Towards a merged satellite and in situ fluorescence ocean chlorophyll product

Abstract

Abstract. Understanding the ocean carbon cycle requires a precise assessment of phytoplankton biomass in the oceans. In terms of numbers of observations, satellite data represent the largest available data set. However, as they are limited to surface waters, they have to be merged with in situ observations. Amongst the in situ data, fluorescence profiles constitute the greatest data set available, because fluorometers have operated routinely on oceanographic cruises since the 1970s. Nevertheless, fluorescence is only a proxy of the total chlorophyll a concentration and a data calibration is required. Calibration issues are, however, sources of uncertainty, and they have prevented a systematic and wide range exploitation of the fluorescence data set. In particular, very few attempts to standardize the fluorescence databases have been made. Consequently, merged estimations with other data sources (e.g. satellite) are lacking.We propose a merging method to fill this gap. It consists firstly in adjusting the fluorescence profile to impose a zero chlorophyll a concentration at depth. Secondly, each point of the fluorescence profile is then multiplied by a correction coefficient, which forces the chlorophyll a integrated content measured on the fluorescence profile to be consistent with the concomitant ocean colour observation. The method is close to the approach proposed by Boss et al. (2008) to correct fluorescence data of a profiling float, although important differences do exist. To develop and test our approach, in situ data from three open ocean stations (BATS, HOT and DYFAMED) were used. Comparison of the socalled "satellite-corrected" fluorescence profiles with concomitant bottle-derived estimations of chlorophyll a concentration was performed to evaluate the final error (estimated at 31 %). Comparison with the Boss et al. (2008) method, using a subset of the DYFAMED data set, demonstrated that the methods have similar accuracy. The method was applied to two different data sets to demonstrate its utility. Using fluorescence profiles at BATS, we show that the integration of "satellite-corrected" fluorescence profiles in chlorophyll a climatologies could improve both the statistical relevance of chlorophyll a averages and the vertical structure of the chlorophyll a field. We also show that our method could be efficiently used to process, within near-real time, profiles obtained by a fluorometer deployed on autonomous platforms, in our case a bio-optical profiling float. The application of the proposed method should provide a first step towards the generation of a merged satellite/fluorescence chlorophyll a product, as the "satellite-corrected" profiles should then be consistent with satellite observations. Improved climatologies with more consistent satellite and in situ data are likely to enhance the performance of present biogeochemical models.

On the vertical distribution of the chlorophyll <i>a</i> concentration in the Mediterranean Sea: a basin-scale and seasonal approach

“…However, this calibration method based on the existence of simultaneous in situ samples is not always applicable. Alternative calibration methods, independent of concomitant HPLC observations, have therefore recently been developed (Boss et al, 2008;Xing et al, 2011;Lavigne et al, 2012). They are based on additional information such as irradiance profiles , ocean color observations (Boss et al, 2008;Lavigne et al, 2012) or the shape of the fluorescence profile .…”

“…Alternative calibration methods, independent of concomitant HPLC observations, have therefore recently been developed (Boss et al, 2008;Xing et al, 2011;Lavigne et al, 2012). They are based on additional information such as irradiance profiles , ocean color observations (Boss et al, 2008;Lavigne et al, 2012) or the shape of the fluorescence profile . Although these new calibration methods do not reach the accuracy of HPLC-based calibration, they offer an acceptable alternative to extract reliable estimates of [Chl a] vertical profiles from a large quantity of fluorescence profiles.…”

Temporal variability of chlorophyll distribution in the Gulf of Mexico: bio-optical data from profiling floats

“…For this specific purpose, FLUO profiles were smoothed, using a 5-point moving median filter, and the MLD was calculated from density profiles, using a 0.03 kg m −3 density criterion (de Boyer Montegut et al, 2004). The relevance of this NPQ correction in oligotrophic areas was specifically addressed in Lavigne et al (2012), which showed it has a positive and significant impact on the estimates of chlorophyll.…”

“…Although this method was not directly validated in the GOM, a comparison between satellite-calibrated profiles and in situ HPLC [CHL] data was performed by Lavigne et al (2012) at the station BATS (32 • N and 64 • W, in the Sargasso Sea). It was shown that calibrated profiles were unbiased with an associated median error of 29 %, which is reduced to 16 % when climatological averages are compared.…”

“…This was performed following the method of Lavigne et al (2012) and using ocean color satellite measurements (8-day composite images at 4 km spatial resolution from the Aqua MODIS satellite ocean color sensor, OCx Algorithm, available on https://oceancolor.gsfc.nasa.gov/cgi/l3). Float and satellite data were matched-up considering 8-day time intervals and ±0.25 • spatial windows centered on the geographical position of the float profile.…”