Macrophyte growth forms shift along the trophic gradient of lakes

Abstract:Phototrophic organisms in lakes change from macrophyte to phytoplankton dominated states along trophic gradients. Before lakes reach a turbid and phytoplankton dominated state, shifts from meadow-forming Characeae to canopy-forming macrophyte species can occur, where Characea are present (i.e. especially in oligo to mesotrophic lakes with sand or gravel substrate). However, eutrophication intensity causing this shift has not yet been estimated.We analysed data from 132 lakes located in Mecklenburg-Vorpommern (Germany). With a Generalized Linear Model (GLM) and Random Forest (RF) models complemented with grid approximation, we analysed (i) if species richness of macrophytes declines along eutrophication gradients, (ii) above which chlorophyll-a concentrations the abundance of Characeae declines, and (iii) above which chlorophyll-a concentrations the abundance canopy-forming species declines.The number of macrophyte taxa declined gradually following a log-linear trend and with increasing chlorophyll-a concentrations. Based on the RF models, the abundance of Characeae already decreased at 5-13 µg L-1 chlorophyll-a, whereas canopy-forming species showed a monotonous and slight unimodal response decreasing at 35-103 µg L-1 chlorophyll-a.The results support the theory of shifts in growth forms along eutrophication gradients in lakes and provides, for the first time, estimations of chlorophyll-a concentrations required for these shifts. Changes in growth forms are obvious indicators for eutrophication and can serve as an additional incentive to improve lake trophic status.Keywords: aquatic plantscanopy forming macrophytesCharaceaechlorophyll-aDisclaimerAs a service to authors and researchers we are providing this version of an accepted manuscript (AM). Copyediting, typesetting, and review of the resulting proofs will be undertaken on this manuscript before final publication of the Version of Record (VoR). During production and pre-press, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal relate to these versions also. AcknowledgementsWe would like to thank the Ministry of Agriculture and the Environment of the federal state of Mecklenburg-Vorpommern for kindly providing the macrophyte and water quality data of the lakes in Mecklenburg-Vorpommern. This study was partly conducted within the AQUATAG project, funded by the German Federal Ministry of Education and Research, grant number 033W046C. Willem Kaijser and Daniel Hering were partly supported by the Collaborative Research Centre 1439 RESIST (Multilevel Response to Stressor Increase and Decrease in Stream Ecosystems; www.sfb-resist.de) funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation; CRC 1439/1, project number: 426547801). The authors declare no conflict of interests.Data availability statementData are available from the respective authority upon reasonable request.