Free-rising, tethered CTD profiler: increased vertical resolution and near surface profiling

The formation of a diurnal thermocline at few meters depth is difficult to document because of the mixing induced by research platforms. We built a free‐rising CTD‐platform, including sensors for chlorophyll fluorescence, turbidity, and light, that profiles to ~0.4 m below the surface with better than 0.03 m resolution and at sufficient distance from the research platform to measure an undisturbed water column. The profiler, with a practical maximum depth of 100 m is controlled mechanically using sacrificial weights and a safety tether that assures easy recovery of the instrument. The design concept is easily adapted to many commercial CTD models. Off Baja California, most profiles showed temperature stratification near the surface and some profiles showed near surface chlorophyll fluorescence thin layers; these features would be difficult to observe with normal CTD profilers.