Reply on RC2

Abstract

In inland settings, groundwater discharge is known to thermally modulate receiving surface water bodies and provide localized thermal refuges; however, the thermal influence of intertidal springs on coastal waters and the thermal sensitivity of these springs 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 south-eastern Canada. We paired in-situ thermal and hydrologic monitoring with analyses of drone-based thermal imagery to estimate the 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 the combined summertime spring inflows (0.047 m3 s-1) were comparable to the combined stream inflows (0.050 m3 s-1). Heat flux analyses indicated that the net advection for the streams and springs were also comparable to each other but were two orders of magnitude less than the downwelling shortwave radiation across the lagoon. Although the 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 investigate seasonal and multi-decadal groundwater temperature patterns to relate measured spring temperatures to their respective aquifer source depths, and to consider long-term groundwater warming. Based on the different climate scenarios used for 2020 to 2100 (5-year averaged air temperature increase up to 4.32 °C), modelled 5-year averaged subsurface temperatures increased 0.08 to 2.23 °C in shallow groundwater (4.2 m depth) and 0.32 to 1.42 °C 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 the potential impacts of groundwater warming.