Episodic particulate fluxes at southern temperate mid-latitudes (42–45°S) in the Subtropical Front region, east of New Zealand

Abstract

Sediment traps were deployed for almost 1 yr at two sites near 178°40′E in 1996–1997 on Chatham Rise (New Zealand). These sites were either side of the Subtropical Front (STF), which is a biologically productive zone, characterised by moderate atmospheric CO 2 uptake. At each site, PARFLUX sediment traps (Mk 7G–21) were deployed at 300 and 1000 m in 1500 m water depth. At 42°42′S, north of the STF, approximately 80% of the integrated total mass, POC and biogenic silica flux at 300 m occurred in a 7-day pulse in austral mid-spring (1064, 141 and 6 mg m −2 d −1, respectively, in early October). This pulse was recorded a week later in the 1000 m trap, indicating a particle sinking rate of 100 m d −1. In contrast, at 44°37′S, south of the STF, the main flux of total mass and biogenic silica occurred 3 weeks later in late spring (289 and 3 mg m −2 d −1, respectively, in early November). Organic carbon, nitrogen and phosphorus fluxes were persistently high over spring at the southern site, although total POC flux integrated over 3 months was only 60 mg m −2 d −1. Thus, up to 2–3 times more material was exported north of the STF, compared with fluxes measured <200 km away to the south. As an integrated proportion of the annual total mass flux, however, more organic carbon was exported south of the STF (17% cf. 5–14%). Furthermore, organic material exported in spring from southern waters was labile and protein-rich (C : N — 8–16, C : P — 200–450, N : P — 13–36), compared to the more refractory, diatom-dominated material sinking out north of the STF in spring (C : N 9–22, C : P 50–230, N : P 5–19). These observations are consistent with anomalously high benthic biomass and diversity observed on south Chatham Rise. Resuspension and differential particle settling are probable causes for depth increases in particulate flux. Estimated particle source areas may be up to 120 km away due to high levels of mesoscale activity and mean flow in the STF region.