In lake pelagic waters, nitrogen (N) and phosphorus (P) are thought to be the most commonly limiting nutrients for biota. The amount of inorganic N and P influences productivity and composition of plankton communities. Ecological stoichiometry theory additionally states that it is not only the absolute quantity of nutrients, but also their supply ratio which affects population growth and species composition at both the producer and herbivore level. This study looks at two common pelagic algal species: the diatom Cyclotella meneghiniana and the green alga Monoraphidium minutum, and focuses on their nutrient usage and competitive interactions along an N:P supply gradient in presence and absence of the grazer Daphnia hyalina. A series of experiments was performed for both species separately to estimate the parameters of N- and P-dependent population growth and grazing mortality. These data were used to parameterize a model describing the dynamics of a system of two nutrients, two algae with flexible elemental (C:N:P) cell composition and a shared grazer with fixed elemental stoichiometry.
Results/Conclusions
The single-species experiments allowed for a precise estimation of algal population growth rate, N and P uptake rates and nutrient cell quotas. The two species were found to differ in their competitive abilities, M. minutum being a better N competitor and C. meneghiniana a better P competitor. The empirically parameterized model gave a very good fit to experimental data of single-species growth and nutrient usage. Despite the differences in competitive abilities, the two-species model predicts that coexistence is impossible along N:P supply gradient, and the competitive outcome in a fairly large range of N:P supply ratios depends on initial conditions. The short-term grazing experiments showed that D. hyalina feeds on both algae with a stabilizing sigmoidal (type III) functional response. The model predicts that the presence of grazers increases algal loss rate through grazing mortality and changes the “realized” inorganic nutrient supply (i.e. total nutrients minus nutrients sequestered by Daphnia) which potentially reverses the outcome of phytoplankton competition. Preliminary data testing these predictions will be discussed.