Present and future thermal regimes of intertidal groundwater springs in a threatened coastal ecosystem

In inland settings, groundwater discharge thermally modulates receiving surface water bodies and provides localized thermal refuges; however, the thermal influence of intertidal springs on coastal waters and their thermal sensitivity to climate change are not well studied. We addressed this knowledge gap with a field- and model-based study of a threatened coastal lagoon ecosystem in southeastern Canada. We paired analyses of drone-based thermal imagery with in situ thermal and hydrologic monitoring to estimate discharge to the lagoon from intertidal springs and groundwater-dominated streams in summer 2020. Results, which were generally supported by independent radon-based groundwater discharge estimates, revealed that combined summertime spring inflows (0.047 m(3) s(-1)) were comparable to combined stream inflows (0.050m(3) s(-1)). Net advection values for the streams and springs were also comparable to each other but were 2 orders of magnitude less than the downwelling shortwave radiation across the lagoon. Although lagoon-scale thermal effects of groundwater inflows were small compared to atmospheric forcing, spring discharge dominated heat transfer at a local scale, creating pronounced cold-water plumes along the shoreline. A numerical model was used to interpret measured groundwater temperature data and investigate seasonal and multi-decadal groundwater temperature patterns. Modelled seasonal temperatures were used to relate measured spring temperatures to their respective aquifer source depths, while multi-decadal simulations forced by historic and projected climate data were used to assess long-term groundwater warming. Based on the 2020-2100 climate scenarios (for which 5-year-averaged air temperature increased up to 4.32 degrees), modelled 5-year-averaged subsurface temperatures increased 0.08-2.23 degrees in shallow groundwater (4.2 m depth) and 0.32-1.42 degrees in the deeper portion of the aquifer (13.9 m), indicating the depth dependency of warming. This study presents the first analysis of the thermal sensitivity of groundwater-dependent coastal ecosystems to climate change and indicates that coastal ecosystem management should consider potential impacts of groundwater warming.

Automated summary

This study investigates the thermal influence of intertidal springs on coastal waters and their thermal sensitivity to climate change through field and model-based research. The results show that intertidal springs dominate heat transfer at a local scale, creating pronounced cold-water plumes along the shoreline. Additionally, a numerical model was used to analyze seasonal and multi-decadal groundwater temperature patterns and predict future groundwater warming.