2001-2006 Regional Estuary Monitoring Programme public report

In April 2001 Waikato Regional Council started a long term monitoring programme in the Firth of Thames and Raglan (Whaingaroa) Harbour, looking at estuary health. The programme monitors sediment-dwelling (‘benthic’) animal communities that live between the low and high tide marks, and sediment characteristics (like the amount of mud, shell and minute algae present).

Estuaries are one of the most sensitive coastal areas, and are at risk from human activities. They are important areas for people culturally, commercially and recreationally, and are highly productive ecosystems that provide important habitats for many fish, shellfish and bird species. Estuaries receive and accumulate sediment, nutrients and contaminants from the surrounding catchment. What happens on land can directly and indirectly affect the health of an estuary. Monitoring will allow early detection of any negative environmental changes, and as such provide a trigger for assessing land management practices.

Five sites are monitored quarterly in the Firth of Thames and Raglan Harbour. Marine worms and shellfish are the most common benthic animals found. Differences in animal communities found between the two estuaries are mainly caused by differences in sediment characteristics (mainly the amount of shell material and proportion of mud in the sediments). Overall, the sediments monitored in the Firth of Thames are sandy with low mud content, and the amount of mud in Raglan Harbour is 2-4 times greater than that found in the Firth of Thames.

In both estuaries, the proportion of mud in the sediment increased between 2001 and 2006. (with a greater rate of increase in Raglan Harbour) - this is a concern. Fewer animals were found at the muddiest sites in Raglan Harbour than at the more sandy sites, which suggests that the mud has a negative effect. However, even though the sediment mud content increased between 2001 and 2006, there is no evidence of declining trends of sensitive animals – at the muddy sites, the number of animals present was low even at the start of the monitoring.

The increase in sediment mud levels could either be caused by activities in the catchment, or it could be part of natural fluctuations. Continued monitoring is needed to determine if the increase in mud is caused by human activities, and if it is having an effect on the benthic animals over time. It is likely that if mud levels continue to increase it is only a matter of time before this will start to impact the benthic animal communities, and the fish and birds that eat them.

Overall the monitoring shows the Firth of Thames and Raglan Harbour are relatively healthy. It is important that monitoring continues in the long term so that any changes in the animal communities resulting from catchment activities can be detected. This will allow better management of these important ecosystems.