Methods forEstimating New Primary Production

In Upwelling Systems

James J. Bisagni

University of Massachusetts, DartmouthPhysics Department &

School for Marine Science & TechnologyNew Bedford, Massachusetts, USA

Talk Outline:

I. List Some Goals

II. Provide Working Definitions

III. Summary of Model Types

IV. Model Descriptions

V. Discussion

Tentative Project Goals:

I. Describe the size of New Primary Production (NPP) in the Peru/Chile upwelling system

II. Describe the mean seasonal cycle of NPP in the Peru/Chile upwelling system

III. Describe interannual variability of NPP in the Peru/Chile upwelling system

IV. Achieve goals I-III with a model-based approach, using both satellite and in-situ data

Total Primary Production

Total primary production in the ocean may be divided into “new” and “regenerated” production based on the source of the nitrogen which is utilized.(Dugdale & Goering, 1967; Eppley & Peterson, 1979).

Allocthonous nitrogen or nitrate (NO3) is input into the euphhotic zone from horizontal and/or vertical advection and diffusion.

Autocthonous nitrogen or ammonium (NH4) is input in the euphotic zone from metabolic recycling caused by biota within the water column and sediments.

Importance of New Primary Production

It is reasonable to state that in the absence of an allocthonous nitrogen supply, any given marine ecosystem will eventually become non-sustainable due to export of nitrogen through sinking of biogenic material and harvesting activities such as fishing, and predation from migratory pelagic species.(Platt et al., 1989)

Thus, it is clear from the standpoints of the global ocean’s ability to sequester atmospheric CO2, along with a local region’s ability to maintain a sustainable ecosystem, that new primary production, rather than total primary production, is the key quantity.

Measurements of New Production

Assimilation of 15N-labeled compounds allows instantaneous estimates of uptake rates of the available nitrogen sources by phytoplankton (Dugdale and Goering, 1967) and an estimate of the so-called “f-ratio” of new production to total production, where

However, estimates of regional or global new production must be done using other techniques such as using the relationship between new and total production and remotely-sensed data in order to discern how the presence of nitrogen species within the euphotic zone varies in space and time.

f–ratio=

[NO3–]

[NO3–] +[NH4

+]

(Eppley and Peterson, 1979)

New Production Models

1) Nitrate Uptake (Shift-Up) ModelsUtilize the inverse relationship between temperature and nitrate and the physiological response of phytoplankton within the euphotic zone.===> Largely “kinetics-based”

Examples include:Dugdale et al., 1989, Northwest AfricaKudela & Dugdale, 1996, CaliforniaDugdale et al., 1997, California

2) Nitrate Bulk ModelsUtilize the inverse relationship between temperature and nitrate and a nitrate budget to account for the amount of nitrate within and entering the euphotic zone.===> Largely “physics-based”

Examples include:Waldron & Probyn, 1992, BenguelaTownsend, 1998, Gulf of MaineBisagni, in-press, Gulf of Maine

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(After Dugdale et al., 1989)

Nitrate Uptake (Shift-Up) Model

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AVHRR (Only) Model (Left-Half)For Each Pixel

Satellite SSTProvides NO3

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AVHRR (Only) Model (Left-Half)For Each Pixel

Satellite SSTProvides NO3

Satellite SSTProvides Time-Base

QuickTime™ and aTIFF (Uncompressed) decompressor

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AVHRR (Only) Model (Left-Half)For Each Pixel

Satellite SSTProvides NO3

Satellite SSTProvides Time-Base

Maximum measuredspecific NO3 uptake(Vmax NO3) at time=tassuming shift-up

Shift-Up: Vmax NO3(t) = VNO3 (i) + A(t) x t

Measured acceleration of NO3 uptake, d/dt(VNO3)

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AVHRR (Only) Model (Left-Half)For Each Pixel

Satellite SSTProvides NO3

Satellite SSTProvides Time-Base

Maximum measuredspecific NO3 uptake(Vmax NO3) at time=tassuming shift-up

Predicted VNO3 attime=t assuming shift-

up & Michaelis-Menton kinetics

Shift-Up: Vmax NO3(t) = VNO3 (i) + A(t) x t

Michaelis-Menton Kinetics:

VNO3(t) = Vmax NO3(t) x [NO3]/(Ks + [NO3])

Measured half-saturation constant

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

AVHRR (Only) Model (Left-Half)For Each Pixel

Satellite SSTProvides NO3

Satellite SSTProvides Time-Base

Maximum measuredspecific NO3 uptake(Vmax NO3) at time=tassuming shift-up

Predicted VNO3 attime=t assuming shift-

up & Michaelis-Menton kinetics

Shift-Up: Vmax NO3(t) = VNO3 (i) + A(t) x t

Michaelis-Menton Kinetics:

VNO3(t) = Vmax NO3(t) x [NO3]/(Ks + [NO3])

Variation in VNO3

versus irradiance& depth of euphotic zone measurements

(or from ocean color)

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

AVHRR (Only) Model (Left-Half)For Each Pixel

Satellite SSTProvides NO3

Satellite SSTProvides Time-Base

Maximum measuredspecific NO3 uptake(Vmax NO3) at time=tassuming shift-up

Predicted VNO3 attime=t assuming shift-

up & Michaelis-Menton kinetics

Shift-Up: Vmax NO3(t) = VNO3 (i) + A(t) x t

Michaelis-Menton Kinetics:

VNO3(t) = Vmax NO3(t) x [NO3]/(Ks + [NO3])

Variation in VNO3

versus irradiance& depth of euphotic zone measurements

(or from ocean color)

Integrate VNO3 over the euphotic zone and multiply by

PONto yield NPP

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

AVHRR (Only) Model (Left-Half)For Each Pixel

Satellite SSTProvides NO3

Satellite SSTProvides Time-Base

Maximum measuredspecific NO3 uptake(Vmax NO3) at time=tassuming shift-up

Predicted VNO3 attime=t assuming shift-

up & Michaelis-Menton kinetics

Shift-Up: Vmax NO3(t) = VNO3 (i) + A(t) x t

Michaelis-Menton Kinetics:

VNO3(t) = Vmax NO3(t) x [NO3]/(Ks + [NO3])

Variation in VNO3

versus irradiance& depth of euphotic zone measurements

(or from ocean color)

Integrate VNO3 over the euphotic zone and multiply by

PONto yield NPP

Compute f-ratio

Stored NO3

NPP is estimated using a bulk quantity termed “potential new production” (PNP) as a proxy “upper limit” for NPP where PNP has been defined in a variety of ways:

For each Benguela upwelling event:

Nitrate Bulk Models

PNP= NO3

–Ze

0

dz ×Redfieldratio×n

(Waldron & Probyn, 1992)

z= 0

z= -Ze

PNP= NO3

–Ze

0

dz ×Redfieldratio

Then sum over “n” upwelling events:

In the Gulf of Maine:

NO3 FluxTPP ×Redfieldratio=PNP

TPP

(Townsend, 1998)

z= 0

z= -Ze

===> PNP = NO3 Flux x Redfield ratio

NO3 Flux=Kz

∂NO3∂z

NO3 Flux

Stored NO3

PNP=

ddt NO3

–Ze

0dz – KZ

∂NO3∂z –Ze

×Redfieldratio

z= 0

z= -Ze

===> PNP = d/dt[Stored NO3 - NO3 Flux] x Redfield ratio

NO3 Flux

(Bisagni, in press)

In the Gulf of Maine:

Model Differences

Nitrate Uptake (Shift-Up) Models

In addition to “standard” hydrographic measurements of temperature, NO3, and PON, such kinetics-based NPP models require “rate measurements” of A(t), KS, and KE, the half saturation constant for NO3 uptake as a function of irradiance (depth). Moreover, measured 15N incubations, if available would allow verification of modeled VNO3(t) values.

Models are sensitive to slope of temperature- NO3 regression and the applied constant heating rate (may not be constant!).

Nitrate Bulk Models

Utilize “standard” hydrographic measurements of temperature and NO3 but require estimates of KZ and upwelling velocity to measure the proxy PNP.

Models are sensitive to slope of temperature- NO3 regression, KZ and upwelling velocity.

Summary & Conclusions

Models exist which are able to estimate NPP

Nitrate uptake (shift-up) models are able to make pixel-by-pixel estimates of NPP, but make some strong assumptions (heating rate) and require many in-situ rate measurements.

Nitrate Bulk Models are able to make pixel-by-pixel estimates of the upper limit of NPP through estimation of PNP as a proxy for NPP, but require some physical oceanographic quantities (KZ and w).

Choosing a model depends largely on the questions being asked and available data.

Summary & Conclusions

Models exist which are able to estimate NPP

Nitrate uptake (shift-up) models are able to make pixel-by-pixel estimates of NPP, but make some strong assumptions (heating rate) and require many in-situ rate measurements.

Nitrate Bulk Models are able to make pixel-by-pixel estimates of the upper limit of NPP through estimation of PNP as a proxy for NPP, but require some physical oceanographic quantities (KZ and w).

Choosing a model depends largely on the questions being asked and available data.

What are the available data?