity and provide an important food source for birds, fish, and people.
Secondly, macrofaunal communities add complexity to sandflat habitats by building burrows, mixing layers of sediment, and pumping oxygen- and nutrient-rich seawater into deeper sediments. In doing so, they stimulate many of the ecological processes and functions that support estuarine. Some species have a greater influence than others, for instance cockles and wedge shells are known to greatly increase the productivity of microscopic algae and enhance the removal of excess nutrients in the sediments around them. Changes in the make-up of macrofaunal communities can therefore have negative effects on how the ecosystem works.
Finally, macrofauna are very good indicators of estuary condition, or ‘health’, and can provide us with clues about what might be impacting an estuary. This is because most macrofauna are not very mobile, meaning they reflect local conditions. They are also relatively long-lived which allows us to get a stable snapshot of health, and there are differences in how sensitive species are to various environmental conditions. For example, some worms cannot live in sediments with even low concentrations of copper, whereas others can tolerate copper but cannot survive in sediments contaminated with lead.
How are estuary macrofauna monitored?
Estuary monitoring sites are usually permanent plots that are visited repeatedly, maybe once a year or every few years, so sampling can document any changes within a known area. The first step when arriving at the monitoring site is to record any observations that may help interpret the data and paint a broader picture of what is happening in the estuary. These observations may include whether birds are on the site (and if so, which species) and if there have been any changes to the land around the estuary. Photos will be taken of the sandflat surface and quadrats may be used to identify and count macrofauna that live on the sediment surface, such as snails, or features like crustacean burrows and shellfish feeding tracks.
To sample intertidal, soft sediment macrofauna, a sturdy plastic core (13 cm in diameter, 15 cm deep) is pushed into the sediment and levered out with a spade to collect a known volume of sediment. This sediment core is sieved over a 0.5 mm mesh to remove the sediment and retain the macrofauna (along with dead shells and other debris), before storing and preserving in a container. In the laboratory, the sample is sorted and the macrofauna species are identified and counted to characterise the community. The distribution of macrofauna across a sandflat can be quite patchy, so multiple cores (usually 10−12) are collected from each monitoring site to capture this.
How is estuary macrofauna data used?
There are multiple metrics that can be determined from ‘raw’ macrofauna data. Certain species can be used alone (‘indicator species’) to indicate the occurrence of a particular, or simple summaries of the entire community, such as species richness (how many species are present) or total abundance, can be calculated. Multivariate metrics, those that combine information on the species present and their quantities, make the greatest use of the available information and can provide a sensitive measure of estuary health.
On LAWA, we are using the Benthic Health Model (mud), referred to as the “Estuary Macrofauna Score”, to assess the health of monitored estuary sites. This model was developed in Aotearoa New Zealand and tells us how relatively impacted a macrofaunal community is by mud, the main stressor of estuaries across the country. The model takes all of the species data and calculate a continuous, unitless score. This score is then converted into one of five categories ranging from “very low impact” to “very high impact”. The categories are used to display relative differences between monitored sites and within a site through time, but the thresholds between categories do not necessarily reflect ecological breakpoints.
Estuary macrofauna data are commonly used in(SOE) reporting by regional councils and unitary authorities, and in national environmental reporting by the Ministry for the Environment. These reports are used to assess and inform coastal and catchment management policies and plans.
Clark DE, Hewitt JE, Pilditch CA, Ellis JI 2020. The development of a national approach to monitoring estuarine health based on multivariate analysis. Marine Pollution Bulletin.