Turbulent supply of nutrients to phytoplankton at the New England shelf break front

Burke Hales,John Marra,Dave Hebert

doi:10.1029/2008jc005011

Abstract

We present observations from deployments of a microstructure turbulence instrument (the Towed Microstructure and Auxiliary Sensor Instrument) aboard a pumping profiling vehicle (the Lamont Pumping SeaSoar) towed behind a research vessel at the New England shelf break front in August 2002. From these we determined coincident fine‐scale vertical eddy diffusivity and gradients of nitrate, phosphate, and silicate on several transects spanning the front. We then quantified vertical turbulent nutrient fluxes through the base of the euphotic zone (defined as the 1% light level), the base of the density transition zone, maximum nutrient gradients (the nutriclines), and the depth of maximum stratification (the pycnocline). Vertical eddy diffusivity estimates spanned a wide range from near‐molecular levels at the pycnocline to values exceeding 10−3 m2 s−1 at depth and in the surface layers. Vertical nutrient fluxes were maximal at the 1% light level and decreased by 2 orders of magnitude as they moved upward through the water column to the depth of the pycnocline. Nutrient fluxes were enhanced shoreward of the front because of high mixing rates and nutrient gradients at the depth of the 1% light level. Nitrate fluxes there averaged about 6 × 10−5 mmol N m−2 s−1, sufficient to support a net community productivity of 30 mmol C m−2 d−1. Seaward of the front, these fluxes averaged about 1 × 10−5 mmol N m−2 s−1 and would support correspondingly lower productivity. A small part of the upward flux appeared to support a silicifying community of phytoplankton that consumed phosphate in proportion to nitrate at about double the canonical Redfield stoichiometry.

Highlights

Introduction andBackground[2] The New England shelf break front separates cold, fresh, low-clarity shelf water from warm, salty, clear slope water
[1] We present observations from deployments of a microstructure turbulence instrument aboard a pumping profiling vehicle towed behind a research vessel at the New England shelf break front in August 2002
It has long been recognized as a region of elevated biomass [Malone et al, 1983; Houghton and Marra, 1983; Marra et al, 1990]. This elevated biomass has been linked to physical concentration mechanisms [e.g., Ryan et al, 1999a] and to enhanced growth rates [Marra et al, 1990]

Summary

Introduction and Background

[2] The New England shelf break front separates cold, fresh, low-clarity shelf water from warm, salty, clear slope water. In order to maintain the spatial resolution and structure of the frontal region, we produced and present the composite of the three front crossings, after the individual sections were shifted to account for the small variability in the position of the front This results in some cross-track averaging of the properties presented but an increase in the along-track data density, as multiple surveys are collapsed into one composite. Because this estimation of turbulent mixing is unreliable for very small vertical temperature gradients, we limited calculation to situations where absolute vertical temperature gradients were >0.01°C mÀ1 This corresponds to about 5 – 10 times the calculation uncertainty in the vertical temperature gradient, given the resolution of the temperature sensors and the smoothing associated with the gridding and differentiation procedures.

Results

Discussion

Findings

Conclusions