Similar zooplankton responses to low pH and calcium may impair long-term recovery from acidification.
Throughout much of the 20th century, unprecedented industrial emissions led to widespread acidification of regions in North America and Europe, and as lake water pH dropped, aquatic ecosystems experienced dramatic declines in biodiversity. International emission-control agreements have led to sweeping increases in lake pH, however acid-structured zooplankton communities still persist in many lakes. Concomitantly, calcium concentrations have been declining as a legacy of acidification and are approaching or have reached concentrations that could represent a barrier to the re-establishment of zooplankton communities similar to non-acidified or circumneutral reference lakes. To understand how declining calcium may influence the re-establishment of zooplankton in acid-damaged lakes we manipulated calcium and pH using a factorial in-lake mesocosm experiment and assessed their individual and combined effects on a regionally-diverse zooplankton assemblage. We found that the impacts of low calcium on zooplankton species were similar to those of acidification, and consequently may prevent the recovery of acid-structured communities. Abundance of the larger bodied and acid-sensitive D. pulex/pulicaria increased in high pH treatments, albeit non-significantly, yet by the end of our experiment only two individuals were sampled among our 10 low calcium enclosures. In contrast, small acid-tolerant cladocerans, such as Daphnia catawba, Daphnia ambigua, and eubosminids maintained significantly higher abundances in low calcium treatments relative to all other treatment combinations. Although we did not detect an effect of calcium on mean body size, the disproportionately high abundance of small cladocerans in low calcium treatments resulted in low calcium communities with higher overall abundance and lower cladoceran evenness. Our results, along with a landscape comparison demonstrating parallel changes in zooplankton relative abundance from 34 historically acidified lakes, suggests that declining calcium will be an important, on-going factor that may limit recovery of zooplankton despite regional improvements in lake pH. This article is protected by copyright. All rights reserved.
['Ross|Alexander J|AJ|https://orcid.org/0000-0003-0340-7407', 'Arnott|Shelley E|SE|https://orcid.org/0000-0002-3834-703X']
recovery mesocosm experiment acid rain calcium decline crustacean zooplankton multiple stressors