Effect of earthquake and storm disturbances on bull kelp (Durvillaea ssp.) and analyses of holdfast invertebrate communities
Thesis DisciplineEnvironmental Sciences
Degree GrantorUniversity of Canterbury
Degree NameMasters of Science
Marine wave-exposed intertidal rocky shores, in the temperate zone, are some of the most productive yet highly stressful biological habitats on earth. In the intertidal zone, marine species experience daily changes in desiccation, temperature and light conditions. Many large canopy-forming seaweeds, including some laminarian kelps and many fucoids, are well-adapted to these conditions, but are typically being limited in their upward distribution on the shore by desiccation tolerances. These rocky shores are also characterized by physical and biological disturbances, such as storm waves, invasions by non-native species and even tectonic events like vertical displacement following earthquakes. Where kelps and large fucoids dominate the biomass, they control and modify ecosystem functions, like productivity, wave attenuation and light levels. Southern bull kelps (Durvillaea spp.), which are actually fucoids, are some of the largest marine habitat formers on earth, often dominating wave exposed intertidal and shallow reefs throughout much of temperate Australasia and South America. Bull kelps support high local primary productivity, attenuate waves and provide food for grazing fish and habitat for invertebrates. A bull kelp is composed of a large holdfast firmly attached to the rocky substratum, a stipe and a flexible buoyant frond. Bull kelp can grow up to 10 m and live up to 10 years. Only a few species live on the stipe and fronds of bull kelp but their large holdfast can provide habitat for many invertebrates. In this thesis I explore, from intertidal reefs along the east coast of the South Island of New Zealand, how bull kelp (Durvillaea poha and D. antarctica) respond to simulated storm disturbances (Chapter 2), how their holdfast provide habitat for invertebrates (Chapter 3), and I describe impacts on bull kelp following a large earthquake and uplift of coastal reefs (Chapter 4).
Firstly, I compared bull kelp responses between undisturbed control plots and three simulated disturbance intensities. To simulate a typical gradient in storm-disturbance effects, I removed either holdfasts, stipes or blades at two reefs at Moeraki and one reef at Oaro. Four months after these disturbances, there were no major effects on density or sizes of adult holdfasts in the stipe removal treatments, demonstrating that after growth has ceased these biological structures can remain intact and attached to the rocky substratum. I found no disturbance-driven effects on either the density or the length of juvenile blades. Overall, juvenile densities were highly variable in space and time. I also found that, at the two Moeraki reefs, the length of juvenile bull kelp show strong increases in all treatments. In addition, most bull kelp with blades showed sign of recovery, with new growing tips along the margins of cut tissue. Finally, all disturbed plots were rapidly colonized by fast growing opportunistic seaweeds, as the green alga Ulva spp. colonized disturbed reefs at Oaro and the invasive kelp Undaria pinnatifida colonized disturbed plots at Moeraki. I also found a negative correlation between the abundance of these early colonizers and the density of bull kelp recruits, indicating either that the former inhibits new bull kelp recruitment, or that small bull kelp recruits can inhibit colonization of opportunistic seaweed.
Secondly, I collected Durvillaea poha holdfasts to test if holdfast-associated invertebrates differ between different sites located along a latitudinal gradient in the South Island of New Zealand and between different holdfast sizes. The result from these collections showed higher biodiversity at the mid-latitudinal reefs, but this could probably be explained by low and high latitudinal holdfasts being either stressed by uplift or by a few size-outliers, respectively. I also found, as in past kelp holdfast studies, strong size-abundance relationship, that is, larger holdfasts were inhabited by more invertebrates than small holdfasts. I then tested, in a factorial short-term transplant experiment, if colonizing invertebrates differed between different holdfast morphologies (round vs. elongated), holdfast types (live vs. 3D printed abiotic models) and holdfast species (Durvillaea poha vs. the early colonizing non-native Undaria pinnatifida). I found, for similar sized holdfasts, that more invertebrates were associated with live than abiotic holdfasts and with Undaria compared to Durvillaea (but with no effects of holdfast shape). These results suggest that invertebrates may partially consume holdfasts and use small interstitial spaces associated with complex biological structures. Although Undaria, for similar sized holdfasts, supported more invertebrates than bull kelp the Undaria habitat is, however, more ephemeral because this holdfast persists for less than one year compared to up to 10 years for bull kelp.
Finally, I surveyed 16 reefs from Oaro to Kaikōura peninsula 3-4 months after a 7.8 Mw earthquake with an epicentre located 4 km from the rural village of Waiau. Of these reefs Oaro experienced 0.2 m submergence, whereas the remaining reefs were uplifted from 0.4 to 2.2 m. At each reef, 50 x 50 cm quadrats were sampled, by taking photos perpendicular to the substratum, in the zone that, prior to the earthquake, was dominated by bull kelp. This zone was subdivided into a higher zone turned white due to decaying calcifying encrusting organisms, a middle zone turned green due to colonization of the opportunistic Ulva spp seaweed, and a lower red zone where the red encrusting understory alga remained relatively intact. In total, 1658 quadrats were analysed for (a) percent cover and (b) density of ‘holdfast scars’ (circular areas of newly exposed fresh rock), (c) stipes without blades, (d) stipes with blades, and (e) percent cover of holdfasts. I found for the 15 uplifted reefs that cover and number of holdfasts scars were greatest in the white zone, that densities of stipes with blades and cover of attached holdfasts were highest in the red zone, and that densities of stipes without blades were highest in the mid-green zone. By contrast, there was no white zone at the single reef experiencing slight submergence (Oaro) and this reef had fewer holdfast scars and stipes without blades. I also tagged stipes from the green zone from 9 uplifted reefs as well as stipes from Oaro. Ca. 8 months later all tags were lost from the bull kelp from the uplifted reefs but all tags survived at Oaro, suggesting that most kelp in the green uplifted zone would eventually die.
Overall, my study documented that bull kelp are resilient to small scale storm-disturbances as they can recover from pruned blades or through formation of a new canopy from rapid growth of understory juveniles. I also found that when perennial kelps were lost they were replaced by opportunistic seaweeds, both following small-scale manipulated and large-scale uplift-related disturbances. Importantly, when bull kelp and their holdfasts are lost, so is the rich fauna that inhabits these biological structures. Finally, I documented extensive loss of bull kelp following an earthquake, a result that likely can be extrapolated to other areas of the coastline that experienced similar or more severe uplifts. These large-scale losses are likely to have long-lasting and wide-ranging ecological effects and it will be of great interest to study these seaweeds beds in the future, to test if these past extensive bull kelp assemblages will recover fully or if remnant surviving beds will remain small.