A central question addressed by the VERtical Transport In the Global Ocean (VERTIGO) study was ‘What controls the efficiency of particle export between the surface and subsurface ocean’? Here, we present data from sites at ALOHA (N Central Pacific Gyre) and K2 (NW subarctic Pacific) on phytoplankton processes, and relate them via a simple planktonic foodweb model, to subsurface particle export (150–500 m). Three key factors enable quantification of the surface–subsurface coupling: a sampling design to overcome the temporal lag and spatial displacement between surface and subsurface processes; data on the size partitioning of net primary production (NPP) and subsequent transformations prior to export; estimates of the ratio of algal- to faecal-mediated vertical export flux. At ALOHA, phytoplankton were characterized by low stocks, NPP, F_v/F_m (N-limited), and were dominated by picoplankton. The HNLC waters at K2 were characterized by both two-fold changes in NPP and floristic shifts (high to low proportion of diatoms) between deployment 1 and 2. Prediction of export exiting the euphotic zone was based on size partitioning of NPP, a copepod-dominated foodweb and a ratio of 0.2 (ALOHA) and 0.1 (K2) for algal:faecal particle flux. Predicted export was 20–22 mg POC m⁻² d⁻¹ at ALOHA (i.e. 10–11% NPP (0–125 m); 1.1–1.2×export flux at 150 m (E₁₅₀). At K2, export was 111 mg C m⁻² d⁻¹ (21% NPP (0–50 m); 1.8×E₁₅₀) and 33 mg POC m⁻² d⁻¹ (11% NPP, 0–55 m); 1.4×E₁₅₀) for deployments 1 and 2, respectively. This decrease in predicted export at K2 matches the observed trend for E₁₅₀. Also, the low attenuation of export flux from 60 to 150 m is consistent with that between 150 and 500 m. This strong surface–subsurface coupling suggests that phytoplankton productivity and floristics play a key role at K2 in setting export flux, and moreover that pelagic particle transformations by grazers strongly influence to what extent sinking particles are further broken down in the underlying waters of the Twilight Zone.

Item Type:	Refereed Article
Keywords:	primary production, subsurface export flux, particle transformations
Research Division:	Earth Sciences
Research Group:	Oceanography
Research Field:	Chemical oceanography
Objective Division:	Environmental Management
Objective Group:	Marine systems and management
Objective Field:	Measurement and assessment of marine water quality and condition
UTAS Author:	Boyd, PW (Professor Philip Boyd)
ID Code:	95542
Year Published:	2008
Web of Science® Times Cited:	30
Deposited By:	IMAS Research and Education Centre
Deposited On:	2014-10-03
Last Modified:	2014-11-11
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