Reactive silicate in shelf waters near Otago Peninsula, New Zealand

Abstract

Reactive Si data from 19 monthly occupations of two transects near Otago Peninsula in 1986-87 showed low surface values (≤1 μM) during spring and summer, and high values (>4 μM) in winter. Concentrations were greatest after flooding in the Clutha River, 100 km south of the study area. In spring and early summer, concentrations were lowest inshore at the surface. At other times of the year, concentrations were highest in inshore surface waters, and usually coincided with the salinity minimum. Vertical gradients across the Southland Front, a near-horizontal thermal front outcropping near the shelf break, were calculated and compared with phosphorus gradients from the same samples. Nine of 21 Si gradients were negative (i.e. concentrations decreased with depth), whereas only 1 of 24 phosphorus gradients was negative. This implies that riverine Si flux is important in establishing Si gradients across the Southland Front. The results are a further indication of the importance of the Clutha River to the Otago shelf system.