Role of Groundwater-Borne Geogenic Phosphorus for the Internal P Release in Shallow Lakes

• Main Authors: Catharina S. Nisbeth, Søren Jessen, Ole Bennike, Jacob Kidmose, Kasper Reitzel
• Format: Article
• Language: English
• Published: MDPI AG 2019-08-01
• Series: Water
• Subjects: internal phosphorus release; geogenic phosphorus; groundwater-borne phosphorus; groundwater-surface water interaction; phosphorus cycling
• Online Access: https://www.mdpi.com/2073-4441/11/9/1783

This study explores the under-investigated issue of groundwater-borne geogenic phosphorus (P) as the potential driving factor behind accumulation of P in lake sediment. The annual internally released P load from the sediment of the shallow, hypereutrophic and groundwater-fed lake, Nørresø, Denmark, was quantified based on total P (TP) depth profiles. By comparing this load with previously determined external P loadings entering the lake throughout the year 2016−2017, it was evident that internal P release was the immediate controller of the trophic state of the lake. Nevertheless, by extrapolating back through the Holocene, assuming a groundwater P load corresponding to the one found at present time, the total groundwater P input to the lake was found to be in the same order of magnitude as the total deposit P in the lake sediment. This suggests that groundwater-transported P was the original source of the now internally cycled P. For many lakes, internal P cycling is the immediate controller of their trophic state. Yet, this does not take away the importance of the external and possibly geogenic origin of the P accumulating in lake sediments, and subsequently being released to the water column.