Chlorophyll a reconstruction from in situ measurements: 1. Method description

Abstract

Understanding the development of primary production is essential for projections of the global carbon cycle in the context of climate change. A chlorophyll a hindcast that serves as a primary production indicator was obtained by fitting in situ measurements of nitrate, chlorophyll a, and temperature. The resulting fitting functions were adapted to a modeled temperature field. The method was applied to observations from the Madeira Basin, in the northeastern part of the oligotrophic North Atlantic Subtropical Gyre and yielded a chlorophyll a field from 1989 to 2008 with a monthly resolution validated with remotely measured surface chlorophyll a data by SeaWiFS. The chlorophyll a hindcast determined with our method resolved the seasonal and interannual variability in the phytoplankton biomass of the euphotic zone as well as the deep chlorophyll maximum. Moreover, it will allow estimation of carbon uptake over long time scales.

Chlorophyll a reconstruction from in situ measurements: 2. Marked carbon uptake decrease in the last century

“…Fründt et al [] showed with a shorter time series from 1980 to 2008, by applying EOF analysis, a correlation between the DCM and the NAO on a period of roughly 5 years. Application of the wavelet coherence between the time coefficient of the first EOF (Figure b) and the NAO showed that the relationship between NAO and the DCM found between 1980 and 2008 was less contributed to the long‐term variability of the DCM since this relationship had not been established before the 1980s (Figure ).…”

“…The calculation of the correlation coefficients ρ between the NAO and the time coefficient of the first EOF showed significant results at a time lag of 0 years ( ρ =0.17, p =0.041) and of 9 years ( ρ =0.21, p =0.016, Figure b) which corresponded to the phase difference as seen in the wavelet coherence. The direct correlation with NAO, i.e., no time lag, was diminished compared to the value found in Fründt et al [] when only the last 28 years were regarded, because a continuous correlation over the entire length was not found. The time lag of 9 years might be related to the transmission of NAO‐induced oceanic signals by slow Rossby waves with speeds of around 1 cm s −1 as reported by Cipollini et al [].…”

“…The study by Boyce et al [] has been intensely discussed by Mackas [], Rykaczewski and Dunne [], McQuatters‐Gollop et al [], and Boyce et al [] because the results were biased by combining different data sets of ocean transparency and in situ measured chlorophyll a . This difficulty was evaded using the method introduced in Fründt et al [] resulting in an unbiased chlorophyll a hindcast. Assuming both the solar radiation flux and the warming of the world's ocean were similarly manifested in the subtropical oceans, it conceded an estimation of the global change of carbon uptake by the decrease of primary production in the last 138 years.…”

“…The in situ measurements were taken from the Azores Frontal Region (29°N–38.5°N and 17°W–27°W) [ Fründt and Waniek , ], a part of the Madeira Basin. The method and the in situ measurements are described in detail in Fründt et al []. As required modeled temperature field, we used the output of the simple ocean data assimilation parallel ocean program (SODA POP) [ Carton et al , ; Carton and Giese , ], version 2.1.6 in its entire length from 1871 to 2008, resulting in a chlorophyll a hindcast of the same length (Figure ).…”

Spatial Variations of Subsurface Chlorophyll Maxima During Thermal Stratification in a Large, Deep Subtropical Reservoir

“…Depth and magnitude of the SCM affect the vertical distribution of zooplankton and fish (Pannard et al, 2015). Therefore, the SCM can play a key role in determining the structure and function of aquatic ecosystems (Frundt et al, 2015;Williamson et al, 1996).…”