Estuarine fronts as conduits for larval transport:hydrodynamics and spatial distribution of Dungeness crab postlarvae

Abstract

Frontal zones are common, hydrographic features in estuaries throughout the world, yet there is no consensus on the importance of fronts to larval transport processes. Frontal circulation may act as a 'barrier' to larval transport since it constrains cross-frontal flows, leading to strong along-frontal flows, which In turn may serve as a type of 'larval conduit' by funneling larvae collected at the front to settlement locations. We examined the larval conduit hypothesis by characterizing hydrodynamic features of apparent frontal structure in Grays Harbor estuary (Washington, USA) and quantifying the effects of frontal structure on the distribution and transport potential of Dungeness crab Cancermagister megalopae dunng spring-time recruitment pulses. CTD casts made perpendicular to fronts indicated that frontal structure was characteristic of buoyant plume and axial convergent fronts at the bay mouth and at a subtidal channel site within the estuary, respectively. Surface drifters deployed seaward of a front at the bay mouth during early flood tide were advected into the front, demonstrating convergent circulation. Surface drifters remained within buoyant plume fronts, which moved up-estuary during flood tide at 0.4 m S-', and were then entrained in axial convergent fronts. Drifters moved an average of 10 km up-estuary during a single flood tide; the direction that drifters tracked corresponded to prevailing winds. The mean concentration of megalopae, as measured with neuston nets, was significantly higher in fronts than in adjacent water, 20 to 30 m upstream of the front, irrespective of year or location within the estuary. Paired plankton nets deployed at the bay mouth during flood tide indicated that the mean concentration and flux of megalopae was higher at the surface than at 3 to 4 m depth; this trend was significant for megalopal flux. Variable wind stress during the recruitment period of Dungeness crab, combined with the suggested effect of fronts serving as larval cond.uits whose direction of transport is influenced by prevailing winds, could significantly influence spatial variation in postlarval supply and subsequent settlement within the estuary.

Structure, variability, and salt flux in a strongly forced salt wedge estuary

“…Channel curvature creates cross-channel shear and sets up a lateral baroclinic gradient, with lower salinities and stronger down-estuary velocities on the southern shoal that oppose the advance of the front. Flow is convergent toward the front, with accumulation of buoyant material and organisms that can be biologically important for larval transport and the concentration of prey [Franks, 1992;Eggleston et al, 1998;Mendes et al, 2002]. After high tide the intrusion front begins to retreat down-estuary, and by mid ebb it has dispersed.…”

Barnacle demography:evidence for an existing model and spatial scales of variation

“…Oscillations in free space ('oscillatory fingerprint') could also be caused by many different mechanisms. For example, recruitment pulses associated with seasonality (Hawkins & Hartnoll 1982) or frontal advection (Eggleston et al 1998), or variable predation (Minchinton & Scheibling 1991). Thus, evidence of pattern does not necessarily imply the applicability of the model mechanisms, as many mechanisms could have caused the pattern.…”

Biological and physical coupling in the lee of a small headland: contrasting transport mechanisms for crab larvae in an upwelling region

“…Temperature increased rapidly shortly after the cessation of the wind (Roughan et al 2005), suggesting that a convergence zone between warm and cold water was not far from this site. Peaks in settlement coinciding with these changes in conditions may have resulted from fronts moving past the collectors because fronts are known to accumulate larvae , Eggleston et al 1998, Shanks et al 2000, Sponaugle et al 2002. The presence of larvae-laden fronts in the Bodega region may help to explain the settlement patterns in years where settlement is not correlated with the arrival of warm water from the lee of Point Reyes.…”