Larval recruitment in a region of strong, persistent upwelling and recruitment limitation

Abstract

Larvae are thought to be highly vulnerable to offshore transport of upwelling regions, limiting recruitment to infrequent wind relaxation and downwelling events. However, larvae could also be transported onshore by upwelled bottom waters, onshore wind-forcing or internal tides throughout the water column. We determined the relative importance of these hypothetical mechanisms for the timing of recruitment of 8 invertebrate taxa during the peak upwelling season in a region of strong, persistent upwelling. Recruitment was determined for 5 yr at an open embayment near the surface and bottom of the water column to examine the interaction of behavioral and physical processes regulating larval recruitment. Postlarvae consistently recruited near the surface or near the bottom depending on the species. Onshore delivery of larvae during wind relaxations and reversals did not best explain recruitment patterns in our area for most taxa. Only mussels consistently recruited more during these events, and they recruited in bottom rather than surface waters. Six crab taxa recruited primarily during upwelling. Recruitment of 7 taxa was intermittently correlated with the maximum tidal range, suggesting that internal waves also may deliver larvae onshore. Thus larvae may recruit by multiple processes in upwelling regions rather than being limited to infrequent relaxation events, leaving the mechanisms responsible for observed spatial patterns in larval recruitment and adult densities unexplained. Comprehensive studies of the behavioral and physical processes underlying larval recruitment and post-settlement mortality are needed to explain observed temporal and spatial variation in population dynamics and community structure in upwelling regions.

Surfzone hydrodynamics as a key determinant of spatial variation in rocky intertidal communities

“…Streaming is suppressed by breaking waves at the seaward edge of the surf zone, but zooplankton, detritus and sediments near the bottom enter the surf zone by entrainment into breaking waves [25][26][27]. Although some species of cyprids have been reported to be more abundant near the sea surface [28][29][30], we previously demonstrated that cyprids of all species recruited almost entirely to the bottom of moorings just outside the surf zone over 5 years on this coast [31]. Hence, cyprids may descend near the bottom as they enter the surf zone regardless of their depth preferences, before they reach the surf zone in response to increased turbulence from shoaling waves, as do other zooplankters in response to turbulence [12,13].…”

Planktonic larval mortality rates are lower than widely expected

“…effectiveness of larval behaviors in retaining larvae near adult populations or enabling migrations between adult habitats and larval nursery areas in dynamic upwelling regimes (Morgan et al 2009a, b, c, Shanks and Shearman 2009, Morgan and Fisher 2010, Drake et al 2013, Miller and Morgan 2013a, b, Nickols et al 2013, Fisher et al 2014, Morgan 2014 as well as in other marine systems (Swearer et al 2002, Queiroga and Banton 2005, Cowen and Sponaugle 2009. Hopefully, methodological advances and finer-scale examinations of larval dynamics in the plankton will enable better resolution of the different factors causing mortality in the planktonic stage.…”

Inverse approach to estimating larval dispersal reveals limited population connectivity along 700 km of wave-swept open coast

“…B 283: 20160370 facilitating retention in slower flows close to shore [26,28,32]. Larvae of most nearshore invertebrates develop in the slowmoving, highly retentive coastal boundary layer by either remaining beneath the fast-moving surface layer or undertaking diel vertical migrations to the surface at night after winds have subsided [25,26,29,31,52]. To evaluate connectivity among populations, all demographic data ideally would be collected during the same reproductive season and year.…”