Episodic Summer Chlorophyll-a Blooms Driven by Along-Front Winds at Aotearoa's Southeast Shelf Break Front

Abstract

This study investigates the influence of along-front wind forcing on chlorophyll-a (Chl-a) at the Otago Shelf Break (OSB) in southeast Aotearoa/New Zealand using remotely-sensed and in situ data. Summer wind stress over the OSB was shelf-aligned, oscillating between upfront and downfront. Surface Chl-a concentrations along the OSB were shown to increase episodically (≤10 days duration) following upfront wind stress. This response occurred over most of the 350 km long shelf break, and was most intense north of the Otago Peninsula. Peak Chl-a enhancement at the shelf break occurred following periods of upfront winds, exhibiting a lagged response of approximately 5 days. Moored thermistor data indicated that upfront wind events were followed by increased thermal stratification over the mid-shelf, whereas downfront wind events were followed by a well-mixed water column. In situ temperature, salinity, and nutrient measurements suggested an offshore movement of the surface expression of the front following upfront winds, and a reduction in nitrate over the shelf break. From these observations a model of Ekman restratification driven by upfront winds is proposed for this system, wherein off-shelf Ekman transport converts strong horizontal isopycnal gradients at the shelf break front into vertical stratification. This stratification holds phytoplankton in the upper water column, allowing increased access to light, which is marked by the increased drawdown of nitrate at the shelf break. Downfront winds break down this stratification, move the shelf break front onshore, and mix phytoplankton through the water column, reducing the surface expression of chlorophyll.