A modelling investigation of the role of phytoplankton in the balance of carbon at the surface of the North Atlantic
Authors: Taylor A. H. , Watson A. J. , Ainsworth M. , Robertson J. E. , Turner D. R. 
Published: April 2008

A simple model is used to examine the way phytoplankton growth influences carbon dioxide concentration in the surface waters of the ocean. The model consists of a one‐dimensional system of two layers, a mixed layer and a thermocline, in which there is a single population of phytoplankton dependent on a single nutrient (represented by nitrate). Beneath the thermocline is a deep layer containing constant nutrient concentration, TCO2, and alkalinity, but almost no phytoplankton. The mixed layer rises and falls seasonally, and the phytoplankton population increases or declines in response to the annual cycle of light and temperature; a diurnal cycle of irradiance is also included. The model calculates the changes in the total inorganic carbon content and the alkalinity resulting from phytoplankton growth and death and hence determines the annual cycle of pCO2. Although simple, the model reproduces the major features of the annual distributions of phytoplankton and pCO2 in the North Atlantic. A spring bloom occurs at nonequatorial latitudes, the bloom appearing progressively later as the site is moved northward. South of 60° a deep‐chlorophyll maximum is present during the summer. At the equator it extends throughout the year and is accompanied by nutrient depletion within the pycnocline. Predicted annual rates of primary production are in agreement with those measured at ocean weather station India (59°N) and at Bermuda (32°N). The model results show the sharp drawdown of CO2 that is associated with the spring bloom. Phytoplankton have the greatest influence on pCO2 at the highest latitudes, the annual cycle of pCO2 at 35°N being dominated by the seasonal changes in temperature. The levels of pCO2 predicted are similar to Geochemical Ocean Sections Study values. The model reproduces the diurnal cycle observed during the U.K. Biogeochemical Ocean Flux Study (BOFS) Lagrangian experiment in 1989. Comparisons of temporal series of chlorophyll‐a and pCO2 obtained from the model with values observed during areal surveys carried out at 47° and 60°N during the BOFS cruise indicate that some of the spatial variability and the differences between latitudes may result from differences in timing of up to 10 days. The model reveals the seasonal cycles of the vertical fluxes of carbon, especially the pronounced effect of the spring bloom. The downward flux of particulate carbon exceeds the upward flux of dissolved inorganic carbon at 60° and 47°N but not at the equator. When a northward transport of water was included in the model, the spring bloom was brought forward because of seeding from the south and was terminated sooner when the water arriving became depleted in nutrients. The most significant feature of the presence of advection was the increased upward flux of dissolved inorganic carbon caused by the sharper vertical gradient of TCO2. Experiments were performed in which the summer phytoplankton were replaced by coccolithophores. The partial pressure of dissolved CO2 was increased, reducing the influx from the atmosphere. Therefore the extra particulate carbon sinking out of the surface waters as coccoliths was supplied by an increased upward flux of dissolved inorganic carbon from the deep ocean.