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

Abstract

A chlorophyll a hindcast in the Madeira Basin from 1871 to 2008 was used to analyze the long‐term variability in the oligotrophic, subtropical gyres in relation to the climate change of the last century. The deep chlorophyll maximum (DCM), as dominant pattern of the chlorophyll a field, showed a fast decrease in its strength in the 1940s. An absolute minimum was reached between 1967 and 1973 when no DCM established with a recovering to the end of the time series. Long‐term variability of the DCM was related to the North Atlantic Oscillation with a time delay of 9 years. The marked decrease in the 1940s was correlated to the drop of the solar radiation in transition from early brightening to global dimming. Caused by the influence of the solar radiation and maybe related to increasing global temperatures in the last century, the integrated chlorophyll a concentration decreased by about 0.7 mg m−2 in 2008 compared to 1871. The high‐resolved chlorophyll a hindcast allowed an estimation of the carbon uptake by the ocean due to primary production in the euphotic zone. A rough calculation over the area of the global subtropical oceans showed 700 megaton less carbon uptake in 2008.

Sclerochronological Study of a Glycymeris vangentsumi Population From the Madeira Islands

“…Probably an even smaller part of the primary production will reach the G. vangentsumi specimens' habitat at 90-160 m depth, a similar case was presented by Witbaard et al (2003). Formation of the DCM in late-spring may also provide food for the shells (Figure 7), however, its formation is not constant in time as it depends on whether the lower boundary of the photic zone reaches the nutricline, or not (Fründt et al, 2015). The significant positive correlation found between March Chl-a surface concentration and shell growth ( Figure 5C) indicates that shell growth in fact depends on food availability.…”

“…In July, surface waters warm up in the region by the mixed layer moving up to a shallower position (from 150 to ∼30 m). The increased stratification is accompanied with a decrease in nutrient concentration and DCM is formed in the lower part of the photic zone (90-130 m, Figure 7), where nutrients are still available (Fründt et al, 2015). Consecutively, the strength of the correlation observed between monthly SSTs and the G. vangentsumi chronology decreases from March to July (Figure 5A).…”

“…Environmental conditions at these depths are not monitored as regularly as at the surface, though in the last decade more data have been collected from cruises and at ocean moorings (Weller et al, 2004;Teira et al, 2005;Waniek et al, 2005). This has enabled the build-up of models of seasonal oceanographic processes (Mason et al, 2011;Fründt et al, 2015).…”

“…Phytoplankton blooms around Madeira are mainly influenced by late-winter mixing, since it enhances nutrient concentration in the euphotic zone. When the surface becomes nutrient-depleted in late-spring, phytoplankton descend to the lower part of the euphotic zone (90-130 m), where nutrients are still available, creating the deep chlorophyll maximum (DCM) (Teira et al, 2005), which usually persists through late summer and early autumn (Fründt et al, 2015), presumably still providing food for the mollusks living at that depth.…”

“…In fact, averaged increment width at the shallower site, where food availability is higher (Figure 7), remains higher during the first 8 years than in any other group ( Figure 3A). Shells from collection site 88 (90 m depth) were not only living closer to the surface phytoplankton bloom, but when surface nutrient limitation forced phytoplankton species to descend to the lower part of the photic zone (90-130 m) they were situated in the depth range of DCM (Fründt et al, 2015) while food availability at the deeper sites must be even more highly seasonal (Figure 7). According with this implied vertical trend, from the Mediterranean, similarly high juvenile growth rates of G. pilosa shells from 1.5 to 3 m depth were observed (Peharda et al, 2016) to the G. vangentsumi shells collected at 90 m depth.…”