Decreasing Groundwater Supply Can Exacerbate Lake Warming and Trigger Algal Blooms

As groundwater depletion becomes a global phenomenon, inland lake ecosystems are being impacted by decreasing groundwater supply. While the current trend of rapid surface warming of inland lakes continues, the deep waters can resist changes, depending on the nature of surface water - groundwater interactions. However, the effects of these interactions on lake processes are not fully understood. Here we investigate the role of groundwater on coupled biophysical processes in a deep, dimictic, groundwater-fed lake using mechanistic models combined with data from field observations. Although excess nutrient inputs are the most commonly cited reason for algal blooms, here we show that algal blooms in inland lakes can also appear due to a decreasing groundwater supply while all other factors remain the same. Results indicate that decreasing groundwater supply to lakes leads to elevated hypolimnetic temperatures, enhanced algal growth rates and algal blooms, and oxygen depletion, thus exacerbating the negative effects of surface warming. Our work suggests that globally declining groundwater supplies may have a significant negative effect on water quality of inland lakes by accelerating water column warming and stimulating algal growth, especially when the groundwater contribution to the lake system is significant compared to riverine discharge. The work provides insights for management efforts to improve the resilience of groundwater-dependent ecosystems in the face of external stressors.