Abstract
Abstract. We analyse long-term trends in marine primary and particle export production and their link to marine phytoplankton community composition for the period 1960–2006 using a hindcast simulation of the Biogeochemical Elemental Cycling Model coupled to the ocean component of the Community Climate System Model. In our simulation, global primary and export production decrease significantly over the last 50 yr, by 6.5% and 8% respectively. These changes are associated with an 8.5% decrease in small phytoplankton biomass and 5% decrease in zooplankton biomass. Diatom biomass decreases globally by 3%, but with strong temporal and spatial variability. The strongest decreases in primary and export production occur in the western Pacific, where enhanced stratification leads to stronger nutrient limitation and a decrease in total phytoplankton. The concurrent decrease in diatom fraction and in zooplankton biomass causes a lower export efficiency in this region. Substantial phytoplankton composition changes also occur in the Southern Ocean and North Atlantic, although these are masked in part by a high degree of interannual variability. In these regions, stronger wind stress enhances mixing, reducing the biomass of small phytoplankton, while diatoms profit from higher nutrient inputs and lower grazing pressure. The relative fraction of diatoms correlates positively with the export efficiency (r = 0.8, p < 0.05) in most areas except for the North Pacific and Antarctic Circumpolar Current, where the correlation is negative (r = –0.5, p < 0.05). However, the long-term trends in global export efficiency are ultimately driven by the reduction in small phytoplankton and particularly decreases in coccolithophore biomass. The diagnosed trends point toward a substantial sensitivity of marine primary production and export to climatic variations and trends.
Highlights
In the recent decades, evidence for impacts of climate variability and change on the global oceans has been accumulating (Denman et al, 2007)
Our model simulation shows a significant decrease in global particle export production (EP) by −0.8 Gt POC yr−1 (−8 %) from 1960–2006, but with strong temporal and regional variability (Figs. 1 and 2d)
EP increases in the North Atlantic (+0.6 mol POC m−2 yr−1 resp. +30 %) and in parts of the Southern Ocean
Summary
Evidence for impacts of climate variability and change on the global oceans has been accumulating (Denman et al, 2007). The trend toward lower salinities observed in the high latitudes enhances the warming-induced stratification there. This overall strengthening of the vertical density gradient has acted to reduce the mixing of surface waters with nutrient-rich deeper waters, resulting in stronger nutrient limitations for phytoplankton growth. This is expected to have resulted in a longterm spatial expansion of the oligotrophic areas of the ocean, similar to what was observed over the period 1998 to 2006 in the Atlantic and Pacific oceans (Polovina et al, 2008). These physical and chemical changes might have already affected marine ecosystems and marine productivity in a substantial manner (Boyd, 2011; Gruber, 2011), but very little is known so far about how marine plankton have changed in the last 50 years
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