AbstractWith increasing urbanization, urban lakes are becoming more common and serve as important flood control infrastructure, recreational uses, as well as habitat for a variety of biota. However, given their construction, function and location, urban lakes are susceptible to disturbances including stormwater runoff from metropolitan catchment areas. For example, nutrient loading from stormwater runoff can cause eutrophication. Chlorophyll‐a concentration is an important water quality parameter because it is used to assess water quality and determine trophic state. Chlorophyll‐a is known to be impacted by a number of environmental factors including precipitation and associated runoff. Previous studies indicate that the relationship between precipitation and chlorophyll‐a is complex and often site‐specific. In this study, we examined the relationship between chlorophyll‐a and precipitation in an urban, tropical lake located in East Kalimantan Province, Indonesia. From October through December 2022, chlorophyll‐a, turbidity, phosphate (PO4), pH, dissolved oxygen, temperature, total dissolved solids and conductivity were measured at five sampling sites in Air Hitam Lake, and precipitation was measured adjacent to the lake. These data indicated that chlorophyll‐a concentrations were negatively correlated with recent precipitation (r2 = .71, p < .05). This relationship was likely a result of dilution from increased lake volume. Although urban runoff is expected to increase nutrient loading, our monitoring indicated that dilution can mediate this process in the short‐term. Our findings suggest that timing matters when monitoring water quality in tropical, urban lakes. Sampling immediately following heavy precipitation and stormwater runoff can lead to an underestimation of normal chlorophyll‐a concentrations due to dilution. As a result, dilution effects should be examined when studying and managing urban lentic ecosystems with strong precipitation dynamics. Further studies are necessary to better understand these urban aquatic ecosystems to continue to improve future management efforts.

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