Temporal coupling between surface and deep ocean biogeochemical processes in contrasting subtropical and subantarctic water masses, southwest Pacific Ocean

Abstract

Surface to deep‐ocean coupling was investigated in subtropical (STW) and subantarctic (SAW) waters off eastern New Zealand. Moorings, comprising a near‐surface fluorometer, temperature loggers, current meters, and sediment trap at 1500 m depth, were deployed at 41° and 46°40′S along 178°30′E between October 2000 and October 2001. Locally validated, remotely sensed data provided areal estimates of surface chlorophyll that were representative of 1997–2004 annual cycles. In STW, early spring chlorophyll peaks were coupled with deposition of labile (molar C:N∼7–8), bio‐siliceous organic matter. Low winter chlorophyll concentrations were associated with high particulate organic carbon (POC) fluxes of moderately refractory material (C:N∼9–10). This indicates that winter flux was affected by heterotrophic recycling processes (zooplankton exuviae, fecal pellets) and/or slowly sinking particles from the preceding autumn. Deep‐ocean POC fluxes off New Zealand were similar to global estimates and Tasman Sea (0.8 cf. 1.0 g C m−2 yr−1). Elevated biogenic silica and lithogenic fluxes probably reflect processes within a warm‐core eddy near an eroding landmass, rather than STW in general. In SAW, POC and biogenic silica flux peaks occurred in spring with moderate surface chlorophyll concentrations. Decoupling between high chlorophyll and low flux in summer may reflect near‐surface organic matter recycling by the microbial‐dominated ecosystem. In spring, moderate chlorophyll levels in SAW, high POC and silica flux, and high C:N ratios (9–13) indicate some coupling with upper water column processes. SAW, east of New Zealand, was characterized by low POC (0.2 g C m−2 yr−1), high biogenic silica, and low carbonate fluxes, unlike other subantarctic sites, which are dominated by carbonate deposition with fivefold higher POC flux.