Sources of excess phosphate leading to cyanobacteria blooms at Lake Temescal, Oakland, CA

Lake Temescal is a dammed sag pond formed as a result of long-term creep along the Hayward Fault. It is a popular swimming, hiking, and fishing spot within the East Bay Regional Parks District (EBRPD). Over the past decade, the lake has experienced numerous cyanobacteria (commonly known as “blue green algae”) blooms and high phosphate (PO [subscript]4 [superscript] 3-) concentrations have been implicated in the blooms, as phosphate is a typically a limiting nutrient. Further water quality, and water chemistry testing, collected over 16 months (November, 2015 – March, 2016), provided insight into sources and cycling of phosphate in the lake and its watershed. Advanced analyses such as the ratio of total nitrogen to total phosphorous (TN:TP), X-ray absorption spectra (XAS) of lake bottom sediments, and 𝛿[superscript]18O of phosphate were applied as tracers of phosphate source and cycling. In general, phosphate concentrations in Temescal watershed creeks and in the lake are well above typical background levels and well above concentrations implicated in blooms. Water quality and chemistry results demonstrate water column stratification and seasonal cycling of the lake, including establishment and breakdown of a thermocline and a persistent redoxcline. Because of rapid sedimentation in recent decades, the lake is only x m deep, and the entire water column is warm by the end of summer. Low dissolved oxygen and high temperatures correlate with higher phosphate concentrations. Samples from late fall show a decrease in phosphate, suggesting that the cycle switches from the internal cycling of phosphorus within the lake system, to receiving external phosphate from the watershed. In June, an external source of phosphate affects the surface of the lake as evidenced by low TN:TP values and an increase in phosphate concentrations. In March and June 𝛿[superscript]18O of phosphate samples were collected at various depths. Expected values for 𝛿[superscript]18O of P were calculated from an empirical formula using parameters collected in the field and then compared to the results; the actual values. The lake would be in equilibrium with the ecosystem if the expected and actual values were the same. The March shallow depth results were closer to the expected values, but the deep samples were extremely far from the expected. External source nutrients are leading to disequilibrium resulting in high 𝛿[superscript]18O of P at the bottom of the lake. Groundwater is a plausible source of phosphate due to the deeper portion being more out of equilibrium and the shallow portion is closer to equilibrium as the groundwater is being diluted. The 𝛿[superscript]18O of P values are also suggesting a more natural phosphate. Lake Temescal is experiencing more natural conditions in the winter when the temperature is low, phosphate is low and TN:TP is high. All June 𝛿[superscript]18O of P values were extremely out of balance with the ecosystem, but most out of equilibrium at the surface. In June, an external source of phosphate is affecting the surface of the lake leading to extreme disequilibrium. The external source is then confirmed by the low TN:TP values (indicative of a non-natural source) and an increase in phosphate concentrations. The June values at deep depth suggest anthropogenic sources and the shallow portion are not in the literature. Some of the 𝛿[superscript]18O of phosphate results found in the waters of Lake Temescal have previously not been reported in the literature. High temperatures, low dissolved oxygen, high phosphate concentration, and low TN:TP were found to be contributing factors on the equilibrium of the 𝛿[superscript]18O of phosphate results. The sources of natural and anthropogenic phosphate can be a result of lake stratification, phosphate movement in the sediment and groundwater or inflow from the surrounding watershed.