Seasonal phytoplankton blooms are an important event in the shelf seas that provides food to higher trophic level marine species and impacts global biogeochemical cycles. The timing and magnitude of seasonal blooms are strongly influenced by light availability, which may be affected by future climate conditions associated with increased sediment re-suspension in shelf seas. On the inner shelf of southeast New Zealand, there are conflicting descriptions of seasonal blooms occurring in either spring or summer, which leaves an opportunity to resolve the seasonal nature of blooms in this area. Additionally, variability in light attenuation and the subsequent drivers are yet to be explored on the inner shelf, which could provide important information regarding controls on seasonal phytoplankton blooms in this area. Through a ten-year synthesis of CTD data (2014-2023), the present thesis characterises the seasonal cycle of stratification, Chl-a, nitrate, and light attenuation on the Otago inner shelf, and uses hindcast meteorological and oceanic data to identify drivers of light attenuation. In addition, a 1-D shelf sea numerical model (S2P3), validated by CTD data, was set up to simulate conditions on the inner shelf and was used to investigate the role of light attenuation in driving Chl-a variability in this area.

Compiling ten years of CTD data reveals a consistent spring and autumn phytoplankton bloom on the Otago inner shelf. The spring bloom in September (2.52 mg m-3) occurs in coincidence with the seasonal drawdown of nitrate, and the autumn bloom in March (1.96 mg m-3) and April (1.78 mg m-3) occurs in coincidence with the seasonal renewal of nitrate. A subsurface Chlorophyll-a maxima is typically present on the inner shelf and is particularly defined during the spring bloom, which surface data alone is unable to resolve. Thermal stratification on the inner shelf is high during December and January (ΔT > 1.2°C) and is low from autumn to early spring (ΔT < 0.32°C), while haline stratification is low during summer (> -0.12 psu) and is elevated from late autumn to early spring (< -0.13 psu). The joint contribution of both temperature and salinity is important in driving the seasonal cycle of density stratification on the Otago inner shelf, which appears to be fundamentally important in governing the timing of seasonal blooms in this system. Light attenuation (KdPAR) on the inner shelf is high from late autumn until early spring (> 0.2 m−1) and is low during summer (∼ 0.1 m−1). Using a linear model of KdPAR estimates alongside ancillary oceanic and atmospheric hindcast data, wave orbital velocities were identified as the primary driver of light attenuation on the Otago inner shelf.

Using a 1-D physics and NPZD model (S2P3), the seasonal cycle of stratification, Chl-a, and nitrate was simulated for the Otago inner shelf. Ten years of in situ data from Chapter 2 was used to evaluate an initial model run, which showed that the model reproduces some seasonal aspects of observations of the inner shelf, but did a relatively poor job in reproducing the seasonal cycle of Chl-a and nitrate. This was driven by the inability of the S2P3v8.0 model to account for the stratifying effect that salinity has on the inner shelf and a lack of zooplankton observations that are required to constrain the NPZ module within S2P3v8.0. To account for this, an empirical 100 Wm−2 was used to amplify the shortwave radiation within the model to account for missing salinity stratification from vertical salinity differences. Additionally, the S2P3v8.0 model was downgraded to S2P3v7.0 which includes a simpler treatment of the zooplankton community. This developed model (S2P3v7.0-D) better reproduced in situ observations of the spring and autumn Chl-a bloom and nitrate drawdown/replenishment on the inner shelf. The S2P3v7.0-D model was subsequently used to investigate the role of light attenuation in driving Chl-a variability on the inner shelf by simulating the seasonal cycle of phytoplankton biomass (Chl-a) under a range of light attenuation scenarios. An increase in light attenuation was found to delay the spring bloom by 64 days for an extreme shift in the optical clarity of the water column, from a relatively clear (0.2 m−1) to highly turbid (0.5 m−1) shelf sea. However, at KdPAR values typical of the Otago Shelf, blooms are not limited to a single summer event, as initially hypothesised. Instead, they occur under favourable stratification conditions during spring and autumn. Collectively, these results demonstrate that high light attenuation does not restrict phytoplankton to a summer bloom in this area, as evidenced by a spring and autumn bloom. The joint contribution of both temperature and salinity are important drivers of stratification in this system, which appears to be fundamentally important in governing the timing of the spring and autumn bloom. Future research would benefit from deploying moored instruments and/or an autonomous wire walker to collect higher frequency temperature, salinity, and Chl-a data on the inner shelf. In addition, a numerical model that can account for two-dimensional horizontal processes, the influence of freshwater on stratification, and multiple phytoplankton species/nutrients would be advantageous in strengthening our understanding of this system.