Spatial and Temporal Variability of Phytoplankton Productivity in the Subtropical Front around New Zealand
The association between enhanced phytoplankton productivity and the Subtropical Front (STF) around New Zealand was investigated using remote sensing images derived from the MODIS-Aqua sensor. The STF is a major circumpolar oceanic system that marks the boundary between subtropical and subantarctic water masses. The STF is accompanied by strong physical and nutrient gradients due to the interactions between these water masses and adjacent coastal processes that lead to elevated phytoplankton biomass. The spatial and temporal variability in the position of the STF due to changes in topographic features and water mass properties regulates the conditions required for growth of phytoplankton. This study provides a long-term analysis of spatial and temporal variability in Chlorophyll-a (Chl-a) concentration with respect to variations in the frontal location. MODIS-Aqua monthly composite images of sea surface temperature (SST) and surface Chl-a concentration from January 2003 to December 2012 were used to estimate the position of the STF and enhanced phytoplankton productivity around the STF to the south and east of New Zealand. The position of the STF mainly followed the shelf-break (between the 200 and 500 m isobaths). The position and strength of the STF changed with season, being located further inshore relative to the 500 m isobath and strengthened during summer, and located further offshore and weakened during winter. Areas of elevated productivity were consistently coincident with the frontal location, with the highest Chl-a concentrations typically occurring inshore of the front. The peak 10-year average concentration along the front was 1 mg/m3 and the peak seasonal average was 1.5 mg/m3. Phytoplankton productivity typically increased heading northward and decreased (< 0.5 mg/m3) with increasing distance from the coast. The locations of enhanced phytoplankton productivity relative to the position of the front varied seasonally. Small patches of high productivity were normally found across regions where the front intensely meandered (e.g. over the Mernoo Saddle and Chatham Rise). Phytoplankton blooms generally occurred at the STF over the Mernoo Saddle and Chatham Rise, following the spring annual cycle, with the Chl-a concentration increasing during spring and dropping during winter. The mean concentration in the blooms over the Mernoo Saddle varied between 1 mg/m3 and 5 mg/m3, being stronger during autumn. Over the Chatham Rise, mean bloom concentration varied between 1 mg/m3 and 3 mg/m3, being stronger during spring. The spring bloom cycle was very obvious inshore of the front, but weaker in the offshore water. Secondary blooms were observed outside the spring blooms during summer or autumn, and were sometimes stronger than the spring bloom within the same year.
Advisor: Vennell, Ross; Rayment, Will
Degree Name: Master of Science
Degree Discipline: Marine Science
Publisher: University of Otago
Keywords: phytoplankton productivity; Subtropical Front; New Zealand; remote sensing
Research Type: Thesis