Measurements of Internal Tides in the Vicinity of Mesophotic Ecosystems in the US Virgin Islands

Mesophotic coral ecosystems (MCE), found at depths of 30 - 150 m, are important ecosystems as they have a high diversity of shallow water and endemic organisms. Moored bottom temperature records at MCEs in the US Virgin Islands show strong variability at frequencies higher than sub-inertial. Additional time series at multiple water column depth show the vertical and seasonal extent of these fluctuations. Nearly stationary time series of ocean glider profiles just offshore of MCEs show strong internal waves with M2 semidiurnal periodicity at and below the upper mixed layer pycnocline, present in three records in November 2023, November 2024, and February 2025. The presence of these waves at the MCE sites would account for much of the variability seen in long records. Lack of variability seen in MCE bottom temperature records during cold moths is due to thermocline deepening and the MCE remaining above the thermocline depth. During warm (shallower thermocline) months, the waves bring semidiurnal thermal relief to the MCEs. Contemporaneous glider measurements of Dissolved oxygen and chlorophyll also suggest that the waves can provide nutrient input to the MCEs as well. The semidiurnal waves are consistent with those seen in a regional ROMS model to be generated by interaction between open ocean barotropic tides and the steep passages into the Northeastern Caribbean Sea.