Otago Regional Council coastal scientist Sam Thomas said climate change appeared to be the major factor in the loss of Otago’s kelp forests, which support pāua, crayfish and inshore fisheries.
However, the first phase of a five-year study on kelp in Otago showed a connection between turbid water and a lack of resilience to increasing sea temperatures for the ecologically important underwater forests, Dr Thomas said.
"We know we can’t manage temperature increase — sea level temperature increase — as ORC," he said.
"Essentially, if we can reduce sediment from land-based activities, whether it’s forestry, agriculture, whatever, it will be of help to the kelp forests in the surrounding coastal zone."
He stressed to the council’s Environmental Science and Policy committee last week that the work under way was in the early stages.
He presented the committee with a 10-year snapshot of Macrocystis pyrifera, or bladder kelp, done for the council by the National Institute of Water and Atmospheric Research (Niwa) using satellite remote sensing.
It showed unseasonably warm temperatures typically had a negative effect on kelp coverage, especially for larger kelp forests.
The study also provided some of the first evidence that the marine heatwave during the summer of 2021-22 caused Macrocystis cover along the coast to shrink.
And while there was not a lot of evidence that varying amounts of sediment in the water "caused" changes in Macrocystis coverage, there were differences in Macrocystis coverage across areas exposed to differing amounts of sediment in the water, his report to the committee said.
"Alongside warm temperature anomalies, Niwa has shown that reduced water clarity is an additive stressor to Macrocystis forests."
The snapshot additionally showed Macrocystis forests tended to be smaller and closer to the shore in the northern end of the study area, at Timaru and North Otago.
Further south, at Moeraki, Waikouaiti, and Blueskin Bay, Macrocystis forests increased in coverage and many large offshore forests were observed, Dr Thomas said.
Macrocystis coverage peaked in what he referred to as the "Waikouaiti Zone".
At the southern extent of the study area, small patches of Macrocystis were observed near Nugget Point and the mouth of the Catlins Estuary.
The initial snapshot Niwa provided was based on satellite imagery over the past 10 years and a follow-up work programme where divers and remote operated submarines would validate the preliminary findings was scheduled for the next financial year.
At present, work was under way to provide a similar snapshot of bull kelp, which unlike Macrocystis could not be mapped by satellite.
But a much more robust understanding would be gained at the end of the five years, he said.
Cr Michael Laws challenged Dr Thomas to "put a percentage on" the impact of land-use compared to climate change, which Dr Thomas could not.
Cr Kate Wilson said the "whole ecology" of the coastal marine environment had appeared to change dramatically a couple of decades ago.
She said she was concerned about jumping to conclusions about what was happening in the coastal environment based on what happened over only the last 10 years.
"Could it be that it is something that has happened in the last 20 years that has had a significant effect as well?"
Cr Gretchen Robertson said floods and storms could also be factors in the loss of kelp forests.
