Fires and drainage associated with the conversion of land to farming are the main causes of degradation of the Wainono Lagoon, according to research commissioned by Environment Canterbury.
University of Otago scientists Marc Schallenberg and Emilie Saulnier-Talbot used sediment cores spanning at least the last 160 years, along with historical information, as the basis of their report.
Their research showed the water had changed from freshwater to brackish, and lost aquatic plant life. There had also been a substantial reduction in the size of the lake and wetland area since the 1850s.
The area was a "mere remnant" of an extensive lowland coastal lake-wetland.
"At the time of European colonisation it provided a diverse mahinga kai resource for hundreds of Maori inhabitants of the area. The lake is currently hypertropic, with high levels of planktonic algae and turbidity."
Nowadays, common aquatic plants were rarely seen in the lagoon.
"However the recent observation of ruppia-type seagrasses growing in the lake indicates that restoration could be possible, at least to the clearwater, brackish state that the lake was in between 1910 and the 1960s-70s."
The report's executive summary said early European visitors described a "vast, quaking swampland" between Willowbridge and the Makikihi River.
The lake was predominantly fresh water during this time.
"Remains of macrophytes, freshwater sponges and freshwater zooplankton were also found in the sediments laid down at that time."
In the 1860s and 1870s, the report said major fires assisted conversion to pasture in areas upland from the lake.
"This caused erosion of soils and an influx of fine and then coarse soil particles and charcoal to the wetland area. The lake continued to be a freshwater system with biological communities remaining more or less unchanged due to the buffering effect of the extensive fringing wetlands."
The next big influence on the wetlands was the building of the Waihao Box in 1910, which lowered the water level and introduced saline water to the lake.
"The opening to the sea allowed the drying, draining, burning and conversion of fringing wetlands into pasture," the report said.
"These changes began a phase of increased fine sediment and nutrient loading to the lake causing eutrophication and increasing turbidity."
Eutrophication is the term for nutrient-rich water that supports a dense plant population.
The decomposition of the plants kills animal life by depriving it of oxygen.
By the 1970s, the report said the lake had become a eutrophic brackish lake with much-reduced water depth.
The average sedimentation rate during the European era was 3mm/year, similar to the rate for Lake Forsyth (Canterbury) and half that calculated for Lake Waihola in South Otago.
- The Timaru Herald