Effects of turbulence conditions on the balance between production and respiration in marine planktonic communities

The control exerted by different intensities and temporal inputs of small-scale turbulence on the balance between C production and respiration in planktonic communities has been studied by means of laboratory microcosms. The different turbulence conditions modified the temporal pattern of phytoplankton (chlorophyll a) and total biomass development. However, the differences between average values of total (after re-suspension) biomass were not statistically significant. Net primary production (NP) was significantly higher at intermittent turbulence (alternating turbulence and calm periods), with maxima coinciding with the turbulence periods. Respiration (R) was less sensitive to turbulence conditions, but on average was significantly higher under continuous turbulence. The relationships between analysed total particulate organic carbon (TPOC) and theoretical estimates of total organic carbon (according to the daily balance between NP and R) indicated that between 30 and 50% of NP could be released as dissolved organic carbon. The quotient between daily C production and R (indicative of auto- or heterotrophic conditions) was higher under turbulence conditions. Autotrophy lasted for longer periods of time under intermittent turbulence. These results confirm the importance of turbulence in the modulation of the trophic status of the system, opening new perspectives for the study of the mechanisms controlling the selection of planktonic food webs.