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

Abstract

[1] 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 0 S along 178°30 0 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 À2 yr À1). 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 À2 yr À1 ), high biogenic silica, and low carbonate fluxes, unlike other subantarctic sites, which are dominated by carbonate deposition with fivefold higher POC flux.

Climate-mediated changes to mixed-layer properties in the Southern Ocean: assessing the phytoplankton response

“…Such a lateral silicic acid gradient is much greater than the relative change in nutrient concentrations due to climate change (see PO 4 , Table 1a). Nelson et al (2001) observed no clear trend in affinity for silicic acid by diatoms (i.e. K s ) with latitude or silicic acid concentration, however there was an apparent seasonal progression -with a lower affinity for silicic acid evident over time.…”

“…b altered N:P ratios for P. antarctica relative to diatoms (more efficient N uptake per unit P (Arrigo et al, 1998). c Nelson et al (2001) study of silica uptake kinetics along a meridional gradient in silicic acid ranging from <5 in the north to 45 µM in the South. d Sedwick et al (2007), effects of iron and light on the growth of natural populations of colonial Phaeocystis antarctica, Ross Sea.…”

“…However, natural gradients in surface nutrient concentrations (<5 to 45 µmol Si L −1 ) such as during a southwards progressing bloom along the 170 • W meridian (i.e. Pacific sector; Nelson et al, 2001) provides estimates of how the silicic acid uptake kinetics of diatoms are altered by changing upper ocean conditions. Such a lateral silicic acid gradient is much greater than the relative change in nutrient concentrations due to climate change (see PO 4 , Table 1a).…”

A hydrodynamic model of Chatham Rise, New Zealand

“…Because of the scientific interest in the STF and Chatham Rise, the National Institute of Water and atmospheric Research (NiWa) has mounted a multidisciplinary research programme focusing on this area for several years, involving repeated meridional transects at c. 178.5°e, sediment trap measurements on the flanks of the Rise, multi-year deployments of moored biophysical instruments in the Hikurangi and Bounty troughs, and detailed surveys of the frontal region Boyd et al 1999;Bradford-Grieve et al 1999;Chiswell 2001;Nodder & Northcote 2001;sutton 2001;Chiswell 2002a;Nodder et al 2005).…”

New evidence links changing shelf phytoplankton communities to boundary currents in southeast Tasmania

“…A recent study focussing on the difference between marine subantarctic and subtropical seasonal production regimes presents a possible explanation. Nodder et al (2005) found that Southwest Pacific subantarctic waters south of the subtropical convergence at 46°40 0 S supported sustained production during summer exceeding that produced during spring and autumn bloom events. This contrasted with production measured from a site north of the subtropical convergence at 41°S, where chlorophyll a values were consistent with a classical temperate cycle of primary production, in which spring and autumn bloom events represented peak production.…”