MEPS 507:69-79 (2014)  -  DOI: https://doi.org/10.3354/meps10809

ABSTRACT: The effect of aerosol deposition on the growth rate (μ) and grazing mortality (m) of different-sized phytoplankton was investigated in microcosm experiments using the dilution technique. Experiments were conducted on 3 August 2011 (Expt 1), 14 November 2012 (Expt 2) and 1 February 2013 (Expt 3) at a subtropical coastal site adjacent to the South China Sea. Significant enhancement in both phytoplankton growth and microzooplankton grazing were obtained after aerosol addition, but the degree of impact was different in each size fraction. Aerosol input induced significant phytoplankton biomass accumulation as well as community structure shift, with 2.0-, 2.7- and 2.5-fold biomass increases of micro-sized cells and 1.3-, 1.5- and 1.9-fold biomass increases of pico- and nano-sized cells in the 3 experiments, respectively. A large increase in growth rate was obtained for micro-phytoplankton in 2 of the 3 experiments, while pico- and nano-sized cells suffered increased grazing pressure in the aerosol treatment. The different responses resulted in a reduced proportion of protist consumption to phytoplankton daily production (i.e. m/μ ratio) and increased net growth rate (μ - m) in micro-phytoplankton but the opposite in nano- and pico-phytoplankon, which explains the clear shift of phytoplankton size structure in response to aerosol addition. The picoplankton populations did not exhibit apparent biomass accumulation due to efficient cropping by heterotrophic nanoflagellates. Our study is the first direct assessment of the impact of aerosol amendment on microzooplankton grazing activity, and our results suggest that it is important to consider the role of protist grazing when evaluating the effects of atmospheric deposition on marine planktonic ecosystems under the scenarios of increasing human activity, atmospheric input and natural perturbations.

KEY WORDS: Aerosol · Microzooplankton grazing mortality · Phytoplankton growth rate · Phytoplankton community structure