Jane E. Robertson

A modelling investigation of the role of phytoplankton in the balance of carbon at the surface of the North Atlantic

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.

Thermal skin effect and the air‐sea flux of carbon dioxide: A seasonal high‐resolution estimate

Understanding the role the oceans play in sequestering anthropogenic CO2 is crucial to understanding global climate change. Correct parameterization of air-sea flux of CO2 is an important challenge to modelers. Recently it has been demonstrated that the thin thermal layer at the surface of the ocean can lead to an underestimate of CO2 solubility (Robertson and Watson, 1992). We re-evaluate the effect of the cool thermal skin and present a high-resolution seasonal estimate of its effect on the air-sea flux of CO2. We compare air-sea flux estimates derived using both a mean wind field and a more realistic Rayleigh distribution of the wind field. Using the mean monthly wind stress and a linear relationship between wind speed and the gas exchange coefficient of CO2 (Tans et al., 1990), we estimate that excluding the southern ocean, the surface skin correction increases the air-sea flux of carbon by 0.48 Gt yr-1. This is 25% lower than the correction suggested by Robertson and Watson (1992) and the difference is attributed to the better temporal and spatial resolution of the present data set. When a more realistic representation of the temporally varying winds is used, the corrected carbon flux decreases to 0.36 Gt yr−1. Conservatively, adding a 10% contribution from the southern ocean, we estimate a mean global increase in CO2 flux due to the skin effect of 0.39 Gt C yr−1. This is 40% lower than the previous estimate of Robertson and Watson (1992). Finally, adopting the gas transfer parameterization of Liss and Merlivat (1984), we estimate a CO2 flux anomaly of only 0.17 Gt C yr−1 which is approximately 50% lower than the analogous estimate using the Tans et al. (1990) formulation and a full 75% lower than the estimate of Robertson and Watson (1992). These results suggest that both a proper representation of the wind speed/flux correlation and a realistic distribution of the wind field is essential in making large-scale flux estimates. We also examine the seasonal variation of the thermal skin effect. The largest negative temperature gradients (-0.75°C) are found during the northern hemisphere winter in the regions of the Kuroshio and the Gulf Stream Currents, whereas the central North Pacific has a small positive temperature gradient during the summer months.

A summer-time sink for atmospheric carbon dioxide in the Southern Ocean between 88°W and 80°E

Abstract Measurements of surface water dissolved carbon dioxide are presented from two cruises in the Southern Ocean from 88°W (Bellingshausen Sea) to 80°E (Princess Elizabeth Trough) with a number of observations close to the ice edge. The data, collected from early to late Austral summer 1992/1993, indicate that the Southern Ocean in these regions was acting as a sink for atmospheric carbon dioxide at this time. No correlation between carbon dioxide levels and the standing stock of phytoplankton or sea-surface temperature was observed, except in isolated regions of high chlorophyll concentration and across small source regions associated with fronts, respectively. Assuming that the observations can be generalised to the region encircled by the cruise tracks (approximately 20% of the Southern Ocean south of 50°S by area), we calculate C0 2 uptake for this region during the four months that the cruises took place. Using transfer velocities based on observed winds, we find an uptake of 0.07 Gt C using the Liss-Merlivat ( The role of air-sea exchange in geochemical cycling ; Reidal Publ., The Netherlands, 1986) parameterisation of transfer velocity, or 0.10 Gt C using that due to Wanninkhof ( Journal of Geophysical Research , 97 , 7373–7382, 1992) or Tans et al. ( Science , 247 , 1431–1438, 1990), including an additional flux from the skin effect. No winter-time data are available to assess the sign of the flux annually (sink or source); however, the size of the sink observed during the summer suggests that, if representative of the whole of the Southern Ocean, there is a drawdown of between 0.35 and 0.50 Gt C over 4 months. The observation of a sink is consistent with the most recent estimates of the regional budgets derived from atmospheric isotope data. Most data sets from earlier years show the region as neutral with respect to atmospheric CO 2 , so it appears possible that the widespread sink we observed, at the very least between 88°W and 80°E, is a recent phenomenon not present in some previous surveys.

Diurnal variation in surface pCO2 and O2 at 60°N, 20°W in the North Atlantic

Abstract A method for rapid determination of the partial pressure of carbon dioxide at the sea-surface is described. The method was employed along with a pulsed oxygen electrode to monitor daily changes in surface pCO 2 and O 2 close to a drifting buoy deployed at approximately 59°N, 20°W. During a 4 day period a gradual rise in oxygen saturation and corresponding fall in pCO 2 was observed in the surface layer. Corrections are made for gas exchange of O 2 using wind speed data, the correction being an important fraction of the supersaturation observed in the water. Estimates of net community production and photosynthetic quotients are derived, giving a range of PQs from 0.9 to 1.5. Though variations in the local hydrography reduce the accuracy of these estimates, the potential of this approach to estimate productivity appears promising.

The influence of the spring phytoplankton bloom on carbon dioxide and oxygen concentrations in the surface waters of the northeast Atlantic during 1989

Abstract By representing growth, decay and vertical mixing during the spring phytoplankton bloom as a series of rate constants, a simple model is constructed that predicts phytoplankton abundance, carbon dioxide concentration and oxygen saturation as continuous functions of latitude and time. The predictions are compared with surface distributions of the partial pressure of carbon dioxide (pCO2), total dissolved inorganic carbon (TCO2), chlorophyll and oxygen saturation mapped during May 1989 on a series of surveys between 47 and 60°N in the eastern North Atlantic near 20°W. Altoough the observations were strongly variable on spatial scales of less than 100 km, the systematic changes revealed in the transects are quantitatively described by the theoretical expressions. Total vertical fluxes of carbon can be calculated from the model, and these can be integrated temporally and spatially. During the course of the spring bloom approximately 5 g m−2 of carbon entered the ocean surface from the atmosphere in the northeast Atlantic and the potential net loss of carbon to the deep ocean was about 16 g m−2.

Surface pH and pC02 distributions in the Bellingshausen Sea, Southern Ocean, during the early Austral summer

Abstract Measurements of the carbon dioxide system (pH, p C0 2 ) from the Bellingshausen Sea, Southern Ocean are discussed in relation to varying biological activity and hydrography. pH was determined using on-line indicator spectrophotometry and varied between 7.65 and 7.85 (at 25°C) and between 8.04 and 8.25 at in situ temperatures. Except for the Bransfield Strait, p CO 2 was below atmospheric p CO 2 for the duration of the cruise, with the average level being 295 ± 32 μatm (1 SE). Large mesoscale variability with no single control on the surface seawater carbon dioxide system was observed, although the majority of the study area was predominantly influenced by the hydrography of the circumpolar waters. The most significant exception being a region with high chlorophyll concentration south of the Southern Polar Front.

Variations in the isotopic composition of particulate organic carbon in surface waters along an 88°W transect from 67°S to 54°S

Abstract The concentration and isotopic composition of carbon was measured in suspended particulate organic matter in surface waters of the Southern Pacific Ocean along an 88°°W transect from 67−54°S during the Austral summer. Concurrent measurements of total dissolved inorganic carbon (ΣCO2), pCO2, particulate organic carbon (POC), salinity, chlorophyll and temperature provide an opportunity to study the covariance of these parameters with changes in the isotopic composition of particulate organic carbon (δ13C-POC). The south-north transect did not show any significant changes in POC (10–19 mmol m−3) or chlorophyll (0.5–1 mg m−3), while δ13C-POC variations of approximately 4%o were recorded. Although surface water CO2(aq) is substantially out of equilibrium with the local atmosphere, the isotopic changes are negatively correlated with the concentration of dissolved carbon dioxide along the transect, and follow the general trends predicted from previously determined relationships. Estimates of ep (e#p) are less well constrained and show a weaker correlation with [CO2 (aq)]. Where e#p varies independently of [C02(aq)] small changes in the concentration gradient between extra- and intra-cellular CO2 can be predicted. The difficulty in obtaining independent measures of growth rate and matching these measurements to the timescales over which the 613C of the phytoplankton is integrated is examined.

Air-Sea Exchange of CO2 and Its Relation to Primary Production

The natural processes which determine how much CO2 is in the atmosphere have never been more intensely and urgently studied than at present. The urgency stems from our realization that we are changing the global climate fundamentally and rapidly by the addition of radiatively active “greenhouse” gases to the atmosphere, CO2 foremost among them. The rate of rise of CO2 concentrations is considerably mitigated, however, by the existence of a net sink for atmospheric CO2 which is about half as large as the total anthropogenic input, (fossil fuel burning and deforestation). Figure 1, based on data in the IPCC scientific assessment (IPCC, 1990) gives estimated net sources and sinks of atmospheric CO2. All of the fluxes shown are, it should be stressed, net. In particular, the ocean uptake of about 2 Gt C yr−1 is the difference between two nearly equal fluxes of order 100 Gt C yr−1 into and out of the ocean.