Drivers and Patterns of Temperatures in Coastal Waters

Abstract:

Many recent hydrology and biology studies have focused on modern and projected warming rates for inland waters and the ramifications for cold-water biodiversity. Concurrent oceanography work has addressed long-term warming and episodic marine heat waves with a focus on ecosystem impacts and sea-level rise implications. These segregated disciplines have often ignored the thermal regimes of coastal water bodies that lie within the transition zone between oceans and inland waters. Coastal water thermal regimes are complex and driven by interplay between atmospheric heat fluxes, heat advection and dispersion from coastal hydrodynamics, and the thermal regimes of up-gradient freshwater aquifers and rivers.

In this study, we present water temperature data from lagoons and estuaries in Prince Edward Island and Nova Scotia, Canada. Spatial thermal diversity within these coastal waters is revealed through drone-assisted thermal infrared imagery. Inter-tidal springs are dominant drivers of small-scale thermal patchiness and may provide thermal refuges for species that behaviourally thermoregulate. Temperature time series from loggers installed at different depths within the water column and at varying positions along the channel reveal dynamic thermal interactions between signals of differing phase and frequency due to solar radiation and tidal action. Frequency domain analyses indicate that most of our study locations exhibit a dominant 24-hour temperature signal as well as a secondary 12.42-hour tidal temperature signal. We propose the adoption of a thermal ratio that captures the relative dominance of diurnal and semi-diurnal thermal signals. Among other factors, the relative importance of tidal thermal signals depends on the water depth, distance from the coastal inlet, channel geometry, freshwater inflow, and tidal range. We propose that the thermal ratio can be applied in conjunction with other information to identify thermally resilient coastal water bodies dominated by cold-water tidal inflow, as well as thermally sensitive coastal waters that may impose future thermal barriers for anadromous fish. The thermal data will help form conceptual and numerical models of coastal thermal regimes to assess the impacts of marine (i.e., sea-level rise and ocean warming) and atmospheric forcing on coastal thermal regimes.