Facilitation of Lake Eutrophication by Altered Feedback Loops Between Submerged Macrophyte Vegetation and Phosphorus Retention

ABSTRACT Charophytes (stoneworts) often dominate the submerged vegetation in nutrient‐poor hardwater lakes, where they support high benthic phosphorus (P) retention. As a consequence, epilimnion P concentrations remain low and water clarity high, resulting in a positive feedback. When perennial charophytes, retaining P all year round, are replaced by vascular macrophytes, P is retained only during summer but rapidly released during decomposition in the autumn and winter. Epilimnion P thus becomes available to phytoplankton in the following growing season, reducing lake water clarity and further shifting plant dominance from bottom‐dwelling charophytes to taller vascular species. We tested the hypothesis that changes in lake P concentrations may be partly related to changes in the structure of submerged macrophytes in a deep hardwater lake that experienced a dramatic increase in total P (TP) concentrations over a decade with no evidence of changes in external P loading. We repeatedly measured water transparency, macrophyte maximum colonisation depth (MCD) and abundance between 2008 and 2022 and determined macrophyte tissue P content in 2020 to test whether changes in submerged vegetation could explain gaps in the lake's P budget and shifts in P sink/source functions of littoral areas. In 2008, charophyte communities were dominated by wintergreen Nitellopsis obtusa . The subsequent decline of the species was negatively correlated with the spring TP concentration in the upper water layer (0–20 m) of the lake and positively correlated with Secchi depth and macrophyte MCD, which decreased from 13.5 to 9.5 m. In contrast, the cover of annual vascular macrophytes (mainly rootless hornwort, Ceratophyllum demersum ) increased and was positively correlated with P losses from the upper water layer in summer and negatively correlated with Secchi depth and MCD. Budget calculations showed that the littoral zone was a P source during the period when N. obtusa declined. Despite the relatively low contribution of P directly released from lost charophyte biomass, declines of N. obtusa partly explained the temporary P source function of the littoral zone and the increasing P concentrations in the upper water layer at the beginning of May. This was most probably due to the positive effects of charophytes on particle retention and the negative effects on sediment resuspension. Conversely, the growth of C. demersum can explain P losses from the upper water layer, turning the littoral zone into a temporal P sink during summer. However, most of this P is likely to return to the upper water layer during the decomposition of plant biomass after the growing season. In conclusion, our data indicate that replacement of charophytes by submerged vascular plants can facilitate rapid lake eutrophication due to changes in the feedback loop between submerged vegetation, benthic P retention, epilimnion P availability, water transparency, and MCD.