Abstract
Abstract. Microzooplankton grazing and algae growth responses to increasing pCO2 levels (350, 700 and 1050 μatm) were investigated in nitrate and phosphate fertilized mesocosms during the PeECE III experiment 2005. Grazing and growth rates were estimated by the dilution technique combined with taxon specific HPLC pigment analysis. Microzooplankton composition was determined by light microscopy. Despite a range of up to 3 times the present CO2 levels, there were no clear differences in any measured parameter between the different CO2 treatments. During days 3–9 of the experiment the algae community standing stock, measured as chlorophyll a (Chl-a), showed the highest instantaneous grow rates (k=0.37–0.99 d−1) and increased from ca. 2–3 to 6–12 μg l−1, in all mesocosms. Afterwards the phytoplankton standing stock decreased in all mesocosms until the end of the experiment. The microzooplankton standing stock, that was mainly constituted by dinoflagellates and ciliates, varied between 23 and 130 μg C l−1 (corresponding to 1.9 and 10.8 μmol C l−1), peaking on day 13–15, apparently responding to the phytoplankton development. Instantaneous Chl-a growth rates were generally higher than the grazing rates, indicating only a limited overall effect of microzooplankton grazing on the most dominant phytoplankton. Diatoms and prymnesiophytes were significantly grazed (12–43% of the standing stock d−1) only in the pre-bloom phase when they were in low numbers, and in the post-bloom phase when they were already affected by low nutrients and/or viral lysis. The cyanobacteria populations appeared more affected by microzooplankton grazing which generally removed 20–65% of the standing stock per day.
Highlights
Atmospheric CO2 levels have increased from about 280 to 380 μatm since the beginning of the industrial revolution. They are projected to reach values as high as 700 μatm by the end of the 21st century (IPCC, 2001). This increase in climate-relevant atmospheric gases including CO2 is predicted to result in e.g. increasing global temperatures, rising sea level and accelerating extreme weather events (IPCC, 2007) Increased atmospheric CO2 levels have already led to increased ocean acidity with a pH drop of 0.1 in the surface ocean since the beginning of the industrial revolution
Since nutrients were limiting in the mesocosms after d13, our results after d13 represent maximal potential growth rates, which were very low
Overall phytoplankton growth and grazing estimates based on chlorophyll a (Chl-a) showed similar patterns in the three CO2 treatments during the incubation experiments (Table 3)
Summary
Atmospheric CO2 levels have increased from about 280 to 380 μatm since the beginning of the industrial revolution They are projected to reach values as high as 700 μatm by the end of the 21st century (IPCC, 2001). This increase in climate-relevant atmospheric gases including CO2 is predicted to result in e.g. increasing global temperatures, rising sea level and accelerating extreme weather events (IPCC, 2007) Increased atmospheric CO2 levels have already led to increased ocean acidity with a pH drop of 0.1 in the surface ocean since the beginning of the industrial revolution. The carbonate saturation in the ocean is decreasing, likely affecting a number of organisms. Those with calcareous skeletons such as coccolithophorids, corals and molluscs are expected to be affected (see discussion and references in Schulz et al, 2007)
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