Development and validation of a DIY profiling float for indirect determination of salinity

Abstract

Ocean salinity, a measure of salt concentration in seawater, is a key variable influencing water density and controls important processes like mixing and stratification. Temporal and spatial salinity variability is high in dynamic coastal systems like Puget Sound where salinity can range annually from 22 to 32 PSU. This variability occurs over small spatial scales and presents a challenge for obtaining high resolution salinity measurements. Salinity is typically measured using conductivity cells, a sensor that quantifies electrical current through seawater. These sensors can be deployed on ship-based CTDs, spatially-limited moorings, or costly AUVs, but can have limited long-term stability due to sensor drift and biofouling. This project assessed the possibility of salinity calculation from buoyancy-driven vehicles, culminating in the development and validation of a cost-efficient buoyancy-driven profiling float, dubbed openFloat, for the indirect determination of salinity. The openFloat measures in situ temperature and pressure through electronic sensors and determines water density at a given depth by achieving neutral buoyancy (e.g., float density equals water density) by adjusting its volume. Salinity can be determined by way of back calculation using the equation of state of seawater using these three known parameters. This proposed method of measuring salinity was tested in situ through multiple deployments of a buoyancy-driven Seaglider in Colvos Passage, WA (Puget Sound). Salinity was determined at the deepest point in the Seaglider’s dive when it achieves neutral buoyancy and salinity is not recorded due to unstable flight conditions. Using Seaglider buoyancy parameters, salinity could be estimated to ± 0.18 PSU in situ, and with the openFloat salinity could be estimated to ± 0.11 PSU in the test pool, demonstrating the proof of concept needed for a more cost-effective option like the openFloat in dynamic systems where the range in salinity is large. Salinity calculation with openFloat can enable the large-scale Salinity determination from buoyancy-driven devices production of floats to obtain the high-resolution spatial and temporal data needed to better quantify change in dynamic coastal systems.