Lagrangian description of 2-D marine larval dispersal kernels Karen Pehrson Edwards, Jon Hare, Cisco Werner, Harvey Seim Ocean Sciences, Feb. 2006
Outline • Background: Dispersal Kernels • Model Region: The South Atlantic Bight • Methods: Lagrangian Particle Tracking • Results: Quantifying the dispersal kernels • Next steps Provided by Harvey Walsh, NOAA Beaufort
1D dispersal kernel -Passive, Mean Flow- Diffusion Dispersal Kernels • A dispersal curve, or dispersal kernel (probability density function), the probability that a larva will settle at a given distance from its release location. Cowen et al. (2002) • Factors affecting dispersal include: • Physical factors: • Advection & diffusion • Intermittent/ episodic events, • interannual variability, • long-term modulations • Biological factors: • Spawning time & location • Larval behavior • Along-trajectory mortality • Larval condition and the ability to respond to cues for suitable habitat. Cowen et al. (2002), Mora & Sale (2002) Provided by Harvey Walsh, NOAA Beaufort
Understanding larval dispersal provides important information about: • Population connectivity • Population persistence or sustainability • Effectiveness and design of management strategies such as MPAs Cowen et al, 2002
Question: what is the relative importance of the following factors on larval dispersal and population connectivity? • time of release, • location of release, • larval duration, • larval behavior, and • larval dispersion • Focusing on a case study in the South Atlantic Bight
Cape Hatteras NC SC GA Cape Canaveral FL The South Atlantic Bight Gulf Stream
The South Atlantic Bight Inner Shelf (<15-20m): • River runoff • Atmospheric fluxes and tides Mid- Shelf (20-40m): • Winds • Tides and Gulf Stream Outer- Shelf (>40m): • Gulf Stream http://www.skio.peachnet.edu/ Lee et al. 1991
Methods: Lagrangian Particle Tracking Model • 3D long-term monthly mean circulation fields (Blanton et al., 2003) • Plus the M2 tide • and turbulent random kick (providing larval dispersion). • 300 particles released in each model run.
Results: GRNMS Dispersal • Monthly releases • Mid-depth releases • 30d passive • Principal Component Analysis (PCA) defines the dispersal kernel.
Larval Behavior Resulting mean dispersal for each behavior: Simple Model Behaviors
Larval Behavior Dispersal for particles released at 60m isobath: Mid-depth Shallow Deep
Physics Tracks of drifters released at GRNMS • Model 2002-2004 to look at interannual variability
www.martinreefs.com/pages/species_index/species_common.html Adult Black Sea Bass Species Occurrence Hare et al., 1999 http://map.mapwise.com/safmc MARMAP survey Modeling specific species
Dispersal in the SAB • Dominant factors: Time and Location of Release • with strong seasonal modulation • Behavioral Implications • Adult behavior (spawning time & location): • most important factor in quantifying dispersal with spawning in (apparently) retentive inner- to mid-shelf • Larval behavior: smaller effects possibly due to • Weak vertical structure on inner-shelf • Long-term (average) flow fields don’t account for transient features.