Macrophyte‐specific effects on epiphyton quality and quantity and resulting effects on grazing macroinvertebrates

Abstract

Aquatic macrophytes can have a significant impact on their associated community of epiphytic algae and bacteria through the provisioning of structural habitat complexity through different growth forms, the exudation of nutrients and the release of allelochemicals. In turn, this effect on epiphytic biofilm biomass and nutrient content has a potential effect on the macroinvertebrates that depend on epiphyton as a food source. We studied the effect of living macrophytes and their growth form on biofilm development in a semi‐controlled replicated microcosm experiment. Conditions of a nutrient‐poor water layer and nutrient‐rich sediment were created to study the effects of nutrient exudation by living macrophytes. We compared biofilm quantity and quality on structurally simple (Vallisneria spiralis) versus complex (Egeria densa) living plants and artificial analogues. Subsequently, the biofilm that had developed on the plants was fed, in a laboratory growth experiment, to two species of macroinvertebrate grazers (the snail Haitia acuta and the mayfly nymph Cloeon dipterum). This enabled us to assess if and how the macrophyte‐induced effects on the epiphyton can influence macroinvertebrate grazers. Living macrophytes were found to have a significant effect on epiphytic algal cover, which was mostly expressed by a lower cover on living macrophytes compared to their artificial analogues. Additionally, epiphyton cover on artificial macrophytes was found to be higher on complex structures compared to simple ones, yet this was not observed on living macrophytes. Plant specific traits, such as the release of allelopathic substances, competition for nutrients and DIC, and the amount of CaCO3 deposition on plant surfaces might explain these results. The density of epiphytic bacteria was found to be negatively correlated with biofilm Ca content from macrophytes in every treatment except living E. densa, which differed in leaf anatomy from the other plants by possessing polar leaves. Furthermore, biofilm on living macrophytes had lower C:N:P molar ratios compared to that on artificial plants, which is likely to be explained by nutrient exudation by the living plants. Although it was expected that a more nutritious biofilm would lead to increased grazer growth, this was observed only for H. acuta on E. densa. Because biofilm quantity was not a limiting factor, this lack of effect may be caused by compensatory feeding. It can be concluded that, depending on their traits, living macrophytes can have a positive effect on macroinvertebrate grazers by providing a large surface area for colonisation by epiphytic algae and bacteria, by improving biofilm stoichiometry and by stimulating bacterial growth.

Epiphyton in Agricultural Streams: Structural Control and Comparison to Epilithon

“…Both biofilm communities showed a dominant heterotrophy similar to previous studies, e.g., [57,58]. However, we also found that epiphyton showed higher heterotrophy compared to epilithon, which suggests macrophytes to be a more favourable substrate for the heterotrophic community compared to the gravel/stone substrate [59].…”

“…The influence of hydrology (i.e., discharge, current velocity, low flow, high flow) on the epilithon structure was highly recognized in many previous studies in lotic systems [6,[67][68][69] and nutrients were identified as an important factor in driving epilithic diatom composition [28,55]. Furthermore, the low association of epiphyton with water nutrient concentrations supports the suggestion that the epiphyton community may depend on nutrients released from the macrophyte [13,59]. Our results support results from Gosselain et al [70] highlighting that epiphyton was related to physical variables such as light, macrophyte architecture and hydrology (i.e., seasonal water level variations) in the order of decreasing importance.…”

Environmental control of macrophyte traits and interactions with metabolism and hydromorphology in a groundwater‐fed river

“…Macrophyte species with a complex morphology have a larger effect on flow velocity reduction and nutrient uptake than macrophytes with a simple morphology (e.g., with strap‐like leaves) (Levi et al, 2015; Sand‐Jensen & Mebus, 1996; Sand‐Jensen & Pedersen, 1999). Moreover, when macrophytes have a large leaf area they can take up more nutrients, provide a larger substrate to epiphytes that take up nutrients (Wolters et al, 2019) and sediment retention increases (Clarke, 2002). Between species, there can be large differences in leaf surface area and morphology (Levi et al, 2015), but differences can also exist within species, showing that there are interactions between macrophyte morphology and the environment.…”

Substratum‐associated microbiota

“…Interestingly, mats of Phormidium were abundant in the afforested streams near the beginning of the experiment, which may have contributed to less grazing because the filamentous morphology, densely layered colonies, and chemical compounds produced by Phormidium make it less palatable for invertebrate grazers. Compared to biofilm on artificial plants, more nutritious biofilm with lower C:N:P developed on living macrophytes, which could be explained by nutrient exudates from the plants (Wolters et al, 2019). However, differences in nutritional content did not lead to increased grazer growth, perhaps because ANNUAL LITERATURE REVIEW grazers fed less nutritious biofilm compensated by consuming greater quantities of biofilm.…”