Nutrient retention in a small reservoir under changed variability of inflow nutrient concentration

ABSTRACT

Within freshwater networks, lakes and reservoirs are reactors that modify nutrient dynamics. Their functioning is based on an interplay of hydrological and biogeochemical processes, rendering them vulnerable to climate change. Future changes in catchment characteristics are likely to alter the timing and magnitude of nutrient concentrations in discharge. This study investigated the impact of changing variability of nutrient concentrations on lake and reservoir dynamics. We examined intraannual nutrient retention and analyzed the role of reservoirs in reconfiguring the variability of nutrients. Utilizing the 1D lake model GOTM-WET, we simulated nutrient processing in a mesotrophic reservoir. Further, we performed scenario simulations by modifying the variability of inflow nitrogen and phosphorus concentrations. Our findings indicate that the reservoir removed ∼4% and ∼12% of total nitrogen (TN) and total phosphorus (TP), respectively. Particulate fractions were retained efficiently, but there was a net export of dissolved organic fractions. Regarding mixing and stratification periods, however, we observed net nitrogen export during stratification in certain years. During stratification, outflow concentration variability remained relatively unchanged for TN and TP compared to inflow concentrations. Conversely, phosphate and nitrate concentration variability increased in the outflow because of in-lake assimilation and the influence of hydrological events. With increasing inflow concentration variability during stratification, there was decreased removal of TN and TP by the reservoir, but increased variability of concentration. By evaluating the lake's capacity to attenuate variability of nutrient inflows under altered conditions, there are opportunities to improve monitoring of nutrient export and evaluate the potential impact of nutrient peaks on downstream drinking water resources and ecosystems.

KEYWORDS:

• catchment–lake interaction; concentration variability
• GOTM-WET
• lake nutrient export
• reservoir biogeochemistry

Acknowledgements

The authors are grateful to 2 anonymous reviewers for their helpful comments and suggestions. The authors thank the Rappbode Reservoir authority, TSB, Sachsen-Anhalt, and the State Administration of Flood Protection and Water Management, LHW, Sachsen-Anhalt, for providing the data. MD wants to thanks M. Shikhani, A. Wachholz, F. Saavedra Melendez, and V. Faréz Roman for their helpful comments.

Data availability statement

Data are available on request.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

MD was funded by the Helmholtz-Centre for Environmental Research via the UFZ PhD-college Dynamo.