Abstract. Microzooplankton grazing and algae growth responses to increasing pCO 2 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 CO 2 levels, there were no clear differences in any measured parameter between the different CO 2 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 prebloom phase when they were in low numbers, and in the post-bloom phase when they were already affected by low Correspondence to: K. Suffrian

The plankton outburst during the so-called late winter bloom in subtropical waters was studied in relation to lunar illumination in the Canary Island waters. Nutrient enrichment by mixing and dust deposition promoted a bloom of phyto-and zooplankton. Mesozooplankton biomass increased as the winter mixing progressed but peaked in every full moon and decreased thereafter because of the effect of predation by interzonal diel vertical migrants (DVMs). The pattern was similar to the one described in lakes due to predation by fishes and confirms that this phenomenon is important in the sea. The estimated consumption and subsequent transport of epipelagic zooplankton biomass by DVMs after every full moon is on the order of the mean gravitational export and is an unaccounted flux of carbon to the mesopelagic zone that may play a pivotal role in the efficiency of the biological pump.Most of the research about the downward flux of carbon in the ocean has centered on the so-called gravitational flux, the transport due to the sedimentation of the particulate organic carbon production from the euphotic layer to the mesopelagic zone. In tropical and subtropical regions this flux is a low number, normally less than 10% of primary production (Karl et al. 1996). Another component of the biological pump is the so-called active flux due to the transport of carbon by vertical migrants. These organisms feed on the shallower layers of the ocean at night and return to their daytime residence at depth where they metabolize carbon or simply are eaten by other organisms. The role of these rather large organisms (mesozooplankton and micronekton) in the ocean carbon sequestration has been almost neglected. Active flux is a rather complex mechanism that involves the gut flux (Angel 1989) (the transport due to the release of feces below the mixed layer), carbon dioxide respiration (Longhurst et al. 1990), dissolved organic carbon excretion (Steinberg et al. 2000), and mortality (Zhang and Dam 1997) at depth. The few values available at present mainly based on respiration at depth indicate that the active downward carbon flux is highly variable, ranging from 4% to 70% of the gravitational flux (Herná ndez-Leó n and Ikeda 2005a). However, diel vertical migrants (DVMs) account for the control of 5-10% of the daily epipelagic zooplankton production (Hopkins et al. 1996), and this ingested food is efficiently transported downward (Pearre 2003). The consumption of epipelagic zooplankton by these organisms and their role in the fate of a bloom are at present poorly known.A way to study the biological pump in subtropical waters is to understand the development of the bloom during winter, when nutrients are present in the euphotic zone. The late winter bloom in subtropical waters is produced by cooling of the shallower layers of the ocean, eroding the thermocline and allowing a small flux of nutrients to the euphotic zone. This process promotes the increase in primary production and the growth of micro-and mesozooplankton. Atmospheric Saharan...

photosynthetic organisms and the subsequent sinking of POC which is sequestered in deep waters (the so-called biological pump) are 2 of the main organism-mediated processes that are involved in this CO 2 sink effect. In pelagic ecosystems, a large proportion of the POC produced by the phytoplankton is re - ABSTRACT: We conducted a microcosm experiment aimed at studying the interactive effects of high CO 2 , nutrient loading and irradiance on the metabolism of a planktonic community sampled in the Western Mediterranean near the coast of Málaga. Changes in the metabolism of phytoplankton and bacterioplankton were observed for 7 d under 8 treatment conditions, representing the full factorial combinations of 2 levels each of CO 2 , nutrient concentration and solar radiation exposure. The initial plankton sample was collected at the surface from a stratified water column, indicating that phytoplankton were naturally acclimated to high irradiance and low nutrient concentrations. Nutrient addition combined with high irradiance resulted in a significant increase in primary production. Nitrate uptake by phytoplankton was also stimulated under high nutrient conditions. High nutrients, high irradiance and the combination of low CO 2 and high irradiance positively affected bacterial production. Light was the main factor affecting the respiration rates of the community, which were higher at the high light level. After 7 d of incubation, nutrient loading was the only factor that significantly affected the amount of particulate organic carbon (POC) accumulated in the microcosms. Therefore, the changes in metabolic rates produced at high CO 2 had no effect on net production of particulate organic matter. If these results are extrapolated to the natural environment, it could be hypothesized that high levels of CO 2 will have a limited impact on biological pump activity in the northern Alboran Sea since it is assumed that POC export towards deeper layers determines the potential for carbon sequestration.

The spatial distribution of meroplankton in Bransfield Strait, Antarctica, and its relationships with hydrographical conditions are described. Biological sampling was carried out with BIONESS sampling gear at 19 stations and at 5 depths between 10 and 300 m. The main hydrographic features were a shallow hydrographic front in the southern part of the strait that separates Transitional Zonal Water with Bellingshausen influence (TBW) from Transitional Zonal Water with Weddell influence (TWW) and a slope front, the so-called Bransfield front, along the South Shetland Islands Slope (SSI). A northeastward baroclinic jet known as the Bransfield Current (BC) originates from this slope front. The meroplankton community was very diverse and included 12 types of larvae, dominated by polychaete and echinoderm larvae. The meroplankton were more abundant closer to the SSI in the BC and decreased in number towards the Antarctic Peninsula. Polychaete larvae were found close to both shores but mainly close to the SSI in TBW; they were very sparse in the central basin, appeared again below 100 m depth in TWW, and increased in abundance in upper layers at stations between the hydrographic front and the Antarctic Peninsula, in TWW. By contrast, echinoderm larvae mainly occupied the central basin and were always associated with the upper 100 m in TBW, just above the polychaete larvae; they were almost absent from TWW. In summary, meroplanktonic larval exhibit a strong dependence on water masses, depth, and fronts.

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