Microzooplankton grazing and phytoplankton growth in marine mesocosms with increased CO<sub>2</sub> levels

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. Phytoplankton and microzooplankton composition were determined by light microscopy. Despite a range up to 3 times the present CO2 levels, there were no clear differences in any measured parameter between the different CO2 treatments. Thus, during the first 9 days of the experiment the algae community standing stock (SS), measured as chlorophyll a (Chl a), showed the highest instantaneous grow rates (0.02–0.99 d-1) and increased from ca 2–3 to 6–12 μg l−1, in all mesocosms. Afterwards the phytoplankton SS decreased in all mesocosms until the end of the experiment. The microzooplankton SS, that was mainly dinoflagellates and ciliates varied between 23 and 130 μg 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 (14–43% of the SS d-1) only in the pre-bloom phase when they were in low numbers and in the post-bloom phase when they were already limited by low nutrients and/or virus lysis. The cyanobacteria populations appeared more effected by microzooplankton grazing, generally removing 20–65% of the SS d−1.

Low CO2 Sensitivity of Microzooplankton Communities in the Gullmar Fjord, Skagerrak: Evidence from a Long-Term Mesocosm Study

“…Complex near-natural systems like the one used in this mesocosms study are associated with a higher buffering capacity towards the effects of OA in comparison with lab studies using simplified food webs [24]. Nevertheless, while we found a high tolerance of most MZP groups to a realistic acidification scenario, we observed effects on both phytoplankton and MZP.…”

“…Previous mesocosm studies of comparable size from the Baltic Sea, the North Sea and the Arctic considered only shorter time spans. This might have masked effects of high CO 2 which are visible only under long-term exposure [15, 23, 24, 67]. Based on the results, MZP communities from coastal regions comparable to the study site are not expected to be strongly affected by end-of-century CO 2 levels.…”

“…Results from a laboratory study applying more realistic OA scenarios on a single MZP species showed no direct effect [13]. In support of that, most mesocosm studies with a CO 2 level expected for the end of the 21 st century also show no or only subtle effects on the MZP community composition and diversity [15, 23, 24]. This can be partly attributed to the high tolerance of coastal communities to frequently occurring habitat pH fluctuations [48, 49].…”

Abundance of the iron containing biomolecule, heme b, during the progression of a spring phytoplankton bloom in a mesocosm experiment

“…Reallocation of heme b resources in response to changes in p CO 2 may be more pronounced in low iron or iron limited regions [22]. Previous studies in iron replete mesocosms report no significant acidification effect upon phytoplankton species composition or succession [31–33], microzooplankton grazing [38] or copepod feeding and egg production [39]. Significant pH-effects have also failed to manifest in field experiments on microzooplankton biomass in a late North Atlantic spring bloom [40] and may further explain the lack of change in heme b and its relation to other parameters.…”

Future Climate Scenarios for a Coastal Productive Planktonic Food Web Resulting in Microplankton Phenology Changes and Decreased Trophic Transfer Efficiency

“…Laboratory and field experiments with non-calcifying unicellular organisms are less frequent and provide contrasting results [15]. In general, when the entire microbial community is analysed using bulk measures (e.g., chlorophyll), only modest or non-significant responses are observed [17]–[19]. Clear acidification effects on natural communities have been reported mostly on picoeukaryotes and some phototrophic bacteria [20]–[22].…”