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

Phytoplankton distribution is relatively constant in large areas of the surface ocean. In order to maintain this apparent stability, phytoplankton production and losses have to be balanced. Indeed, growth (mu(o)) and grazing (g) rates obtained simultaneously with the dilution technique are often tightly coupled. One problem with this approach is that growth and grazing are not independent in the ecological model on which the method is based (net growth rate = mu(o) - g). We evaluated to which extent this methodological artefact may influence the correlation between mu(o) and g estimated using the dilution technique. Following a Monte-Carlo approach, we show that the methodological correlation can be substantial depending on: (1) the % error in the measurement of the state variable N-D (e.g. chlorophyll a) and (2) the range (+/- SD) of the mu(o) and g considered. As long as the error of N-D is small (< 10 %), the measured correlation 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 decreases as the range of growth and grazing rate values increases. We developed a procedure to evaluate the ecological versus the methodological nature of the correlation observed between mu(o) and g. The application of this procedure to a data set obtained from a coastal site revealed that the high correlation observed (rs = 0.881, p < 0.0001) reflected a true ecological relationship.