37-2 Caught in the Carbonates: Exploring Phosphorus Dynamics Beyond EPC in Marl Lakes

Session: Geoscience Research Poster Showcase by 2YC and 4YCU Undergraduate Students

Poster Booth No.: 125

Presenting Author:

Maddie Westover

Authors:

Westover, Maddie¹, Bickmore, Barry R.², Sorensen, Mary A.³, LeMonte, Joshua J.⁴

(1) Geological Sciences, Brigham Young University, Provo, Utah, USA, (2) Geological Sciences, Brigham Young University, Provo, Utah, USA, (3) Brigham Young University, Provo, Utah, USA, (4) Geological Sciences, Brigham Young University, Provo, Utah, USA,

Abstract:

Excess phosphorus (P) negatively impacts surface water globally, particularly those near population centers where it often feeds harmful algae blooms that can disrupt local ecosystems and economies. The equilibrium phosphorus concentration (EPC) framework is commonly used for predicting whether lakebed sediment will act as a source or a sink of P and to improve water quality models. However, when applied to carbonate-rich, alkaline marl lakes with sediments containing more than 50% calcite, the EPC framework may not adequately represent phosphorus dynamics because it assumes sorption processes dominate P mobility and does not account for calcite co-precipitation within the system.  

To investigate P dynamics in marl lake systems, sediment and water samples were collected from seven sites across Utah Lake, a shallow, hypereutrophic marl lake located in northern Utah at the eastern edge of the Basin and Range province. The EPC for each site was determined using sorption isotherm experiments conducted at pH values of 7.5, 8.0, 8.5, and 9.0 with total P additions ranging from 0–762 mg/L. The samples were analyzed using ion chromatography and inductively coupled plasma optical emission spectrometry and the resulting data were modeled using a modified Langmuir approach (Zhou et al, 2005). 

The EPC values across all sites and pH levels were found to be substantially higher (EPC avg = 0.53 mg/L) than the average lake P concentration (P = 0.03 mg/L), suggesting that the sediment should act as a P source. However, an estimated 90% of the P that enters the lake remains within the system (Merrit and Miller, 2016). This suggests that phosphorus mobility within the lake is not dominated by sorption processes, but rather by another mechanism that leads to P retention. This mechanism is likely to be calcite co-precipitation. Using the Danen-Louwerse model, calcite co-precipitation was estimated to account for 159–434 mT P/yr, significantly exceeding the sorption-based estimate of 6–18 mT P/yr.  

These findings demonstrate that calcite co-precipitation can be underrepresented when modeling P dynamics in marl lakes, and that accurate models and predictions require the use of a framework that includes both sorption and co-precipitation processes. Improved representation of these processes is necessary for understanding P cycling and developing management strategies for marl lakes, especially those near populated areas that are particularly vulnerable to eutrophication. 

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