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Abstract
Abstract. Availability of phosphate for phytoplankton and bacteria and of glucose for bacteria at different pCO2 levels were studied in a mesocosm experiment (PeECE III). Using nutrient-depleted SW Norwegian fjord waters, three different levels of pCO2 (350 μatm: 1×CO2; 700 μatm: 2×CO2; 1050 μatm: 3×CO2) were set up, and nitrate and phosphate were added at the start of the experiment in order to induce a phytoplankton bloom. Despite similar responses of total particulate P concentration and phosphate turnover time at the three different pCO2 levels, the size distribution of particulate P and 33PO4 uptake suggested that phosphate transferred to the >10 μm fraction was greater in the 3×CO2 mesocosm during the first 6–10 days when phosphate concentration was high. During the period of phosphate depletion (after Day 12), specific phosphate affinity and specific alkaline phosphatase activity (APA) suggested a P-deficiency (i.e. suboptimal phosphate supply) rather than a P-limitation for the phytoplankton and bacterial community at the three different pCO2 levels. Specific phosphate affinity and specific APA tended to be higher in the 3×CO2 than in the 2×CO2 and 1×CO2 mesocosms during the phosphate depletion period, although no statistical differences were found. Glucose turnover time was correlated significantly and negatively with bacterial abundance and production but not with the bulk DOC concentration. This suggests that even though constituting a small fraction of the bulk DOC, glucose was an important component of labile DOC for bacteria. Specific glucose affinity of bacteria behaved similarly at the three different pCO2 levels with measured specific glucose affinities being consistently much lower than the theoretical maximum predicted from the diffusion-limited model. This suggests that bacterial growth was not severely limited by the glucose availability. Hence, it seems that the lower availability of inorganic nutrients after the phytoplankton bloom reduced the bacterial capacity to consume labile DOC in the upper mixed layer of the stratified mesocosms.
Highlights
Rising atmospheric CO2 concentration changes seawater carbonate chemistry by lowering seawater pH, carbonate ion concentration and carbonate saturation state, and increasing the dissolved CO2 concentration
Samples for dissolved and particulate nutrients, chlorophyll-a (Chl-a), and bacterial abundance and production were collected from all nine mesocosms (Paulino, et al, 2007; Schulz, et al, 2007; Allgaier, et al, 2008), while those for particulate P, turnover times of glucose and phosphate, and alkaline phosphatase activity (APA) were taken from one mesocosm of each pCO2 level (M2: 1×CO2, M5: 2×CO2, and M8: 3×CO2) because of logistic constraints
These nutrient dynamics can be summarized as follows: (1) no obvious nutrient depletions between Days 0–6, (2) only Si depleted between Days 7–9, (3) Si and phosphate depleted between Days 10–12, (4) Si, phosphate, and nitrate depleted from Day 13 onward (Table 1)
Summary
Rising atmospheric CO2 concentration changes seawater carbonate chemistry by lowering seawater pH, carbonate ion concentration and carbonate saturation state, and increasing the dissolved CO2 concentration (reviewed by Riebesell, 2004). Studies dealing with biological responses to increasing CO2 partial pressure (pCO2) and related changes in carbonate chemistry range from a single-species level in laboratory cultures up to a semi-natural community level in outdoor mesocosms. Tanaka et al.: Availability of phosphate and labile organic carbon pCO2 can enhance primary production (Zondervan, et al, 2001; Leonardos and Geider, 2005), release of dissolved carbohydrates by phytoplankton (Engel, et al, 2004), and modify phytoplankton species composition and succession (Tortell, et al, 2002) Such pCO2 dependent changes in phytoplankton parameters further enhance growth rate and production as well as α- and β-glucosidase activity of heterotrophic bacteria, especially of particle-attached bacteria (Grossart, et al, 2006a). It seems that pCO2 dependent changes in phytoplankton and bacterial parameters are not necessarily consistent
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