Flexible nutrient stoichiometry mediates environmental influences, on phytoplankton and its resources

The elemental composition of plants, which affects ecosystem processes such as energy transfer to consumers and nutrient recycling, varies with the supply of nutrients and light. In contrast to most terrestrial systems, aquatic plants compete with abiotic light absorbents. Light supply to pelagic producers and, consequently, algal carbon to nutrient stoichiometry are therefore affected by background turbidity (light absorption by non-algal components, K-bg) and mixing depth (vertical extension of the mixed water column, z). Still, light as a dynamic variable has been neglected in models exploring the ecological consequences of flexible algal stoichiometry. In addition, there are hardly any field experiments exploring effects of light supply on planktonic systems. We present a dynamical model that accounts for the simultaneous dependence of algal production on light supply and internal algal nutrient stores and derive predictions on how a suite of state variables (algal biomass, light climate, algal nutrient stoichiometry, dissolved nutrient concentration, and nutrients in sediment) should be affected by z and K-bg. We compare these predictions with results of an enclosure experiment in a phosphorus-deficient lake, in which we manipulated z and K-bg. Algal biomass decreased at higher K-bg and showed a unimodal response to z (being limited by sinking losses at shallow z and by light at deep z). The biomass peak shifted toward lower z with increased K-bg. The seston carbon: phosphorus ratio decreased with increasing z and K-bg. Dissolved mineral phosphorus was undetectable in all treatments. Total water column phosphorus and sedimented phosphorus increased with increasing z but were unaffected by K-bg. These data are in almost complete qualitative congruence with model predictions over the examined range of mixing depths (1-15 m). The model thus provides a useful framework for the continued mechanistic exploration of how environmental drivers influence producer stoichiometry in pelagic environments.