Temperature Inverted Haloclines Provide Winter Warm-Water Refugia for Manatees in Southwest Florida

Abstract

Florida manatees (Trichechus manatus latirostris) overwintering in the Ten Thousand Islands and western Everglades have no access to power plants or major artesian springs that provide warm-water refugia in other parts of Florida. Instead, hundreds of manatees aggregate at artificial canals, basins, and natural deep water sites that act as passive thermal refugia (PTR). Monitoring at two canal sites revealed temperature inverted haloclines, which provided warm salty bottom layers that generally remained above temperatures considered adverse for manatees. At the largest PTR, the warmer bottom layer disappeared unless significant salt stratification was maintained by upstream freshwater inflow over a persistent tidal wedge. A detailed threedimensional hydrology model showed that salinity stratification inhibited vertical convection induced by atmospheric cooling. Management or creation of temperature inverted haloclines may be a feasible and desirable option for resource managers to provide passive thermal refugia for manatees and other temperature sensitive aquatic species.

Passive thermal refugia provided warm water for Florida manatees during the severe winter of 2009−2010

“…The TTI region included the 2 PTR studied by Stith et al (2011), both associated with dredged canal systems southeast of Naples; one at Port of the Islands (POI), and the second at Big Cypress National Preserve Headquarters (BCNP) (Fig. 2) The western coastal Everglades region has a nearly continuous coastline with 5 large rivers connecting the Gulf of Mexico to an extensive set of inner bays, the largest being Whitewater Bay at the southern end.…”

“…As a continuation of prior research (Stith et al 2011), we monitored the temperature and salinity at POI and BCNP for 2 additional winters (2008−2009 and 2009−2010) to characterize the vertical temperature and salinity regime. At each site we deployed 2 YSI 600 XLM water quality sondes within 15 cm of the surface using a floating mount, and the other within 15 cm of the bottom.…”

“…Recently Stith et al (2011) studied 2 PTR used by manatees in extreme southwestern Florida and identified the hydrological mechanism that maintained warm water temperatures. The PTR were part of canal systems that received freshwater from upstream and interaction with tidal waters downstream (Fig.…”

“…An understanding of the mechanism that maintains the thermal properties raised the possibility of managing or creating halocline-driven PTR (Stith et al 2011). However, lack of hydrological data for winters with severe cold periods left open the possibility that factors such as tidal mixing, wind, varying freshwater inputs, or conduction could disrupt the temperature inversion during severe cold.…”

“…The unprecedented cold of winter 2009−2010 provided data to evaluate the hydrodynamics under extreme conditions. Stith et al (2011) also noted evidence for a source of bottom heat, indicated by multi-day periods when the bottom layer was warming while the surface layer was cooling, but the source of heat was not determined. We examined additional hydrological data, including canal tidal stage and groundwater level, to look for potential hydraulic head differences and evidence that tidal pumping of groundwater could be a source of heat.…”