Relationship between changing sea levels 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
https://doi.org/10.3390/geohazards204001641

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 sea-level change caused by tectonic uplift and evaluated 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 vertical uplift from the earthquake as close as possible to our post-earthquake study sites in the new intertidal zone. Challenges for this included the availability of elevation data within this area since it was previously covered by water at high tide.We used a methodology based on LiDAR data from the closest adjacent areas to landward that also incorporated an assessment 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 included two different sensitivity analyses to validate the approach used, and assessed interactions with substrate types.

Findings

We found that co-seismic uplift accounted for the majority of  the sea-level change at most locations. However, some changes were detected in the period after the initial earthquake that result from the effects of reef weathering and movement of mobile gravels along the coast. 

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 indicate a functional threshold of approximately one-quarter of the tidal range above which major impacts were sustained. This pattern wasn't entirely explained by the previous position of zone boundaries between the main habitat-forming species in relation to their intertidal position, suggesting that other factors  (additional to sea-level changes) were involved.

One of the interesting effects involved previously subtidal algae such as bull kelp (Durvillaea spp.) individuals that were uplifted into the low intertidal zone where they ought to persist - but did not. This suggests that additional post-dearthquake stressors had contributed to the degree of impact, since otherwise we would have expected to find more survivors in our lower intertidal study areas. Similar effects were found for Hormosira in the mid-intertidal zone. Continuing research has been investigating the nature of these factors. These 'double whammy' situations are evidently important to the regeneration of ecosystems and ecosystem services following a major disturbance, and may also affect the severity of observed mortality events.

Recover newsletter issue 6 on Kaikōura coastal recovery

#RECOVER Issue 6 features the ‘new land’ created by the earthquake uplift of the coastline, recreational uses of beaches in Marlborough, and pāua survey work and hatchery projects with our partners in Kaikōura.

Available online here https://hdl.handle.net/10092/101857

Whitebait spawning grounds at non-tidal rivermouths

We recently published a study of īnanga (Galaxias maculatus) spawning grounds at non-tidal rivermouths in Pacific Conservation Biology. 

Article freely available here

Non-tidal rivermouths are common on the east coast of the South island and south-east of the North Island where they are commonly associated with high energy mixed sand-gravel beaches that form perched lagoons or hāpua at the rivermouth. 

At the outset of the study the spatial pattern of spawning in these environments grounds was unknown. Some of the interesting findings were that the location of spawning was close to the rivermouth despite the lack of salt water intrusion or tidal influence, both of which are associated with spawning grounds in tidal situations. 

A downstream fish migration evidently still occurs prior to spawning events and fish movements were surprisingly rapid in response to spawning cues in many instances. Spawning events are triggered by water level changes as occurs in tidal rivermouths, but appear to be more haphazard due to their origins being rain events rather than regular tidal cycles.Geographical aspects of the spawning locations were remarkably consistent across all seven study sites in key aspects such as the position in the catchment, relationship with water changes and presence of riparian vegetation. 

This provides a solid basis for identifying and managing spawning habitat in other non-tidal rivers. Effects we recorded included spawning on flood events that resulted in spawning grounds being located some distance from the active channel. As with tidal situations this places the eggs at risk from human activities in those same zones, but potentially presents a more difficult management proposition due to these events being less predictable. Their irregular nature suggests they are less likely to be accommodated in riparian zone planning and management. Importantly, the spawning grounds are located close to the peak water levels experienced on these events, which coincides with areas that are only briefly inundated. 

Recognising and protecting these emphemeral flood-zone areas is the key to effective conservation.

Read the full article here