Roles of phosphorus and ultraviolet radiation in the strength of phytoplankton–zooplankton coupling in a Mediterranean high mountain lake

Bullejos FJ, P Carrillo, M Villar-Argaiz, JM Medina-Sánchez. - 2010

Ultraviolet solar radiation (UVR) and atmospheric nutrient inputs associated with aerosols are major worldwide stressors that simultaneously affect species and the interaction among them. A 2 x 5 field experimental design was used to determine how variations in light regimes (presence and absence of UVR [+UVR and -UVR]) and nutrients might influence the strength of phytoplankton–zooplankton coupling (PZC). We observed unimodal curves for zooplankton biomass in response to increased food supply from nutrient enrichment. These results challenge the ‘‘more is better (or at least never worse)’’ concept, since high food levels resulted in weakened PZC. The effect of UVR on zooplankton was nutrient dependent, significantly reducing zooplankton abundance at intermediate phosphorus (P) supplied levels but not at the two ends of the trophic gradient generated (control and highest P level). Neither food quantity nor food quality explained observed differences in zooplankton biomass between light treatments, suggesting a deleterious direct effect of UVR on zooplankton at intermediate food ranges, resulting in a weakening of PZC. The location of this lake in the Mediterranean region has shown an increasing intensity and frequency of aerosol depositions over the past three decades (1973–2003), resulting in higher phytoplankton biomass. A combination of these higher atmospheric dust depositions with the high UVR levels characteristic of high mountain lakes might underlie the interannual decoupling between phytoplankton and zooplankton dynamics observed in these oligotrophic ecosystems.