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

Abstract

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.

The relative importance of species diversity and functional group diversity on carbon uptake in phytoplankton communities

“…Two of the most important threats to aquatic ecosystems, global warming and eutrophication, may not only drastically alter the species composition in pelagic communities, for example by changing nutrient availability or stratification, but also the performance of entire functional algal groups. Climate-related changes in stratification patterns will presumably influence the population dynamics of diatoms because of their need for deep and continuous water-column mixing (Cermeno et al 2008;Jä ger et al 2008). Because diatoms are one of the most abundant phytoplankton groups in large parts of the marine pelagial (Falkowski et al 2004), environmental factors that could lead to a considerable decline might impair whole pelagic food webs.…”

Effects of nutrients and physical lake characteristics on bacterial and phytoplankton production: A meta‐analysis

“…In addition, large shallow lakes often have a larger Zeu relative to mean depth, which is the case in this data set (R 2 5 0.254, p 5 0.0027, n 5 135). This could increase light and nutrient availability and reduce sinking rates of phytoplankton cells (Thebault et al 1999;Jä ger et al 2008).…”

Artificial mixing to control cyanobacterial blooms: a review

“…Both the depth of the mixed layer and the intrinsic settling velocity of the algae determine the extent of sedimentation losses of the negatively buoyant species in a lake (Visser et al 1996a;Jäger et al 2008). In a stratified lake with only a shallow mixed layer, sedimentation losses of nonbuoyant algae are generally high.…”