Abstract
Forests of rimurimu (giant kelp) found over the inner-shelf (3-30 m depth) in temperate coastlines around the Pacific Ocean support numerous economic and culturally important species. However, the persistence of giant kelp is threatened as anthropogenic climate change drives long-term warming and more intense and frequent marine heatwaves that elevate water temperatures above their thermal tolerance. Understanding the processes that govern temperature dynamics over the inner-shelf is imperative for forecasting the fate of giant kelp forests and preparing coastal communities that rely on them for future climate scenarios. Previous studies have examined temperature dynamics over the East and West coast of the United States of America with specific studies done over giant kelp forests along the inner-shelf of Oregon and California. These inner-shelf systems are characterised by relatively uniform coastal bathymetry and persistent wind-driven upwelling and/or internal wave forcing that drives regular cross-shore fluxes of heat. The present thesis aims to characterize the controls of temperature variability, over the inner-shelf of East Otago, southern Aotearoa/ New Zealand. This system presents an important contrast to the oceanographic regimes in California and North Carolina, with a kelp forest located downstream of a large (25 km) coastal headland and quasi-permeant mesoscale eddy, within a predominantly wind-driven downwelling environment.
Moored in-situ timeseries of depth-resolved temperature and velocity were collected over one month during Austral summer (February) 2024 around a forest of giant kelp at Ahuriri rock located on the inner-shelf of southeast Aotearoa/ New Zealand. These data were used in conjunction with timeseries of air-sea heat flux and alongshore wind stress derived from an atmospheric reanalysis to investigate the characteristics and controls on temperature within this case-study kelp forest.
The dominant periodicities of temperature variability were analysed using data from the offshore side of the kelp forest using a power spectral density analysis. This power spectral density analysis revealed that the dominant temperature variability occurred at timescales that exceed the semi-diurnal and diurnal frequencies, defined in this the present study to be the subtidal timescales (>33 hours). Further correlative analysis showed that at the subtidal scale, the depth structure of temperature was linked to the local currents. Alongshore currents drove barotropic (constant through depth) temperature changes, while cross-shore velocities set up a baroclinic (depth varying) temperature profile, including increased stratification. These velocities showed a time-dependent relationship with alongshore wind forcing, which suggested a correlation between alongshore wind stress and temperature dynamics that occur in the kelp forest.
To mechanistically link the observed subtidal temperature variability in the kelp forest to its drivers, an observation-based heat budget was constructed over a cross-shelf ‘wedge’ that extended 2050 m offshore. This budget considered the contributions of surface heat flux (SHF), cross-shore heat flux (CHF) and alongshore heat flux (AHF) to the temperature variability. From this analysis, the mean heat budget appeared to be largely three-dimensional, in which SHF warms the system and the advective terms AHF and CHF cool the system. A correlative analysis suggested the time-varying heat budget was strongly linked to both the AHF and CHF. Subsequent analysis suggest the CHF was likely linked to upwelling dynamics driven by alongshore winds, whilst the AHF was linked to wind-forced spin-up / spin-down of the headland eddy downstream of the Otago Peninsula. This research is the first to identify the role of this eddy on time-dependent temperature dynamics over a kelp forest in East Otago and the first to use in-situ data to demonstrate the eddy's response to alongshore wind stress as a driver. This is contained within a single kelp forest, over a single summer period (one month). Additional studies would be required to extend upon the spatial and temporal (seasonal, intra-annual) variability of this ‘eddy effect’.
Overall, this research contributes valuable insight into the oceanographic regime of East Otago which govern the temperature dynamics experienced by the Ahuriri rock giant kelp forest. More broadly, these results emphasise the importance of horizontal advection, in moderating property and material transport (e.g. heat, nutrients and larvae) into and out of this kelp forest which may aid in their persistence. Furthermore, the results from this thesis suggest that any changes in atmospheric pressure systems that moderate the local wind field under future anthropogenic forcing are essential to consider in future research focused on inner-shelf dynamics and thereby, the persistence of kelp under future climate conditions.