Examination of the effects of nutrient regeneration mechanisms on plankton dynamics using aquatic biogeochemical modeling

The prolonged stratification of lakes due to climate warming is expected to increase the dependence of planktonic food webs on internal nutrient regeneration mechanisms (i.e., microbial mineralization, zooplankton excretion). Our current conceptualization of aquatic communities, however, suggests that while the strength of the recycling feedback loop is indeed related to climate forcing, other biotic factors (e.g., zooplankton community composition) along with the system productivity may also be equally important. What do the contemporary operational models predict about the role of recycling rates in different trophic environments? How tight is the relationship between mineralization rates and lake warming? How realistically do modelers describe the mechanisms by which nutrients in non-living organic matter are recycled into inorganic forms? Our study addresses these questions using a complex biogeochemical model that simulates multiple elemental cycles (C, N, P, Si, O), multiple functional phytoplankton (diatoms, green algae and cyanobacteria) and zooplankton (copepods and cladocerans) groups. We relaxed the assumption of strict zooplankton homeostasis by allowing nutrient use efficiency to vary with food quality. Our analysis shows that the nutrient regeneration rates can play a major role in planktonic food webs, but their relative importance is somewhat inconsistent with the existing paradigm. We provide evidence that the recycled material and the associated energy fluxes can be significant drivers in low as well as in high-productivity ecosystems depending on the period of the year examined. Warmer climatic conditions and longer stratification periods will increase the dependence of lakes on nutrient regeneration rates. The lake productivity response, however, is non-linear and non-monotonic and is modulated by the type of nutrient limitation (nitrogen or phosphorus) experienced. Our study concludes by pinpointing some problems of the existing mathematical representation of the recycling rates, and emphasizes the need to improve our understanding of the interplay among microbial metabolism, trophic state, and lake thermal structure.