Coupling between phytoplankton growth and microzooplankton grazing in dilution experiments: potential artefacts

Abstract

Phytoplankton distribution is relatively constant in large areas of the surface ocean. In order to maintain this apparent stability, phytoplankton produc- tion and losses have to be balanced. Indeed, growth (μo) and grazing (g) rates obtained simultaneously with the dilution technique are often tightly coupled. One prob- lem with this approach is that growth and grazing are not independent in the ecological model on which the method is based (net growth rate = μo - g). We evaluated to which extent this methodological artefact may influ- ence the correlation between μo and g estimated using the dilution technique. Following a Monte-Carlo ap- proach, we show that the methodological correlation can be substantial depending on: (1) the % error in the mea- surement of the state variable ND (e.g. chlorophyll a) and (2) the range (± SD) of the μo and g considered. As long as the error of ND is small (< 10%), the measured corre- lation between growth and grazing closely reflects a true ecological relationship. For large errors, the dilution technique can yield a substantial correlation between both variables, regardless of their ecological relation. The influence of this methodological correlation de- creases as the range of growth and grazing rate values increases. We developed a procedure to evaluate the ecological versus the methodological nature of the corre- lation observed between μo and g. The application of this procedure to a data set obtained from a coastal site re- vealed that the high correlation observed (rS = 0.881, p < 0.0001) reflected a true ecological relationship.