Stormwater infrastructure as landscape hydrology: Contrasting runoff-fed and groundwater-fed urban ponds reveals design-driven harmful algal bloom risk

Residential stormwater ponds are common features of urban and suburban landscapes, yet their vulnerability to harmful algal blooms (HABs) remains insufficiently understood despite their close proximity to residential communities. This study examined how stormwater routing pathways and pond water-level dynamics influence underwater light conditions and subsequent bloom development, to inform stormwater management strategies beyond nutrient source control. From July to November 2024, we monitored four groundwater-fed residential stormwater ponds and one surface-runoff-fed lake in Florida, USA, during a hydrologically active, warm season. Field observations were integrated with geochemical, optical, and microbiological measurements, including chlorophyll-a, water level, cyanobacterial community composition, nutrients, calcium, and colored dissolved organic matter (CDOM), to identify environmental conditions associated with bloom occurrence. Substantial HABs developed exclusively in the groundwater-fed ponds, where chlorophyll-a concentrations reached up to 21 times Florida's algal bloom threshold and were dominated by toxin-producing genera such as Microcystis and Dolichospermum. In contrast, no blooms were observed in the runoff-fed lake despite similar nutrient concentrations. Groundwater-fed ponds consistently exhibited lower CDOM and shallower, hydrologically stable water columns, resulting in enhanced underwater light availability that promoted rapid phytoplankton growth. These results indicate that stormwater designs that increase groundwater connectivity and maintain clear, shallow, and stable water columns can elevate HAB susceptibility independent of nutrient enrichment. Bloom risk should therefore be recognized as an emergent consequence of hydrologic routing and optical conditions, underscoring the need for management approaches that reduce light exposure, avoid prolonged stable stages, and support routine monitoring and risk communication in residential environments.