The tattered curtain hypothesis revised: Coastal jets limit cross-shelf larval transport

Abstract

Exchange and retention of coastal waters modulate dispersal of marine larvae, affecting marine ecosystem dynamics. A hypothesis was put forward in the 1980s describing the coastal upwelling front as a “tattered curtain” that retains larvae. This front was envisioned to be broken up by squirts and eddies, hitting the coast under upwelling relaxation events. Here we revise this hypothesis by using an idealized ocean model of an eastern boundary upwelling current, and an idealized particle/larvae model appropriate for shelf-spawning benthic species. Modeled larval settlement patterns were controlled by retention in the core of the upwelling jet, bounded by regions of high-velocity shear on the flanks of the jet. Squirts, filaments, poleward-moving eddies, and meanders modulated settlement patterns locally, while dense packets moved equatorward within the upwelling jet. Correlation between settlement (i.e., particles 20–40 d old <10 km from shore) and wind was low for a lagged wind product (r=0.33) and moderate for a 20-d integrated wind product (r=0.62). We determined that it is not upwelling relaxation but sustained, moderate upwelling that can result in a highly retentive jet that entrains larvae and acts as a barrier to cross-shelf transport; however, the amount of retention is highly variable. Settlement was low after strong, persistent upwelling completely tattered the jet. Jet cores in general should act as important retentive transport barriers across diverse coastal systems, a view supported by dynamical theory, modeling studies, and larval recruitment observations.