Phytoplankton, Light, and Nutrients in a Gradient of Mixing Depths: Field Experiments

Abstract

We studied the effects of water column mixing depth and background turbidity on phytoplankton biomass, light climate, and nutrients in two field enclosure experiments designed to test predictions of a dynamical model. In 1997 and 1998, we created gradients of mixing depth by enclosing the 100-m-filtered phytoplankton community of a phosphorus-deficient lake in cylindrical plastic bags of varying depth (1.5-15 m) which were continuously mixed. To mimic different levels of background turbidity, we surrounded the transparent enclosure walls with a layer of opaque white (1997) or black (1998) plastic. The experiments were run for 4 wk (1997) and 6 wk (1998). The results supported two key assumptions of the model: specific production and specific sedimentation losses both decreased with increasing mixing depth. At all mixing depths, fast-sinking diatoms dominated the communities. In accordance with model predictions, algal biomass concentration and standing stock (summed over the mixed layer) showed a unimodal relationship to mixing depth when background turbidity was high (1998). When background turbidity was lower (1997), only the ascending limbs of the corresponding relationships were found, which supports the prediction that the mixing depth at which biomass peaks (i.e., becomes predominantly limited by light) increases with decreasing background turbidity. Also in accordance with predictions, light intensity at the bottom of the mixed layer decreased with increasing mixing depth and with increasing background turbidity. Finally, the data supported only the ascending limbs of the predicted inverse unimodal relationships among mixing depth and dissolved inorganic and total water column phosphorus. The absence of descending limbs in these relationships at low mixing depths was probably due to deviations of the experimental systems from two model assumptions. First, the remineralization rate of sedimented phosphorus may have been too slow to equilibrate with sedimentation losses over the experimental periods. Second, biomass yield per unit nutrient (the ratio of seston carbon to phosphorus) was not constant, but decreased with increasing mixing depth. To our knowledge, these are the first field experiments in which the effects of mixing depth on phytoplankton and its resources have been investigated systematically along a large gradient.

Artificial mixing to control cyanobacterial blooms: a review

“…3). The biomass per surface area of lightlimited phytoplankton decreases with increasing mixing depth, because more light is absorbed by water, and hence, less is available for growth of the plankton (Huisman et al 1999;Diehl et al 2002).…”

“…This can be explained by nutrient limitation of the phytoplankton: more nutrients become available for growth per unit surface area, and moreover, the concentration of nutrients in the hypolimnion is often higher than in the epilimnion, so that when the two are combined through destratification the availability of nutrients in the euphotic zone increases. Also decreased sedimentation losses of the phytoplankton are suggested as an explanation for increased biomass per surface area (Toetz 1981;Visser et al 1996b;Diehl et al 2002). In order to establish a decrease in total algal biomass, the mixing depth should be further increased.…”

The role of interactions between Prorocentrum minimum and Heterosigma akashiwo in bloom formation

“…Although there have been various studies on the time-space distribution and eco-physiological characteristics of algal populations (Karentz and Smayda, 1984;Haigh et al, 1992;Smayda, 1998;Diehl et al, 2002;Bruggeman and Kooijman, 2007), it is not always clear how certain phytoplankton species come to dominate a community. Thus, various environmental factors may influence competition between phytoplankton, and allelopathy and cell contact may also be involved (Rice, 1984;Uchida, 2001;Cembella, 2003;Legrand et al, 2003;Granéli and Hansen, 2006).…”

Extracellular polysaccharide-protein complexes of a harmful alga mediate the allelopathic control it exerts within the phytoplankton community

“…Although there have been various biological and non-biological studies of the distribution and growth of flagellate populations (Erga and Heimdal, 1984;Karentz and Smayda, 1984;Haigh et al, 1992;Smayda, 1997;Miralto et al, 1999;Diehl et al, 2002;Ianora et al, 2004;Bruggeman and Kooijman, 2007), it is not always clear how certain phytoplankton species can come to dominate a community. Empirical approaches to studying the allelopathic effects and inter-specific interactions among phytoplankton have developed relatively slowly (Figueredo et al, 2007;Gentien et al, 2007;Strom, 2008).…”