Effects of Egeria densa Planch. beds on a shallow lake without piscivorous fish

Abstract

Submerged plants are thought to negatively affect phytoplankton crops in the temperate zone by a number of mechanisms, including nutrient and light limitation, and enhancement of top-down control by offering diurnal refuge for zooplankton against visual predation, and by favouring piscivores. In 1997–1998, Lake Blanca (34° 54′ S, 54° 50′ W), a yellow-brownish shallow lake in Uruguay, suffered a severe water level reduction (associated with El Niño events between 1995–1997) that resulted in a massive fish kill and an extensive colonisation by Egeria densa. A clear water phase is established nowadays in the system (Secchi depth > 1 m), despite a fish community restricted to two small omnivorous–planktivorous fish: Jenynsia multidentata and Cnesterodon decemmaculatus. We studied the effects of E. densa on bottom-up and top-down controls on phytoplankton by comparing physical, chemical, and biological characteristics between submerged plant beds and sites without plants, from autumn 2000 to autumn 2001. The water column had low to intermediate nutrient concentrations, and phytoplankton community was highly diverse with a low to moderate biomass (mean Chl-a = 10.6 μg l⁻¹). The water level, recovered during the study, promoted a dilution process that explained the temporal pattern of many chemical variables. Macrophyte PVI represented 28–39% of the lake volume (annual mean biomass = 174 g DW m⁻²). The zooplankton community was generally dominated by copepods in terms of biomass. Fish and zooplankton were significantly associated with submerged plant beds. In spite of the high biomass and density of omnivorous-planktivorous fish (115 kg ha⁻¹, 13 ind m⁻²), zooplankton strongly affected phytoplankton spatial and temporal variation. The most important differences of algal biomass between zones coincided with a high herbivorous zooplankton biomass and/or with plants occupying the entire water column during the low level period. Medium-sized zooplankton declined with fish reproduction. The consequent stronger predation of juvenile fish seemed to decrease macrophyte efficiency as a zooplankton refuge in summer. E. densa bottom-up mechanisms would also be present, contributing to maintaining clear water. Besides the usually described nutrient and light limitation, the internal production of humic substances could enhance the observed top-down effect.