Dissolved organic carbon affects the occurrence of deep chlorophyll peaks and zooplankton resource use and biomass

Abstract

Abstract Concentrations of dissolved organic carbon (DOC) are increasing in many parts of the world and vary considerably among lakes. As a result, it is important to understand the effects of DOC on zooplankton, which are a key component of freshwater food webs. Deep chlorophyll maxima (DCMs) also are common in many lakes and numerous studies suggest that they may be an important resource for zooplankton. In a survey of eight boreal lakes spanning a gradient of DOC (3.5–9.2 mg/L), we assessed variations in the occurrence of DCMs, zooplankton biomass and their use of basal resources using stable isotopes of carbon (C), nitrogen (N) and hydrogen (H). DCMs occurred only in lakes with lower DOC concentrations and Bayesian stable isotope mixing models indicated that they contributed between 22% and 72% of the assimilated diet of zooplankton. Food quality as indicated by C:P and chlorophyll a: C ratios increased with depth and in DCMs. The proportion of C ultimately derived by zooplankton from terrestrial sources increased from 6% to 27% with DOC. Zooplankton biomass among lakes declined with increasing DOC concentrations, the absence of DCMs and with increasing allochthony. Our results suggest that increasing inputs of DOC may suppress zooplankton in many lakes by affecting both resource quality and the availability of metalimnetic phytoplankton resources. Because zooplankton occupy a central position in freshwater food webs, declines in their biomass may further affect phytoplankton dynamics and fish productivity.