Abstract
Eastern Boundary Upwelling Ecosystems (EBUEs) are associated with high biological productivity, high fish catch and they highly contribute to marine carbon sequestration. Whether coastal upwelling has intensified or weakened under climate change in the past decades is controversially discussed and different approaches (e.g. time-series of chlorophyll, wind, sea surface temperature, modelling experiments) have been considered. We present a record of almost two decades of particle fluxes (1991-2009) from ca. 600 to 3100 m water depth in the Canary Basin at site ESTOC (European Station for Time series in the Ocean Canary Islands; ca. 29°N, 15°30.W, ca. 3600 m water depth), located in the offshore transition zone of the northern Canary Current-EBUE. We compare these flux records with those measured at a mesotrophic sediment trap site further south off Cape Blanc (Mauretania, ca. 21°N). The deep ocean fluxes at ESTOC in ca. 3 km recorded the evolution of the coastal Cape Ghir filament (30-32°N, 10-12°W) due to lateral advection of particles, whereas the upper water column sediment traps in ca. 1 km reflected the oligotrophic conditions in the overlying waters of ESTOC. We observed an increased emphasis in spring-time fluxes since 2005, associated with a change in particle composition, while satellite chlorophyll biomass did not show this pattern. Due to its northern location in the CC-EBUEs, spring biogenic fluxes at ESTOC provide a better relationship to the forcing of the North Atlantic Oscillation than those recorded further south off Cape Blanc. Off Cape Blanc, deep fluxes showed the best overlap with the deep ESTOC fluxes during the spring season before 2005. On the long-term, both chlorophyll and particle fluxes showed an increasing trend at ESTOC which was not observed further south at the mesotrophic Cape Blanc site. This might indicate that, depending on their location along the NW African margin, coastal upwelling systems react differently to global change.
Highlights
Eastern Boundary Upwelling Ecosystems (EBUEs) are among the most productive ecosystems in the world oceans and are economically important
We investigated the relationships between the environmental data (e.g., sea surface temperature (SST), chlorophyll, modeled mixed layer depths (MLD)), around ESTOC and Cape Ghir and the particle fluxes at ESTOC
An almost two-decade long flux record from ca. 1 and 3 km at the oligotrophic ESTOC site combined with environmental data and compared to the offshore flux record off Cape Blanc (Fischer et al, 2016, 2019) revealed the following major findings:
Summary
Eastern Boundary Upwelling Ecosystems (EBUEs) are among the most productive ecosystems in the world oceans and are economically important. Bakun (1990) and Bakun et al (2010, 2015) proposed a scenario of a general intensification of coastal upwelling due to increasing pressure gradients and wind fields associated with global warming. This hypothesis is under discussion and differing findings were recently summarized by Garcìa-Reyes et al (2015); Bonino et al (2019), and Sylla et al (2019). A progressive warming and decreasing productivity over the last two decades was suggested for the CC-EBUEs as a whole (Arístegui et al, 2009)
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