Effect of CO2, nutrients and light on coastal plankton. I. Abiotic conditions and biological responses

We report on results of a microcosm experiment to study the interactive effects of elevated CO2, high organic and inorganic nutrient loading, and high irradiance on phytoplankton and bacterioplankton from the Mediterranean coastal ecosystem of the Alboran Sea. This experiment was part of the Group for Aquatic Productivity 9th international workshop and was conducted by Working Group 1 (WG1: Phytoplankton of coastal waters, www.gap9.uma.es). Over a 7 d period, we measured the variation in physical and chemical variables and the characteristics of phytoplankton and bacterioplankton in microcosms incubated under 8 treatments, representing full factorial combinations of 2 levels each of CO2 supply, nutrient concentrations and solar radiation exposure. For each treatment combination, we incubated triplicate microcosms consisting of 20 l polyethylene bags which were transparent to ultraviolet radiation. Sustained growth of phytoplankton biomass (chl a) occurred in all treatments. The absence of mesozooplankton in the microcosms resulted in a trophic cascade. Picophytoplankton were initially stimulated but then decreased, apparently due to microzooplankton grazing, and were largely replaced by diatoms. Bacteria were also initially stimulated and then decreased, but eventually recovered. Responses were modified markedly by nutrient enrichment and light availability, with moderate effects of elevated CO2. Relative to ambient CO2, elevated CO2 resulted in higher chl a under low irradiance, but lower chl a under high irradiance.