Maximal sustainable sinking velocity of phytoplankton

Abstract

Most phytoplankton species have a tendency to sink. Phytoplankton require light for photosynthesis, however. Therefore, phytoplankton species that sink too fast will not be able to sustain a viable population in the euphotic zone. This points to the existence of a maximal sustainable phytoplankton sinking velocity. Using a reaction-advection-diffusion model of phytoplankton growth in a stratified water column, we derive that this maximal sinking velocity is inversely proportional to the turbidity of the water column. In other words, clear waters can sustain species with high sinking rates, whereas turbid waters can sustain species with low sinking rates only. We show that this prediction is both qualitatively and quantitatively supported by empirical data. An intriguing implication is that export production of sinking phytoplankton might be sensitive to the turbidity of the water column.

Environmental control of diatom community size structure varies across aquatic ecosystems

“…By contrast, nutrient concentration and supply rate can become somewhat uncoupled from stratification in aquatic systems impacted by significant external nutrient inputs, such as many North American small, shallow, inland lakes. These systems are often characterized by high chlorophyll a concentrations, sediment loads and/or coloured organic materials, which can contribute to shallow euphotic zone depths and light limitation of phytoplankton growth (Smol et al 1984;Huisman & Sommeijer 2002). As a result, physiological and ecological strategies to avoid or reduce light limitation can become the primary determinants of the size structure of these phytoplankton communities.…”

“…Light limitation should select for smaller diatoms, which are less susceptible to light limitation due to their higher pigment-specific absorption efficiencies ( Finkel 2001;Finkel et al 2004) and lower sinking rates (Smayda 1970;Huisman & Sommeijer 2002), and which will tend to increase the proportion of the day they spend in the upper sunlit waters compared with larger diatoms (Smol et al 1984). The southeastern Ontario lakes examined in this study have higher nutrient inputs and chlorophyll a concentrations (0.6-8.4 mg m K3 ) than typical oceanic systems (global average: 0.2-0.3 mg m K3 ) or the oligotrophic Lake Tahoe (maximum concentration approx.…”

Late Pleistocene paleohydrography and diatom paleoecology of the central basin of Lake Malawi, Africa

“…These species are often the dominant forms reported from the open waters of the modern lake (Hecky and Kling, 1987;Patterson and Kachinjika, 1995); when smaller stephanodiscoid forms dominate, this may be an indication of decreased turbulence or perhaps increased turbidity of the surface waters (Huisman and Sommeijer, 2002).…”

“…Because their frustules have a greater silicate mass, large euplanktic diatoms generally are denser than smaller diatoms, and as a result, they tend to sink at much faster rates. Consequently, turbulence in the epilimnion or deeper light penetration is required to maintain populations of larger-diameter euplanktic diatoms (Huisman and Sommeijer, 2002). During periods of low turbulence, some euplanktic species may persist near the base of the thermocline, where denser waters with elevated nutrient concentrations may help keep them from completely sedimenting out of the water column (Tilman and Kilham, 1976).…”

Body size distributions signal a regime shift in a lake ecosystem

“…Plankton obtains nutrients from the water column, whereas benthos can derive some of their nutrients from the substrate that they are attached to, which can result in differing rates of productivity [12]. The shapes and sizes of planktic diatom species are also strongly limited (or selected for) by convective mixing strength, which determines the extent to which valves can stay suspended in the water column [13,14]. By contrast, benthic species do not have these same constraints on size and shape, because they live attached to a substrate [15,16].…”