Acoustic Doppler Current Profiler (ADCP) records from multiple “L.M. Gould” (LMG) supply transits of Drake Passage to the Antarctic Peninsula from 1999 to present demonstrate spatial and temporal variability in acoustics backscattering (e.g., SONAR). Acoustics backscattering strength in the upper water column corresponds to zooplankton and nekton biomass that relates to seabird and mammal distribution and abundance. This project added biological sampling to the LMG time series with the expectation that backscattering data, calibrated with net-tow data, depth-referenced underwater videography and seabird/mammal visual surveys, could be used to extend the spatio-temporal coverage of environmental sampling in Drake Passage.  

Biological sampling was conducted during 3 cruises (LMG14-10, October-November 2014; LMG15-04, April-May; LMG15-09, October-November, 2015).  Small sized zooplankton taxa dominated samples north of the Polar Front, larger zooplankton dominated in the Antarctic Coastal Zone (Figs. 1, 2), and variable mixtures of large and small taxa occurred in the Antarctic Zone between the Antarctic Peninsula and Polar Front.  Myctophids, fish that are major components of the vertically migrating (nekton) deep scattering layer and important predators of zooplankton, were concentrated near the Polar Front.  Marine mammals (fin and humpback whales, Antarctic fur seals) were most abundant in waters off of the Antarctic Peninsula, particularly during LMG15-04 when we encountered large concentrations of Antarctic krill, a major prey item.  During all transits greatest numbers of sea birds were observed in coastal waters off the tip of South America (Cape Horn) and the Antarctic Peninsula in association with feeding and nesting areas.

Net samples, coincidental underwater videorecordings and ADCP records all demonstrated that greatest concentrations of zooplankton occurred in the upper waters of Subantarctic and Polar Frontal Zones north of the Polar Front (Fig. 3, 4).  These results conform to long term ADCP records which indicate significantly more backscattering organisms here compared to the Antarctic Zone south of the Polar Front. Over the 1-year study period, small sized zooplankton abundance (specifically copepods, chaetognaths and shelled pteropods) increased significantly in the Antarctic Zone; coincidental increases in ADCP backscattering strength occurred here and to a lesser extent north of the Polar Front (Fig. 4).  The zooplankton abundance increase coincided with increased primary production across Drake Passage as indicated by satellite-derived maps of sea surface Chlorophyll-a concentrations during spring (November) 2014, fall (March ) 2015 and spring (November) 2015 (Fig. 5A) and the presence of dense Phaeocystis phytoplankton in net samples during fall and spring 2015 (LMG15-04, LMG15-10; Fig. 5B).  This increased primary and secondary productivity followed a period characterized by cool La Niña conditions and extensive winter sea ice development and in conjunction with reduced westerly winds and northward movement of the Polar Front.  These results are consistent with interannual fluctuations of backscattering strength in the long term ADCP record that have been shown to be strongly correlated with atmospheric influences such as the El Nino Southern Oscillation (ENSO) and the Southern Annular Mode (SAM) which describes the north–south movement of the westerly wind belt that circles Antarctica.  Together the biological sampling and underway ADCP records demonstrate large scale atmospheric-oceanic coupled processes underlie Southern Ocean ecosystem productivity.

Specific significant rewards from this effort include:

 (1)  augmentation of scientific output from costly ship time invested in Drake Passage supply transits by acquiring biological information coincidental with routinely collected physical data,

(2)  establishing basic information on the taxonomic composition and abundance of zooplankton/nekton assemblages across Drake Passage, an area that has generally remained biologically under-sampled since the 1925-36 British “Discovery Expeditions”,

(3)  documenting seasonal changes in the composition and distribution of seabird/mammal assemblages represented across Drake Passage and the Antarctic Coastal Zone,

(4)  incorporation of underwater videorecordings with net sampling that provides critical information on the depth distribution of  “light scatterers” that correspond to sound scatterers recorded by the ADCP,

(5)  proof of concept that the addition of biological sampling to coincidental physical monitoring during Drake Passage transits can provide a great deal of insight into seasonal and between-year changes in Southern Ocean ecosystem structure with respect to underlying coupled atmospheric-oceanic processes.

Unexpected discoveries include, but are not limited to, the following:

(1)  observations suggesting the importance of the Cape Horn Current  off the tip of South America in transporting large concentrations of phytoplankton and zooplankton (particularly the early life stages of euphausiids and fish) from the southeast Pacific into the south Atlantic,

(2) observations suggesting the importance of seasonal changes in the abundance and maturity stage composition of myctophids that, in conjunction with ADCP records, may be used to monitor interannual and longer term variability in primary productivity-related reproduction and recruitment success.

Last Modified: 12/30/2016
Modified by: Valerie J Loeb