Nutrient Interactions of Deep Phytoplankton Biomass Layers in Lake Michigan and Modeling of Layer Conditions

Primary producers in aquatic ecosystems live in spatially and temporally complex environments. They depend on a consistent supply of light and nutrients, which are not evenly distributed in a water column. Spatial heterogeneity that is sufficiently stable over time leads to formation of discrete layers of varying compositions. Due to these layers and availability of light and nutrients, phytoplankton have different rates of production and amounts of biomass accumulation. Phytoplankton abundance, composition and production has changed greatly during mussel-driven oligotrophication of the large Laurentian Great Lake Michigan since 2003. Mussels have enhanced the role of ammonia in deep lake metabolism, both through excretion and decomposition of mussel tissue. We hypothesize that ammonium, which is scarce in Lake Michigan (<1µM), is preferred and more energetically efficient in comparison to nitrate, which is readily available (>25µM). We prepared a series of nutrient enrichments using water obtained from distinct layers in the lake. Ammonium provided at chronic low addition similar to excretion rates (500nM/day) completely replaced nitrate as an N source, and suggestion of better growth at low light was found. Substitution by ammonium at up to 15µM over 1 week occurred in each of 6 different surface and deep biomass maximum samples. Layers dominated by diatoms or picocyanobacteria occupied specific, slightly-overlapping zones near the 1% light depth in 4-6°C water. As many as 5 discrete layers have been found at any one time – further analytical work (microscopy) may reveal even more. We are currently building a model to define and simulate these conditions. This model considers biomass of two species of phytoplankton, interaction of two nutrient sources including terms to describe preferential uptake, and a term for the light field. Boundary conditions describe inflow and outflow of the biomass and nutrients in the water column.