AGU/ASLO 1994: Lascara

Seasonal Changes in the Mesoscale Distribution of Antarctic Krill (Euphausia superba) in waters West of the Antarctic Peninsula

Cathy M. Lascara

Center for Coastal Physical Oceanography. (Old Dominion University)

  • Objectives
    • MAP: Space-Time Distribution of Krill using BioAcoustics
    • INTERPRET: Krill Patterns relative to other Habitat Characteristics
      • Concentration and Composition of Food Resources
      • Ice History
      • Large-scale Flow Regimes
      • Hydrographic and Optical Properties of Upper Ocean
  • Acoustics Field Collection
    • Single Frequency, 120 kHz
    • Replicate (n=2,3), short (1-3 km) acoustic tows centered on station
    • Coincident with oblique zooplankton net tow.
  • Generate 2-D matrix of krill biomass (g/m3)
    • Binned Vertically into 2-m strata from 6 to 185 m depth
    • Binned Horizontally into 3-ping records (2-8 meters)
  • Compute vertically-integrated mean biomass (g/m2) for
    • Each acoustic event
    • Each station (pool replicates)
    • Each cruise (pool all events)
  • Seasonal Characterization:
    • Mean biomass by season
      • Spring - 38 g/m2
      • Summer - 65 g/m2
      • Fall - 5 g/m2
      • Winter - 2 g/m2
    • Percent Positive Events
      • Spring - 90%
      • Summer - 90%
      • Fall - 40%
      • Winter - 15%
    • Swarms per kilometer
      • Spring - 5
      • Summer - 10
      • Fall - 1
      • Winter - 1
  • Spring: November 1991 (n=15 stations)
    • Range of biomass = 0-95 g/m2
    • 4 Stations with Biomass > 50 g/m2
      • Inside or Near Dallman Bay, coincident with high concentrations of fucoxanthin
      • Offshore over Circumpolar Deep Water (> 1.5 C)
      • Adjacent to Ice Edge
  • Summer: Jan-Feb 1993 (n=39 stations)
    • Range of biomass = 0-460 g/m2
    • 9 Stations with Biomass > 100 g/m2
    • Mesoscale hydrography characterized by two major regions
      • Coastal regime - stratified surface waters influenced by meltwater inputs (seasonal water mass)
      • Oceanic regime - isohaline upper ocean overlying Circumpolar Deep Water
    • Strong relationships between hydrographic structure and
      • Highest concentrations of krill.
      • Length frequency and maturity stage of krill.
      • Relative abundance of salps and pteropods.
  • Fall: Apr-May 1993 (n=100 stations)
    • Range of biomass = 0-44 g/m2
    • 4 Stations with Biomass > 30 g/m2
    • Dramatic reduction in krill biomass from summer (3mos)
    • High biomass stations isolated from each other
    • Coastal regime with stratified surface waters extends further offshore overlying Circumpolar Deep Water for most of survey grid
  • Winter: Aug-Sep 1993 (n=33 stations)
    • Range of biomass = 0-80 g/m2
    • 1 Stations with Biomass > 10 g/m2
    • Annual sea-ice covered almost entire survey grid.
    • Krill absent from open water below sea ice at 80% of stations. Divers observed krill associated with ice.
    • Large swarms observed at inshore station on two separate days.
  • Summary
    • Acoustically-detectable krill biomass varied greatly between seasons, with high concentrations during spring and summer followed by low concentrations during fall and winter
    • During the summer, strong relationships were observed between the mesoscale hydrographic structure and several biological distributions.
    • The spatially averaged krill biomass decreased by an order of magnitude over a 2-month period between summer and fall 1993, despite prescence of stratified surface waters in which krill were so abundant during the summer.