In situ comparisons of glider bio-optical measurements to CTD water properties

ANFOG has completed over 175 glider missions, making sustained oceanographic observations around Australia. While conductivity, temperature, depth (CTD) data from gliders are highly accurate and readily interpreted, there is a... [ view full abstract ]

ANFOG has completed over 175 glider missions, making sustained oceanographic observations around Australia. While conductivity, temperature, depth (CTD) data from gliders are highly accurate and readily interpreted, there is a great need to improve our understanding of seawater bio-optical properties and to validate bio-optical sensor measurements. Despite their importance, direct in situ comparisons between bio-optical instruments (such as the glider Ecopuck) and water sample measurements are rare. We aimed to increase our understanding of bio-optical data by comparing Ecopuck data (such as fluorescence and backscatter) from the Slocum glider with water samples collected simultaneously by Niskin bottle.
In January 2015 we attached a Slocum gilder to a Seabird CTD rosette and made 19 casts on a cruise between Exmouth and Darwin. Seawater was sampled at the surface, the bottom and at the chlorophyll maximum and analysed for photosynthetic pigments, particulate suspended matter and phytoplankton community composition as determined by CHEMTAX. The glider simultaneously measured CTD and bio-optical parameters (fluorescence, backscatter, irradiance and CDOM).
Phytoplankton communities were remarkably similar across the sampling area, with most chlorophyll biomass occurring as deep chlorophyll maxima at the thermocline. Prasinophytes, haptophytes and cyanobacteria (e.g. Prochlorococcus and Synechococcus) dominated inshore (the Montebello Islands) and mid shelf regions that included a tidal front west of Karratha and a cyclonic eddy north of Broome. Diatoms dominated the communities only at 3 sites west of Broome. Otherwise diatoms, dinoflagellates, and cryptophytes were minor components of the community. Niskin bottle chlorophyll concentrations were generally less than 1 mg m-3. However, biomass reached values of 1 - 2 mg m-3 where diatoms were abundant. Chlorophyll a estimates by the Ecopuck correlated well with bottle chlorophyll (r2 = 0.81), but relationships between Ecopuck backscatter and organic and inorganic suspended matter were poor.
Bio-optical instruments are now widely deployed on gliders, moorings and Bio-Argo floats around the world, and it is critical we improve our understanding of these bio-optical data. Our study provides useful biological insights into bio-optical data and ocean productivity and provides empirical data for modellers. We recommend similar studies be incorporated into transit voyages around Australia.