Zooplankton response to shading effects of free-floating plants in shallow warm temperate lakes: a field mesocosm experiment

Abstract

Dense mats of free floating plants (FFP) produce severe underwater light attenuation and strong oxygen depletion in the water column. In this study we experimentally assessed the zooplankton response to artificial shading by using field mesocosms. We simulated during thirty days three different light scenarios by mimicking the persistence, absence and fluctuation of FFP typically encountered in vegetated shallow lakes. We used dark meshes to simulate the abiotic effects engineered by FFP. Both in the permanently covered and fluctuating situations anoxia impaired zooplankton development. Anoxia constituted a major driving force in shaping the zooplankton response, whereas the feeding resource availability (phytoplankton) played a minor role; no top down effect on phytoplankton occurred in anoxic situations. In the fluctuating cover regime (periodic darkness and anoxia) nanophytoplankton temporal variation was not affected by zooplankton; once again oxygen availability was the main force shaping the zooplankton dynamics. Either periodical or permanent shading, associated to anoxic conditions, impaired the success of small herbivores. Large herbivores and microphytoplankton were only negatively affected under persistent shadowing and anoxia. Conversely, when neither light nor oxygen limitation occurred, such as in scenarios without shading, a top-down control occurred. This study underscores the role that the oxygen regimes engineered by FFP exert on the structure and dynamics of zooplankton assemblages and on the top down cascading effects on phytoplankton.