Estimation of Size-Fractionated Primary Production from Satellite Ocean Colour in UK Shelf Seas

Kieran Curran,Gavin Tilstone,Robert Brewin,Anna Hickman,Heather Bouman

doi:10.3390/rs10091389

Abstract

Satellite ocean-colour based models of size-fractionated primary production (PP) have been developed for the oceans on a global level. Uncertainties exist as to whether these models are accurate for temperate Shelf seas. In this paper, an existing ocean-colour based PP model is tuned using a large in situ database of size-fractionated measurements from the Celtic Sea and Western English Channel of chlorophyll-a (Chl a) and the photosynthetic parameters, the maximum photosynthetic rate ( P m B ) and light limited slope ( α B ). Estimates of size fractionated PP over an annual cycle in the UK shelf seas are compared with the original model that was parameterised using in situ data from the open ocean and a climatology of in situ PP from 2009 to 2015. The Shelf Sea model captured the seasonal patterns in size-fractionated PP for micro- and picophytoplankton, and generally performed better than the original open ocean model, except for nanophytoplankton PP which was over-estimated. The overestimation in PP is in part due to errors in the parameterisation of the biomass profile during summer, stratified conditions. Compared to the climatology of in situ data, the shelf sea model performed better when phytoplankton biomass was high, but overestimated PP at low Chl a.

Highlights

Photosynthesis by phytoplankton is the primary source of organic carbon to pelagic ecosystems, which is determined and modified by the intensity and spectral quality of light within the water column and the efficiency with which it is absorbed by phytoplankton
The model assumes that as total Chl a increases, the community size structure changes from picophytoplankton dominated in low-chlorophyll regions such as the gyres to microphytoplankton dominated in high-chlorophyll regions such as temperate shelf seas or upwelling zones
Nano and picophytoplankton reach asymptotic Chl a values Cnm, Cmp, such that Cp does not increase above 0.13 mg m−3, and Cn does not increase above 0.82 mg m−3, beyond which additional Chl a is apportioned to microphytoplankton

Summary

Introduction

Photosynthesis by phytoplankton is the primary source of organic carbon to pelagic ecosystems, which is determined and modified by the intensity and spectral quality of light within the water column and the efficiency with which it is absorbed by phytoplankton. Photosynthesis by phytoplankton is the principal source of organic carbon for pelagic ecosystems in shelf seas [6], supporting secondary production by heterotrophic protozoan and metazoan grazers and regenerative production by a range of heterotrophic bacteria [7]. Ecosystem and biogeochemical models often include phytoplankton size classes and functional types and primary production to estimate group- or size-specific carbon and nutrients cycling rates [22,23,24,25,26] to evaluate the impact of phytoplankton size classes on elemental stoichiometry and grazing rates in the ecosystem [27,28,29,30]

Methods

Results

Discussion

Conclusion