Surveys led by Niwa marine biologist Daniel Leduc (pictured) have shown considerable improvement in the seabed quality off the Tairāwhiti and Hawke's Bay coasts that were badly impacted by sediment in the wake of Cyclone Gabrielle.
Surveys led by Niwa marine biologist Daniel Leduc (pictured) have shown considerable improvement in the seabed quality off the Tairāwhiti and Hawke's Bay coasts that were badly impacted by sediment in the wake of Cyclone Gabrielle.
Scientists found evidence of sediment impact on seafloor ecosystems off the Tairāwhiti coast in the wake of Cyclone Gabrielle but surveys have since shown recovery in those ecosystems.
Niwa benthic (sea floor) 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 are no comparable pre-cyclone camera observations, we were not 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.”
De Leduc said 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.
This image, taken in October, shows dramatic improvement in seabed quality on the Tokomaru shelf reefs.
“However, nine months after the cyclone, we saw that populations had recovered to pre-cyclone levels,” said Dr Leduc.
Fisheries NZ’s director of science and information, Simon Lawrence, said marine ecosystems were complex and could 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.
“It will help us plan for, and respond to, future events like this.
