Subtidal research

Robyn Dunmore and PhD student Dan Crossett from the Cawthron Institute have been tracking recovery in the nearshore subtidal zone. This has involved largescale surveys and experiments in the field and lab. They found that initially, there was clear disturbance to areas with medium to high uplift, especially around Waipapa Bay but also including areas like Ward, Wharanui and Okiwi Bay. Bare rock had been uplifted through sand and gravel, and shifts in sand and gravel
into seaweed habitats had also occurred. More recently, they observed some declines in large brown algae across many sites (even those with low uplift), with changes from brown seaweed-dominated habitats to areas characterised by red seaweed.

These effects could result from a combination of a changing wave climate (from uplift and specific weather events) and stress from marine heatwaves.

After the earthquake, slips along rivers increased runoff of turbid water into the sea, and erosion from unstable rock along the coast also contributes to increased turbidity. Lab experiments were done to test the effects of temperature, light and turbidity on the early life stages of several species of large brown algae. These showed that some species can handle a range of temperatures, while others are restricted to growing only at cooler temperatures. Low light and turbidity also slowed growth in some species. Clearance experiments are helping us understand how seaweed recruitment is affected by substrate type, encrusting coralline algae and the presence of large brown seaweeds. Sites at Waipapa have had little or no recruitment of coralline algae and large brown seaweeds, but other sites with nearby reproductive adult plants are quick to recover. The information from the clearance and lab experiments is important to understand how these seaweeds will recruit into and grow in areas disturbed by the earthquake.

Experimental clearance plots at start (left) and after 9 months (right) showing fast recovery in a plot with surrounding adult plants (top), and very little recovery at a bare area in Waipapa (bottom). Picture by the Cawthron Institute.

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