Variation beneath the surface: Quantifying complex thermal environments on coral reefs in the Caribbean, Bahamas and Florida

Abstract

Analysis of in situ temperature records collected on six coral reefs in the Caribbean, Bahamas, and Florida Keys reveal significant variability across a range of temporal and spatial scales from minutes to seasons, across depths, and among sites. Subsurface variability occurring at daily and faster frequencies is prevalent across the region, likely driven by combinations of diurnal heating and cooling, wind driven advection, and internal waves at tidal and faster frequencies. This high frequency variability is not detected in records of remotely-sensed sea surface temperature alone. Diurnal variability likely caused by diurnal solar heating and cooling and possibly by advection associated with diurnal winds (daily sea breeze) was significant at all sites and showed greatest magnitude of variation at shallowest depths. Temperature fluctuations at tidal and faster frequencies were common at 5 out of the 6 sites. The magnitude of this variability is not well explained by measured vertical temperature stratification combined with oscillations of the water column associated with barotropic surface tides. Rather, the magnitude and nature of the temperature changes point to the presence of internal waves generated at tidal and faster frequencies. Power spectra calculated seasonally show greatest variability within both diurnal and semi-diurnal frequency bands in Spring and Summer at Florida, Bahamas, Jamaica, and St. Croix. Variability within the semi-diurnal frequency band at Belize and Bonaire was greatest in Winter. Warming in Summer estimated as degree-hours per day above 29.0°C increased with increasing latitude and varied significantly among sites and depths in a manner not predictable from remotely sensed SST data alone. Site latitude was directly related to the amplitude of the seasonal thermal variability, but was not tightly related to variability at daily and faster frequencies which was greatest at the highest and lowest latitude sites. The interactions of depth, site, and season across the study region are associated with distinct signals of thermal variability, and have significant implications for the physiology and ecology of corals and other reef organisms.

The physiological response of reef corals to diel fluctuations in seawater temperature

“…Our study was motivated by the observation that coral communities can flourish in thermally unstable habitats (Craig et al, 2001;Lee et al, 1999;Leichter et al, 2006;Sheppard, 2009), including the back reef of Moorea, and had the objective of contrasting the effects of fluctuating and steady temperatures on two species of corals. We went to great lengths to create fluctuating temperature treatments with temporal replication, but the logistical challenges of this task resulted in a time effect preventing an analysis replicated in time.…”

“…The widespread use of remotely sensed SST to characterize seawater temperatures, specifically those associated with coral bleaching (e.g., Gleeson and Strong, 1995) may not detect aspects of the underwater thermal regime that are also important to corals (Brown, 1997b;Leichter et al, 2006;Sheppard, 2009). Critically, SST in reef environments routinely differs from underwater measurements by as much as ≈3°C (Leichter et al, 2006), and sub-surface temperatures are more dynamic over short periods.…”

“…Critically, SST in reef environments routinely differs from underwater measurements by as much as ≈3°C (Leichter et al, 2006), and sub-surface temperatures are more dynamic over short periods. For instance, underwater temperature fluctuates rapidly (i.e., up to~5°C in b24 h) throughout the Florida Keys, the Bahamas, St. Croix, Belize, and Bonaire (Leichter et al, 2006), largely as a result of diurnal warming in shallow water (10 m), and tidal forcing and internal waves at greater depths (20-30 m).…”

“…Critically, SST in reef environments routinely differs from underwater measurements by as much as ≈3°C (Leichter et al, 2006), and sub-surface temperatures are more dynamic over short periods. For instance, underwater temperature fluctuates rapidly (i.e., up to~5°C in b24 h) throughout the Florida Keys, the Bahamas, St. Croix, Belize, and Bonaire (Leichter et al, 2006), largely as a result of diurnal warming in shallow water (10 m), and tidal forcing and internal waves at greater depths (20-30 m). Daily fluctuations as large as 10°C also affect the near-shore, shallow reefs of Oman and Taiwan (Coles, 1997;Lee et al, 1999), and daily fluctuations of~6°C are common in the lagoon and over fringing reefs (1-10 m depth) in American Samoa and other Pacific islands (Brown, 1997b;Craig et al, 2001).…”

Physical environments of the Caribbean Sea

“…The environment of the Caribbean Sea was defined in terms of sea surface temperature, water clarity, salinity, wind-driven wave exposure, and hurricane incidence. Surface physicochemical variables may differ from those measured at the subsurface (Leichter et al 2006). The magnitude of these differences depends on local atmospheric and oceanographic conditions, particularly the level of stratification of the water column, which is spatially and temporally variable (Donlon et al 2002).…”