Estuaries in the Mataura

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Information to come on Haldane, Waikawa, Toetoes (Fortrose) estuaries, and Lake Brunton.​ For information on Waituna Lagoon, please see www.waituna.org.nz.

Fortrose (Toetoes) Estuary Broad Scale Habitat Mapping 2017-18

Fortrose Estuary is one of the key estuaries in Environment Southland's State of Environment Monitoring Programme. Part of this monitoring includes broad scale habitat mapping in the estuary on a 10 year cycle, in estuaries showing signs of concern focused monitoring of macroalgae and seagrass coverage is carried out annually, where possible. Seagrass is an important habitat in estuaries because it provides a nursery for fish species, habitat for macroinvertebrates, stabilises the estuary bottom and filters nutrients from the water column. Macroalgae, including Gracilaria Chilensis found in Fortrose Estuary, is natural in New Zealand Estuaries. However in the presence of excess nutrients and mud macroalgae can become opportunistic and grow into thick mats across large areas which can displace seagrass habitat and reduce the sediment quality leading to areas known as "gross eutrophic zones". A "Gross Eutrophic Zone" is defined as an area that has low sediment oxygenation (<1cm aRPD), soft mud (>25% mud content) and the presence of high macroalgal cover (>50% cover), these areas are in poor condition and can no longer support most estuarine animals and shellfish.

The 2018 Fortrose (Toetoes) Estuary Broad Scale Habitat Mapping report summarises data collected since 2003 to 2018 (including the years 2003, 2009-2013, 2016 and 2018). Fortrose estuary is a dynamic system that experiences large sediment deposition and erosion events from the two main riverine inputs, the lower reach of the estuary is well flushed. Since monitoring began the estuary had very limited seagrass coverage (0.3ha), however these small seagrass patches have decreased by 33% to 0.2ha. The expansion of mud in the estuary occurred in 2013 and areas of the estuary experiencing poor conditions (no oxygen, excess macroalgal growth and soft mud), also known as gross eutrophic conditions, established in 2016 and remained stable in the 2018 monitoring. Between 2003 and 2018 these areas have increased from 0% (0ha) to 3.7% (9ha). This means that 3.7% of the estuary is currently in poor condition and estuarine animals and shellfish in these areas will struggle to survive. Of particular concern are the southern sand flats of the estuary that are experiencing low oxygen conditions just below the sediment surface. These areas normally well oxygenated and support many estuarine animals (crabs, shellfish, snails etc.), low oxygen conditions in these areas will have a negative impact on sediment dwelling animals. The cause of these low oxygen conditions is uncertain, however it could be related to water column nutrients from the Mataura River. The estuary was assessed to be in a poor state and showing signs of eutrophic symptoms. 

 Fortrose (Toetoes) Estuary Broad Scale Habitat Mapping 2017-18​

Page reviewed: 18 Apr 2019 3:39pm