Villefranche sur Mer: multistressors experiment March 2012

The effect of ocean warming and acidification was investigated on a natural plankton assemblage from an oligotrophic area, the bay of Villefranche (NW Mediterranean Sea). The assemblage was sampled in March 2012 and exposed to the following four treatments for 12 days: control ( 360 µatm, 14°C), elevated pCO2 ( 610 µatm, 14°C), elevated temperature ( 410 µatm, 17°C), and elevated pCO2 and temperature ( 690 µatm, 17°C). Nutrients were already depleted at the beginning of the experiment and the concentrations of chlorophyll a (chl a), heterotrophic prokaryotes and viruses decreased, under all treatments, throughout the experiment. There were no statistically significant effects of ocean warming and acidification, whether in isolation or combined, on the concentrations of nutrients, particulate organic matter, chl a and most of the photosynthetic pigments. Furthermore, 13C labelling showed that the carbon transfer rates from 13C-sodium bicarbonate into particulate organic carbon were not affected by seawater warming nor acidification. Rates of gross primary production followed the general decreasing trend of chl a concentrations and were significantly higher under elevated temperature, an effect exacerbated when combined to elevated pCO2 level. In contrast to the other algal groups, the picophytoplankton population (cyanobacteria, mostly Synechococcus) increased throughout the experiment and was more abundant in the warmer treatment though to a lesser extent when combined to high pCO2 level. These results suggest that under nutrient-depleted conditions in the Mediterranean Sea, ocean acidification has a very limited impact on the plankton community and that small species will benefit from warming with a potential decrease of the export and energy transfer to higher trophic levels.

(Table 1) Annual gross and new primary production rates obtained for areas centered on the AWI-HAUSGARTEN mooring (2000-2005)

The lack of extended dataset has so far prevented an inclusive understanding of the long-term relationships between primary production (PP) and vertical export in the Arctic Ocean. It is urgent to investigate these connections as Arctic ecosystems are on the verge of climate-related shifts, which could be caused by the combined effects of increase in Pacific and Atlantic inflow, climate warming, and sea ice decline. For a period of 6 years we investigated the degree of coupling between PP and export by making use of modelled PP rates and vertical particle fluxes collected with sediment traps moored at ~300 m depth in the eastern Fram Strait. Our analyses indicate that total and new simulated PP averaged for different areas centered on the mooring location (5-200 km radius) explain at best 20-44% of the observed biogenic particle fluxes at 300 m, when applying extended time-lags (55-90 days) between PP and vertical fluxes. Based on this phasing, we define a conceptual framework that presents the temporal dimension as a prime determinant of the maximum strength of the PP-export coupling at a given depth. Our results support that planktonic food webs in the Fram Strait process heavily biogenic material in the epipelagic zone, but we further suggest that Atlantic-Arctic water interactions induce a particular ecological setting responsible for the extended turn-over. In conclusion, we hypothesize that global warming could promote a transition toward a more retentive ecosystem in the Fram Strait region despite the likely increase of pelagic PP in the Arctic Ocean.

PeECE I - Pelagic Ecosystem CO2 Enrichment Study, Raunefjord, Bergen, Norway, 2001

The effects of CO2-induced seawater acidification on plankton communities were also addressed in a series of 3 mesocosm experiments, called the Pelagic Ecosystem CO2 Enrichment (PeECE I-III) studies, which were conducted in the Large-Scale Mesocosm Facilities of the University of Bergen, Norway in 2001, 2003 and 2005, respectively. Each experiment consisted of 9 mesocosms, in which CO2 was manipulated to initial concentrations of 190, 350 and 750 µatm in 2001 and 2003, and 350, 700 and 1050 µatm in 2005. The present dataset concerns PeECE I.