Abstract
The coastal waters of Aotearoa New Zealand (ANZ) are experiencing increasingly frequent, strong and long-lasting marine heatwaves as a result of long-term ocean warming. Waitati Inlet, north of Otepoti Dunedin in south-eastern ANZ was chosen as a study site to examine how physical processes associated with the inlet might amplify the impacts of marine heatwaves over the inner-shelf. An improved understanding of the dynamics of extreme heating over the inner-shelf is important as coastal marine heatwaves can strongly impact ecosystem structure, such as kelp forests.
This thesis utilised oceanographic and meteorological data of Waitati Inlet from February 2022, together with 1D numerical models, to quantify a comprehensive heat budget of this estuarine system. Additionally, this thesis investigated whether the presence of the inlet increased exposure time of warm water in a nearby kelp forest. The hypotheses of the current study were that (1) vertical heat fluxes in the inlet were dominated by air-sea heat fluxes, with secondary contributions by sediment-sea heat fluxes; (2) that these vertical heat fluxes had a greater contribution to the heat budget than the horizontal heat fluxes; and (3) that a coastal inlet with the physical characteristics of Waitati Inlet acts as a heat incubator over summer months and therefore increases exposure time of a nearby kelp forest to warm water.
The contribution that vertical terms (air-sea and sediment-sea heat fluxes) played in the inlets heat budget were first investigated using in situ timeseries of water and sediment temperature, together with meteorological and sea level parameters. During the experiment, air-sea heat fluxes were found to warm the inlet’s main channel and cool the tidal flats. In comparison, sediment-sea heat fluxes acted to warm water within the inlet. The largest components of the air-sea heat exchange for both areas was the heating by shortwave radiation and cooling by longwave radiation and latent heat flux, with sensible having the weakest contribution. The vertical terms largely balanced over the experiment’s 28-day duration; however large residuals that had a periodicity of 12.3 hours were also evident, suggesting that advective heat fluxes linked to the semi-diurnal tide were also important for the inlets heat budget on shorter timescales.
A one-dimensional (1D), cross-flat, numerical model of Waitati Inlet was then setup to evaluate the relative importance of vertical and horizontal processes in governing the inlet’s heat budget, and explore interactions between them. The model incorporated air-sea and sediment sea heat fluxes, together with air-sediment heat fluxes through a 1D sediment module and horizontal heat fluxes from advection. The model was tuned through comparison with in situ temperature timeseries from the inlet. The model was then run for a 28-day period over February 2022. During this period, air-sea and advective heat fluxes were found to cool the main channel in the inlet, whilst sediment-sea heat fluxes acted to warm. In contrast, on the tidal flats, sediment-sea and advective heat fluxes added heat, whilst air-sea heat fluxes removed heat. In both locations, the vertical heat fluxes were found to play a greater role in controlling the local heat budget than advection, which largely balanced over a tidal cycle. The influence of solar and tidal phasing was also found to determine the relative contribution of vertical versus horizontal terms on a given day.
The influence that the presence of Waitati Inlet had on exposure time of warm water in a nearby kelp forest was then investigated. Exposure time, in the context of the current thesis, relates to time (hours) above a thermal threshold. Three short (5-day) case study periods during February 2022 were selected for this investigation. Timeseries of water temperature inside and outside the inlet, and within the kelp forest, demonstrated regular impact of warm inlet water on the neighbouring kelp forest. A similar pattern was evident in each case study with a noon peak in net air-sea heat flux leading to an afternoon peak in water temperature exiting the inlet on the ebb tide and the subsequent arrival of warm water in the kelp forest during the evening/night. Observed temperature variability within the kelp forest was then compared to temperatures predicted from a null model. The null model represents the water temperature variability in the kelp forest that can be accounted for by air-sea heat fluxes. Empirical cumulative distribution functions were subse2 quently constructed from the observed and predicted temperature timeseries to compare exposure time of the kelp forest to water temperatures above a threshold temperature of 16.25◦C. In all three case studies, the exposure time of warm water in the kelp forest was increased by ≈ 84, 73 and 19 hours due to presence of the coastal inlet, compared to the null model driven solely by air-sea heat fluxes.
Collectively, these results highlight the importance of having an improved understanding of dynamics of heating over the inner-shelf, and the ways in which inner-shelf temperatures can be amplified by the presence of coastal inlets that act as heat incubators. As warming over the inner-shelf continues, together with predicted increases in MHW conditions, negative impacts on coastal systems, including morbidity and mortality of kelp forests, are likely to be exacerbated. Future local studies should incorporate measurements from within the kelp forest to have greater confidence in the contribution of advective heat fluxes from the inlet. An improvement to the 1D inlet model could account for vertical stratification to incorporate more complex nuances of the physical system. Finally, a range of thermal threshold temperatures could be studied to assess the potential impact on a variation of kelp and benthic coastal species.