**Collin J. Weber and Christoph Weihrauch \***

Department of Geography, Philipps University of Marburg, 35037 Marburg, Germany; collin.weber@geo.uni-marburg.de

**\*** Correspondence: christoph.weihrauch@geo.uni-marburg.de

Received: 10 February 2020; Accepted: 3 May 2020; Published: 7 May 2020

**Abstract:** Climate change is projected to aggravate water quality impairment and to endanger drinking water supply. The effects of global warming on water quality must be understood better to develop targeted mitigation strategies. We conducted water and sediment analyses in the eutrophicated Antrift catchment (Hesse, Germany) in the uncommonly warm years 2018/2019 to take an empirical look into the future under climate change conditions. In our study, algae blooms persisted long into autumn 2018 (November), and started early in spring 2019 (April). We found excessive phosphorus (P) concentrations throughout the year. At high flow in winter, P desorption from sediments fostered high P concentrations in the surface waters. We lead this back to the natural catchment-specific geochemical constraints of sediment P reactions (dilution- and pH-driven). Under natural conditions, the temporal dynamics of these constraints most likely led to high P concentrations, but probably did not cause algae blooms. Since the construction of a dammed reservoir, frequent algae blooms with sporadic fish kills have been occurring. Thus, management should focus less on reducing catchment P concentrations, but on counteracting summerly dissolved oxygen (DO) depletion in the reservoir. Particular attention should be paid to the monitoring and control of sediment P concentrations, especially under climate change.

**Keywords:** eutrophication; phosphorus; water quality; sediment; dissolved oxygen; phosphorus mobilization; climate change; algae bloom
