Biological modulation of induced pH changes. An experimental study with contrasting plankton communities and trophic conditions

Abstract

Two mesocosm experiments were conducted to examine the dynamics of microbial communities in different artificial and natural induced conditions of pH, and nutrient availability. Natural seawater from the Blanes Bay Microbial Observatory, NW Mediterranean, was enclosed in eight 200 L tanks and maintained in a temperature-controlled chamber, with a light: dark cycle of 12:12 h. The experiments were performed during two different seasons of the year (winter 2010, and summer 2011) to test the importance of the initial conditions on the responses of the plankton communities to acidification. Each experiment consisted in four experimental conditions: KB (controls), KA (decreased pH) NB (nutrient addition), and NA (decreased pH nutrient addition). Seawater pH was adjusted by bubbling small amounts of pure CO2 directly to the mesocosms. This addition was performed manually every morning, in order to maintain the levels of pH in the acidified tanks ∼0.3 pH units lower than the controls. The values in the controls were not adjusted, thus, pH changes due to biological activities occurred throughout the experimental period. The temporal changes of phyto- and bacterioplankton as well as nutrients and pH were followed during 9 days. Chlorophyll reached, in general, higher values in the acidified mesocosms although the differences in chlorophyll concentration between acidified and control mesocosms were less conspicuous at higher nutrient concentrations. The response of the microbial communities differed depending on initial plankton composition. Differences on biological variables between treatments were standardized respect to control values in order to compare the results obtained in winter and summer experiments. The sign and the significance of normalized differences between acidified and control conditions varied depending on the phase and the magnitude of the phytoplankton bloom. The variability in responses was interpreted to be consequence of the natural changes in pH mediated by biological activities