Transient Dynamics of Pelagic Producer–grazer Systems in a Gradient of Nutrients and Mixing Depths

Abstract

Phytoplankton-grazer dynamics are often characterized by long transients relative to the length of the growing season. Using a phytoplankton-grazer model parameterized for Daphnia pulex with either flexible or fixed algal carbon:nutrient stoichiometry, we explored how nutrient and light supply (the latter by varying depth of the mixed water column) affect the transient dynamics of the system starting from low densities. The system goes through an initial oscillation across nearly the entire light-nutrient supply space. With flexible (but not with fixed) algal stoichiometry, duration of the initial algal peak, timing and duration of the subsequent grazer peak, and timing of the algal minimum are consistently accelerated by nutrient enrichment but decelerated by light enrichment (decreasing mixing depth) over the range of intermediate to shallow mixing depths. These contrasting effects of nutrient vs. light enrichment are consequences of their opposing influences on food quality (algal nutrient content): algal productivity and food quality are positively related along a nutrient gradient but inversely related along a light gradient. Light enrichment therefore slows down grazer growth relative to algal growth, decelerating oscillatory dynamics; nutrient enrichment has opposite effects. We manipulated nutrient supply and mixing depth in a field enclosure experiment. The experimental results were qualitatively much more consistent with the flexible than with the fixed stoichiometry model. Nutrient enrichment increased Daphnia peak biomass, decreased algal minimum biomass, decreased the seston C:P ratio, and accelerated transient oscillatory dynamics. Light enrichment (decreasing mixing depth) produced the opposite patterns, except that Daphnia peak biomass increased monotonously with light enrichment, too. Thus, while the model predicts the possibility of the "paradox of energy enrichment" (a decrease in grazer biomass with light enrichment) at high light and low nutrient supply, this phenomenon did not occur in our experiment.

Spring phenological responses of marine and freshwater plankton to changing temperature and light conditions

“…Model investigations of phytoplankton-Daphnia dynamics predict that the magnitude of algal and Daphnia spring peaks is fairly insensitive to changes in temperature (De Senerpont Domis et al 2007;Schalau et al 2008) but rather sensitive to changes in algal carrying capacity as mediated by light supply (Jäger et al 2008;Schalau et al 2008). These expectations were confirmed in the freshwater experiments, that is, the magnitudes of phytoplankton and Daphnia peaks were unaffected by temperature, but increased with increasing light supply.…”

“…Blooms are triggered by individual species' life history and physiological responses to changing abiotic conditions. Timing and magnitude of blooms are controlled by population feedbacks and mediated through resource dynamics and predator-prey interactions (Carpenter et al 2001;Jäger et al 2008;Sommer et al 1986). In aquatic systems, seasonally changing abiotic conditions involve both changes in the supply of basal resources, most notably light, and in physical conditions, most notably temperature.…”

“…In contrast to temperature, light supply affects not only the growth rate but also the carrying capacity of primary producers (Jäger et al 2008(Jäger et al , 2010. Increased primary production at high light and sufficient nutrient supply can enhance zooplankton production, which may then feed back on phytoplankton.…”

“…Increased primary production at high light and sufficient nutrient supply can enhance zooplankton production, which may then feed back on phytoplankton. Under this scenario, earlier occurrence of the peak of a phytoplankton spring bloom is not necessarily just a direct phytoplankton growth response to increased temperature and light supply, but may result from faster zooplankton growth and higher grazing rates that terminate the phytoplankton bloom earlier (Berger et al 2010;Jäger et al 2008). Grazing impacts depend, however, on the taxonomic identity of the dominant primary consumers, which typically differ between marine and freshwater systems.…”

Effects of spring warming and mixing duration on diatom deposition in deep Tiefer See, NE Germany

“…For example, zooplankton grazing can terminate diatom blooms in spring through complex interactions with temperature, bloom timing and magnitude, and lake mixing (Jäger et al 2008;Berger et al 2010). Warmer winter/spring conditions were found to cause earlier starts and terminations of phytoplankton blooms.…”