Background Information

Understanding and predicting how the Southern Ocean responds to change is a globally relevant issue that requires circumpolar scale analyses.

Sustained multidisciplinary observations are required to detect, interpret and respond to change. Key is an understanding of baseline levels of ocean circulation and hydrography, and primary and secondary productivity, as it is from these baseline levels that change can be identified.

Through global observation programmes, in particular the WOCE (World Ocean Circulation Experiment), syntheses of global observations have been undertaken for temperature, salinity, oxygen concentration and surface chlorophyll. However, despite their wide spatio-temporal distribution, huge abundance, and crucial importance for ocean biogeochemical cycling and the distribution and abundance of top predators, information on the large scale distribution of mid- trophic levels has lagged behind.

Mid-trophic level organisms range in size from small plankton (<2 cm), which drift with currents, to larger nekton (>10cm), which have the ability to swim freely. They are a taxonomically diverse group that include cephalopods, gelatinous organisms, euphausiids and mesopelagic fish. They play a critical role in Southern Ocean ecosystems by transferring biomass and energy through the food web from primary producers to predators.

Antarctic food web – Image courtesy of British Antarctic Survey (BAS).

Acoustic methods offer a means to collect high resolution data over small to large spatial and temporal scales of key mid-trophic level species and/or functional groups from a number of different platforms. However, acoustic measurements cannot be used without standardisation of systems, appropriate validation and quantification of basic relationships between biological variables of interest and acoustic energy.

To that end a systematic and long-term strategy to establish an acoustic sampling programme including the implementation of standards and protocols for basin-scale ecosystem comparisons coordinated through international organisations and programmes is required. The Southern Ocean Network of Acoustics (SONA) aims to address this.