Periphyton and benthivorous fish affect charophyte abundance and indicate hidden nutrient loading in oligo‐ and mesotrophic temperate hardwater lakes

Abstract

Charophytes are a key group of submerged macrophytes in temperate hardwater lakes, but have declined or been replaced by angiosperms as a result of eutrophication for more than 100 years. At present, this process is continuing in many European lakes despite an overall reduced nutrient loading. In eutrophic lakes, light attenuation by periphyton is a major factor responsible for declining angiosperm abundance. For charophyte declines, the impact of benthivorous and herbivorous fish has been discussed often, yet periphyton shading has been largely neglected. In our study, we investigated 11 temperate oligo‐mesotrophic (total phosphorus [P] concentrations 9–22 μg/L) hardwater lakes along a gradient of charophyte abundances. Charophyte abundance was mapped using underwater cameras or scuba diving, and coverage was estimated in five classes. Periphyton biomass, nutrient stoichiometry and invertebrate grazer abundance were measured twice on artificial substrates exposed inside and outside fish exclosures for 4 weeks in July and August. Fish biomass was sampled once using multi‐mesh gill netting. We analysed the relationships between water chemistry (concentrations of nutrients and dissolved organic carbon), periphyton biomass and nutrient stoichiometry, periphytic invertebrate grazers and fish biomass to test (a) whether periphyton biomass was controlled bottom‐up by nutrient availability or top‐down by an omnivorous fish–invertebrate–grazer cascade and (b) whether charophyte abundance was affected by periphyton biomass, benthivorous and herbivorous fish. Multiple linear regressions revealed that charophyte abundance was significantly predicted by the biomasses of periphyton and benthivorous bream (Abramis brama L.), whereas herbivorous rudd (Scardinius erythrophthalmus L.) had no significant effect in the investigated lakes. Bream biomass and its proportion in the total fish biomass were linearly related to lake total P concentrations. Periphyton biomass in the tested lakes reached 0.1–21 g dry weight/m2 and primarily was controlled bottom‐up by P availability as indicated from stoichiometry and positive correlations with grazer abundance. Top‐down control was found in only three lakes, which had significantly lower periphyton biomass inside fish exclosures as compared to controls. Our data imply that periphyton shading and negative fish impacts on charophytes increase with P loading. Low charophyte abundances at moderate P concentrations in the lake water indicate a high sensitivity of charophytes and/or unnoticed nutrient loading due to nutrient retention by charophyte stands. Management measures thus must focus on reducing nutrient loading and/or benthivorous fish biomass at early stages of impact to preserve charophyte stands in oligo‐mesotrophic hardwater lakes.

Periphyton in urban freshwater facilitates transformation of trace organic compounds: A case study on iodinated contrast media

“…Facilitation of TrOC removal by periphyton is thus a natural ecosystem service and can potentially be optimized in surface waters affected by treated wastewater, but also used for drinking water production. High periphyton biomass per substrate area has been found both in oligo-and eutrophic lakes and rivers (Roberts et al, 2003;Köhler et al, 2010;Alirangues Nuñez et al, 2022). Periphyton biomass used in our experiments was similar to these values.…”

Adaptive responses and transgenerational plasticity of a submerged plant to benthivorous fish disturbance

“…Those may be attributed to their studies' experimental periodicity and benthivorous fish density. Through excretion and disturbance, benthivorous fishes increase the nutrient content of overlying water, promote the growth of phytoplankton and periphyton, and form a shaded effect on submerged plants (Alirangues Nuñez et al, 2023 ; Ren et al, 2022 ). Therefore, benthivorous fish disturbance hurts the growth and reestablishment of aquatic plants in shallow lakes (Pacheco et al, 2021 ; Ren et al, 2022 ).…”

Biodiversity, Ecology and Distribution of Mediterranean Charophytes in Southern Italy

“…In freshwater ecosystems, charophytes are responsible for a high primary production, providing trophic and spatial niches for other aquatic organisms [3,[29][30][31]. Moreover, by counteracting resuspension of sediment particles [3,29,32], by acting as nutrient sinks [21,29,33], and by hampering phytoplankton biomass growth [4,10], charophytes participate in the establishment and maintenance of clear-water states [3,29]. As a consequence, charophytes characterize the pattern of ecological successions (in which they often represent pioneer species [6]) and the community structure of several types of water bodies [3,29].…”