Performance of sinking and nonsinking phytoplankton taxa in a gradient of mixing depths

According to a recent dynamical model, the depth of a well-mixed water column should have contrasting effects on the abundances of sinking and nonsinking phytoplankton taxa. Because of increasing light limitation, nonsinking taxa should decline monotonically with increasing mixing depth, and because of sinking loss limitation at low mixing depths, sinking taxa should peak at intermediate mixing depths. Along a gradient of mixing depths, the position of this maximum should increase with increasing taxon-specific sinking velocity and decrease with increasing background turbidity. In two field-enclosure experiments, we investigated the effects of mixing depth and background turbidity on a variety of sinking and nonsinking phytoplankton taxa. We exposed the natural, 100-mmscreened phytoplankton community of a clear, unproductive, but silica-rich lake to a gradient of mixing depths (1.5– 15 m) during 4–6 weeks. To mimic two different background turbidities, the transparent enclosure walls were surrounded by either white or black foliage. Although diatoms suffered from high sedimentation losses at low mixing depths, they dominated biomass at all mixing depths throughout both experiments. Results were largely in accordance with model predictions. Specific gross growth rates of most common taxa were negatively related to mixing depth. In both experiments, the abundances of most sinking taxa showed a unimodal pattern along the mixing depth gradient, while two of three motile taxa declined monotonically with mixing depth. The depths where these taxa reached their maximal abundances were positively related to taxon-specific sinking velocity and negatively related to background turbidity. The vertical extension of the mixed surface layer of pelagic aquatic systems (also called mixing depth) is an important parameter in the environment of phytoplankton. Most phytoplankton is passively moved within the mixed surface layer. Its average vertical position therefore decreases with increasing mixing depth. Because light decreases exponentially in the water column, the average availability of photosynthetically active radiation also decreases with increasing mixing depth. As a consequence, primary production (per unit volume) and the concentration of phytoplankton biomass are expected to decrease with increasing mixing depth (Riley 1942; Sverdrup 1953; Huisman and Weissing 1995). Such a pattern has been found in controlled laboratory experiments and in comparative lake data (Petersen et al. 1997; Huisman 1999; Soto 2002). Sverdrup (1953) suggested that, beyond a specific mixing depth (which he called critical depth), the average light intensity is insufficient to support a phytoplankton population. This concept has been successfully applied to oceanic systems with deep and intensively mixed surface layers (Sverdrup 1953; Smetacek and Passow 1990). Recently, the generality of the expectation of a monotonic 1 To whom correspondence should be addressed. Present address: Institute for Marine Research, Düsternbrooker Weg 20, 24105 Kiel, Germany ([email protected]).