Intertidal spring discharge to a coastal ecosystem and impacts of climate change on future groundwater temperature: A multi-method investigation

Abstract

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.

Influence of precipitation event magnitude on baseflow and coastal nitrate export for Prince Edward Island, Canada

“…The assumed similarity between groundwater discharge to streams and directly to the ocean (Figure 2) has testable implications for surface hydrology on the island. Previous local studies at two sites on the island (Danielescu et al, 2009; KarisAllen et al, 2022) indicated that a large portion and potentially the majority of freshwater groundwater discharge to the PEI coastline occurs in a focussed manner via springs, as opposed to diffuse discharge. While no quantitative data on the contribution of focussed groundwater discharge to streams is available on an island‐wide scale, studies have also reported the widespread presence of springs across the island (Brandon, 1966) and in individual watersheds (Francis, 1989).…”

“…Based on the assumption detailed above, available hydrometric data between 2000 and 2019 for six PEI watersheds were analysed to evaluate the effects of precipitation event magnitude on groundwater discharge and nitrate loading to streams, and by extension, on SGD to coastal waters. Such SGD has been shown to significant in PEI coastal zones (e.g., Danielescu et al, 2009; KarisAllen et al, 2022; KarisAllen & Kurylyk, 2021). For example, 13% of the island's land area is closer to the coast that to any mapped (Government of PEI, 2011) stream (Figure 1b), favouring direct groundwater discharge to the coast from these areas.…”

“…Springs are common in PEI because of the fractured bedrock aquifer setting (Brandon, 1966). A study of small PEI estuaries (Danielescu et al, 2009) estimated direct groundwater discharge (dominated by springs) to be 15% of stream inputs, while a similar study in a coastal lagoon in PEI (KarisAllen et al, 2022) found that the cumulative direct focussed groundwater discharge from intertidal springs was comparable to the total freshwater inflows from streams. Lastly, PEI lies in the area affected by Atlantic hurricanes and associated heavy rainfall events with seven named hurricanes/posttropical storms passing over the island between 2001 and 2019 (Knapp et al, 2010(Knapp et al, , 2018.…”