Submerged macrophytes affect the temporal variability of aquatic ecosystems

Abstract

Abstract Submerged macrophytes are important foundation species that can strongly influence the structure and functioning of aquatic ecosystems, but only little is known about the temporal variation and the timescales of these effects (i.e. from hourly, daily, to monthly). Here, we conducted an outdoor experiment in replicated mesocosms (1,000 L) where we manipulated the presence and absence of macrophytes to investigate the temporal variability of their ecosystem effects. We measured several parameters (chlorophyll‐a, phycocyanin, dissolved organic matter [DOM], and oxygen) with high‐resolution sensors (15‐min intervals) over several months (94 days from spring to autumn), and modelled metabolic rates of each replicate ecosystem in a Bayesian framework. We also implemented a simple model to explore competitive interactions between phytoplankton and macrophytes as a driver of variability in chlorophyll‐a. Over the entire experiment, macrophytes had a positive effect on mean DOM concentration, a negative effect on phytoplankton biomass, and either a weak or no effect on mean metabolic rates, DOM composition, and conductivity. We also found that macrophytes increased the variance of dissolved organic carbon composition and metabolic rates, and, occasionally in the observed period, increased the variance of phytoplankton biomass and conductivity. The observation that macrophytes decreased the mean but increased the variance of phytoplankton biomass was consistent with the model that we implemented. Our high‐resolution time series embedded within a manipulative experiment reveal how a foundation species can affect ecosystem properties and processes that have characteristically different timescales of response to environmental variation. Specifically, our results show how macrophytes can affect short‐term dynamics of algal biomass, while also affecting the seasonal build‐up of DOM and the variance of ecosystem metabolism.