Phytoplankton carbon biomass (Cphyto) x growth rate (µ)

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

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Growth-based NPP (C × µ)
Approach: phytoplankton C biomass × specific growth rate
Context: incubation, lab
Spatial scale: point sample
Temporal scale: 24 h
Units: µmol C L-1 d-1; (x)mol C biomass-1 d-1
Community captured: bulk, group-specific
Co-measurements: POC, chlorophyll, PAR, temperature; cell counts or biovolume (flow cytometry)

Method Overview

Net primary production is estimated as the product of phytoplankton carbon biomass (C_phyto) and the specific growth rate (µ, d^-1): NPP = C_phyto × µ. C_phyto is determined from cell counts and biovolume-to-carbon conversion relationships, from flow cytometric sorting followed by elemental (CHN) analysis, or from optical backscatter-derived relationships. Growth rates are estimated by cell cycle analysis (¹⁴C incorporation into chlorophyll pigments; Redalje & Laws 1981), or from the dilution method or other growth rate assays.

The approach can be applied to the bulk community or to individual taxonomic groups when combined with group-specific biomass and growth rate methods such as HPLC pigment-based sorting, flow cytometric sorting, or cell-cycle analysis.

Scale of measurement

As a bottle-based or cell-based measurement, this provides a point estimate in space. Integration time is typically 24 h, but can vary with the growth rate method used.

Data generated

The method yields carbon-specific growth rates (d^-1) and volumetric NPP (µmol C L^-1 d^-1). When applied to size fractions or taxonomic groups, it provides group-specific production estimates.

Units & currency

Units are µmol C L^-1 d^-1 or (x)mol C biomass^-1 d^-1. The currency is carbon.

Sample size

Typical samples are ~1 L in volume.

Limitations

Uncertainties in both biomass and growth rate estimates propagate multiplicatively into the NPP estimate. Biovolume-to-carbon conversion factors vary significantly across taxa and physiological states. Disentangling net versus specific growth rates (the latter requires knowledge of grazing losses) requires additional information. The method is sensitive to the choice of growth rate method; each has distinct assumptions.

Example Applications & Protocols

Classic examples

• Redalje & Laws (1981) A new method for estimating phytoplankton growth rates and carbon biomass
• McDuff & Chisholm (1982) The calculation of in situ growth rates of phytoplankton populations from fractions of cells undergoing mitosis
• Goericke & Welschmeyer (1993) The chlorophyll-labeling method: measuring specific rates of chlorophyll a synthesis in cultures and in the open ocean

Recent applications

• Graff et al. (2012) The measurement of phytoplankton biomass using flow-cytometric sorting and elemental analysis of carbon

Common calculations/conversions

• NPP (µmol C L^-1 d^-1) = C_phyto (µmol C L^-1) × µ (d^-1).
• Biovolume-to-carbon: C (pg) = 0.261 × V^0.860 (Menden-Deuer & Lessard 2000) for most non-diatom protists.