A Physiological Interpretation of the Dynamic Responses of Populations of a Planktonic Diatom to Physical Variability of the Environment

Summary The dynamic responses of Fragilaria crotonensis populations to artificial variations in the mixed depth of the water column of a Lund Tube (Enclosure C, Blelham Tarn) are shown to conform to a descriptive model involving only two environmental variables: mixed depth and optical depth. The physiological interpretation of these responses invokes a second model, relating the rates of net population change to photosynthetic productivity, corrected for respiration, and to in situ sinking loss rates. Population increases coincide with mixing episodes, when it is shown that the maximum photosynthetically‐derived growth rate exceeds the sinking low rate by an amount similar to the observed rate of net gain. Abrupt declines occur when the column rapidly restratifies, at rates explicable in terms of accelerated sinking and sinking‐loss rates, i.e. that there is also an abrupt cessation in growth. Evidence that this effect is due to extreme near‐surface photoinhibition is presented. The ecology of Fragilaria populations is briefly discussed in relation to these findings.