Submerged macrophytes affect the temporal variability of aquatic ecosystems

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.

Non‐additive effects of foundation species determine the response of aquatic ecosystems to nutrient perturbation

“…The suppression of phytoplankton responses to increased nutrient loading by either macrophytes or mussels alone is consistent with a large body of previous theoretical and empirical work (van Nes et al 2007, Iacarella et al 2018, Yamamichi et al 2018, Lürig et al 2020). For example, macrophytes can outcompete phytoplankton under certain nutrient loading and light conditions (Iacarella et al 2018, Lürig et al 2020), and produce allelopathic substances that inhibit phytoplankton growth (Nakai et al 2001, 2012, Korner and Nicklisch 2002, Hilt and Gross 2008). However, these mechanisms are typically insufficient to suppress phytoplankton when nutrient loading is high and light transparency is sufficiently low (Scheffer et al 1993, van Nes et al 2007, Kéfi et al 2016, Yamamichi et al 2018).…”

“…Both macrophytes and mussels can have important effects on aquatic ecosystems due to their capacity to limit phytoplankton biomass in the face of increasing nutrient loading (Jeppesen et al 1998, Bierman et al 2005, Ibelings et al 2007, Lürig et al 2020). Macrophytes, which are considered to be important foundation species (Scheffer et al 2003, Kéfi et al 2016), can be competitively dominant over phytoplankton at low nutrient loading (Lürig et al 2020), and may persist at intermediate nutrient loading via a positive feedback between macrophyte growth and water transparency (Carpenter and Lodge 1986, Jeppesen et al 1998). By comparison, mussels have high grazing rates on phytoplankton, (Johengen et al 1995, James et al 1997) and can dramatically increase water clarity in some lake ecosystems (Ibelings et al 2007).…”

“…As foundation species, macrophytes and mussels are also likely to affect other important ecosystem properties, either independently or interactively. For example, if foundations species affect the buildup of dissolved organic matter (DOM) and the overall oxygen metabolism of ecosystems (Scheffer et al 1993, Kéfi et al 2016, Lürig et al 2020), then this could affect how external disturbances propagate through the network of biological and abiotic interactions in aquatic ecosystems (Olff et al 2009). Pulsed ecosystem disturbances can characteristically affect both the mean and variance of ecosystem conditions (Hillebrand et al 2020), and previous nutrient addition experiments in aquatic ecosystems have documented such effects (Carpenter et al 2011, Scheffer et al 2012, Gsell et al 2016).…”

“…Ideally, we would want the capacity to make high‐resolution ecosystem measurements in well‐replicated experiments that are conducted over ecologically relevant time scales for the communities of interest. Automated sondes deployed in each replicate ecosystem of an experiment can provide the appropriate resolution to capture, for example, diurnal changes in phytoplankton biomass concurrently with abiotic changes (e.g., temperature and conductivity) and rates of ecosystem metabolism, such as net primary productivity and respiration (Carpenter et al 2011, Batt et al 2013, Nielsen et al 2013, Lürig et al 2020). These processes are largely driven by the autotrophic lake community, both benthic (e.g., macrophytes) and pelagic (e.g., phytoplankton) but can also be affected by DOM dynamics associated with the growth and decay of biomass (Catalán et al 2014).…”

“…To capture these ecosystem dynamics, the relevant experimental scale might be multiple months to years. Methods have been developed to quantify whole‐ecosystem metabolism from high‐frequency measurements (Staehr et al 2010, Lürig et al 2020), and using such approaches in experimental settings will undoubtedly reveal new insights into the resistance and resilience of aquatic ecosystems (Batt et al 2013).…”