Influence of water-column depth and mixing on phytoplankton biomass, community composition, and nutrients

We independently manipulated mixing intensity (strong artificial mixing vs. background turbulence) and water-column depth (2 m, 4 m, 8 m, and 12 m) in order to explore their separate and combined effects in a field enclosure experiment. To accentuate the vertical light gradient, enclosures had black walls, resulting in a euphotic depth of only 3.7 m. All enclosures were placed in a well-mixed water bath to equalize temperature across treatments. Phytoplankton responded to an initial phosphorus pulse with a transient increase in biomass, which was highest in the shallowest, least light-limited water columns where dissolved mineral phosphorus subsequently became strongly limiting. As a consequence, the depth-averaged mineral phosphorus concentration increased and the seston carbon (C) : phosphorous (P) ratio decreased with increasing water-column depth. Low turbulence enclosures became quickly dominated by motile taxa (flagellates) in the upper water column, whereas mixed enclosures became gradually dominated by pennate diatoms, which resulted in higher average sedimentation rates in the mixed enclosures over the 35-d experimental period. Low turbulence enclosures showed pronounced vertical structure in water columns .4 m, where diversity was higher than in mixed enclosures, suggesting vertical niche partitioning. This interpretation is supported by a primary production assay, where phytoplankton originating from different water depths in low-turbulence treatments had the relatively highest primary productivity when incubated at their respective depths of origin. Physical conditions, notably the depth of the water column and the intensity of mixing, influence population dynamics of pelagic primary producers by affecting the average light climate, sedimentation loss, and the availability of nutrients (Huisman et al. 1999; O’Brien et al. 2003; Huisman et al. 2006). For example, under well-mixed conditions phytoplankton are passively entrained in the entire water column and, over time, each algal cell experiences the depth-averaged light intensity, which is a decreasing function of water-column depth. Consequently, depth-averaged specific primary production decreases with increasing water-column depth (Huisman 1999; Diehl et al. 2002, 2005) and will become insufficient for the maintenance of a viable population when the mixed water column exceeds a ‘‘critical depth’’ (Sverdrup 1953). In contrast, in a weakly mixed water column the velocity of entrainment will often be slower than the rate of algal reproduction. Consequently, algal cells can remain in the well-lit upper part of the water column for long enough to maintain a population even if the water column exceeds the critical depth (Huisman et al. 1999). Water-column depth and mixing intensity also affect algal sedimentation losses. The probability that an individual algal cell or a colony will sink out of the water column increases with increasing sinking velocity and decreasing water-column depth, but decreases with increasing mixing intensity (Visser et al. 1996; Condie and Bormans 1997; Ptacnik et al. 2003). The latter occurs because turbulent mixing disperses phytoplankton in the water column and therefore partly counteracts sedimentation; overall, sinking losses should thus be most severe for fast-sinking algae in shallow and weakly mixed water columns (Diehl 2002; Huisman et al. 2002). Algal sedimentation constantly removes particular nutrients from the water column, which are subsequently mineralized and recycled back into the water column from below. In sufficiently deep and weakly mixed water columns nutrients may therefore become strongly limiting 1 Present address: Department of Ecology and Environmental Science, Umeå University SE-90187 Umeå, Sweden (christoph.jager@ emg.umu.se).