Thursday, 30 September 2021

Relationship between sea-level change and loss of seaweed on a rocky shore

In this paper recent published in GeoHazards we evaluated the relationship between sea-level change and the severity of impacts in the major habitat-forming seaweed beds that sustain life on rocky shores.

Threshold effects of relative sea-level change in intertidal ecosystems

The 7.8 Mw Kaikōura earthquake affected a large section of the South Island’s east coast and led to a major re-assembly of ecological communities and coastal resource use. To understand the drivers of change and recovery in nearshore ecosystems, we quantified the variation in relative sea-level changes caused by tectonic uplift and evaluated their relationships with ecological impacts with a view to establishing the minimum threshold and overall extent of the major effects.

For this assessment we needed to quantify the degree of uplift as close as possible to our post-earthquake study sites in the new intertidal zone. Challenges for this include the availability of elevation data within the landforms and ecosystem types of interest.

We used a methodology based on LiDAR data from adjacent areas to landward that incorporates the consideration of tilt effects that could lead to uneven ground level displacements, and two time periods to address the potential for continued displacement subsequent to the main seismic event. We also assessed interactions with substrate types and inlcuded two different sensitivity analyses to validate the approach used. 

We found that co-seismic uplift accounted for the majority of relative sea-level change at most locations. However, some changes were detected after the initial earthquake that result from the effects of reef weathering and accumulation of mobile gravels. Intertidal vegetation losses were evident in equivalent intertidal zones at all of the uplifted study sites. Nine of ten uplifted sites suffered severe (>80%) loss in habitat-forming algae and they included the lowest uplift values (0.6 m). 

The results are consistent with a functional threshold of approximately one-quarter of the tidal range above which major impacts were sustained. One of the interesting effects is that previously subtidal algae  such as bull kelp (Durvillaea spp.) were uplifted into the low intertidal zone where they ought to persist - but did not. This suggests that additional post-disturbance adversities have contributed to the degree of impact, since otherwise we would have expected to find more survivors in our lower intertidal study areas. Continuing research has been investigating the nature of these factors. They are obviously important to the regeneration of ecosystems and ecosystem services following a major disturbance.

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