Seafloor Impacts Of Cyclone Gabrielle Revealed

Scientists have found evidence of sediment impacts on seafloor ecosystems following Cyclone Gabrielle.

The research was done by NIWA on behalf of Fisheries New Zealand to support fisheries management decisions.

NIWA benthic ecologist Dr Daniel Leduc said that while there were noticeable impacts on the seafloor, some areas were already showing positive signs of recovery.

"Analysis of satellite images suggest that the influence of Cyclone Gabrielle on water quality lasted approximately two to three months across the two regions. We also found that of the 36 locations we surveyed with an underwater camera, 11 showed signs of sediment impact. However, because there aren’t comparable pre-cyclone camera observations, we weren’t able to directly link these to the cyclone," said Dr Leduc.

Extreme weather events can create sediment deposits in the ocean because of accelerated soil erosion and transport from rivers into the sea. Increased sedimentation is one of the main threats to marine ecosystems in New Zealand and the most important land-based stressor on our coastal fisheries.

NIWA conducted two research surveys in June and October 2023 across Hawke’s Bay and Tairāwhiti. Several methods were used in the study, including satellite imagery, seafloor mapping, and underwater cameras.

In addition to the vessel surveys, NIWA adapted an existing seafloor model to explore the impacts of cyclone disturbance on animals living on the seafloor.

"We fed the model lots of information, including history of bottom trawling and sedimentation prior to the cyclone, biological characteristics of seafloor organisms, and how much sediment was deposited after the cyclone. The model suggested that habitat forming organisms have been impacted by pre-cyclone fishing and sedimentation, and only limited additional declines in habitat-forming organisms, such as sponges, were predicted from the cyclone.

"We also took sediment core samples and saw evidence of cyclone impacts on seafloor animal communities off Poverty Bay, where small fauna sampled four months after the cyclone had decreased markedly compared to samples dating back to 2010. However, nine months after the cyclone, we saw that populations had recovered to pre-cyclone levels," said Dr Leduc.

Fisheries New Zealand’s Director Science & Information, Simon Lawrence says marine ecosystems are complex and can be impacted by a range of factors including direct human impacts, extreme weather events such as the recent cyclones and the effects of climate change.

"The results of this survey provide an important baseline for understanding the impacts of these cyclones on our coastal marine environment and help us plan for, and respond to, future events like this. Some of the early observations from this research have already been used for management settings. The research will undoubtedly prove useful for ongoing fisheries management decisions," Simon Lawrence said.

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