Net community production (NCP)

• Page authors: PRIMO
• Responsible curator: Hagen Buck-Wiese

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NCP (nitrate drawdown, BGC-Argo)
Approach: seasonal nitrate drawdown from BGC-Argo float profiles
Context: in situ, autonomous
Spatial scale: regional to basin
Temporal scale: days to months
Units: µmol NO3- L-1 season-1; mmol N m-2 season-1
Community captured: bulk
Co-measurements: mixed layer depth, float trajectory, salinity; Redfield ratio for C conversion

Method Overview

BGC-Argo floats equipped with nitrate sensors (ISUS or SUNA UV spectrophotometers) profile dissolved nitrate continuously through the water column over months to years. The seasonal decline in mixed layer nitrate from its winter maximum to the summer minimum is used as a proxy for net community production (NCP): the drawdown reflects biological nitrate uptake (new production) assuming negligible physical supply of nitrate from below the mixed layer during the productive season.

NCP is calculated by integrating the nitrate deficit (relative to the winter reference) over the mixed layer depth and the productive season duration. This approach is particularly valuable in remote or seasonally ice-covered regions where ship-based sampling is limited.

Scale of measurement

Each float provides depth profiles at its ascent frequency (typically every 5–10 days), integrated over the mixed layer. Because floats drift with ocean currents, horizontal coverage is Lagrangian and varies with regional circulation. Basin-scale coverage requires arrays of floats.

Data generated

The primary output is depth-integrated NCP expressed in mmol N m^-2 season^-1, or converted to carbon using the Redfield C:N ratio (≈ 6.6). Conversion to volumetric units requires knowledge of the mixed layer depth and its temporal evolution.

Units & currency

Units are µmol NO₃^- L^-1 or mmol N m^-2. The currency is carbon (derived from nitrogen via Redfield) or nitrogen.

Sample size

In situ sensor measurement; no discrete sample collection required.

Repositories & databases

BGC-Argo float data are available through the Argo Global Data Assembly Centre (GDAC): argodatamgt.org.

Limitations

Float trajectories may cross water masses with different initial nitrate inventories, complicating the interpretation of drawdown as local biological uptake. Results are sensitive to the choice of winter reference concentration and the depth of integration. Physical processes (diapycnal nitrate supply, lateral advection, nitrification) can contribute to nitrate changes independently of biological uptake, introducing uncertainty in NCP estimates.

Example Applications & Protocols

Recent applications

• Wyatt et al. (2022) Ecosystem impacts of marine heat waves in the northeast Pacific

Common calculations/conversions

• NCP (mmol N m^-2) = ∫ (NO₃^-_winter − NO₃^-_t) dz, integrated over the mixed layer.
• NCP (mmol C m^-2) = NCP_N × 6.6 (Redfield C:N ratio).