Constrained nearshore larval distributions and thermal stratification

Abstract

Vertical and cross-shore distributions and abundances of shallow-water barnacle larvae were characterized in La Jolla, southern California (USA), during a 2 yr period. Five stations located within 1 km of shore and ranging from 4−12 m water depths were sampled intensively in 2 m depth intervals during 27 cruises throughout spring−summer (April−July) and fall−winter (October−December) of 2014 and 2015. Larval abundances significantly decreased from 2014 to 2015, which could be related to the arrival of a warm-water anomaly (the so-called 'Blob') in 2014 and El Niño conditions in 2015. Despite the presence of these large-scale regional disturbances, vertical and cross-shore larval distributions were consistent throughout the 2 yr study period. Early-stage nauplii and Chthamalus fissus cyprids tracked bottom depth, and cyprids were on average deeper than nauplii. Vertical distributions were not related to the mid-depth of the thermocline or thermal stratification. Early-stage nauplii had a broader cross-shore distribution than cyprids, which were concentrated at inshore stations. Nearshore cyprid concentration had a positive relationship with thermal stratification, and the center of distribution of cyprids was farther offshore during fall−winter when stratification decreased. These results suggest that thermal stratification elicits enhanced behavioral control of cyprids to remain close to shore and reach the adult habitat.

Delivery of zooplankton to the surf zone during strong internal tidal forcing and onshore winds in Baja California

“…It has also been suggested that earlier stages of larvae located throughout most of the water column avoid the surf zone by detecting turbulence and shear from breaking waves near the coast (Fuchs & Gerbi 2016, Morgan et al 2017, explaining their low numbers in our samples. On the other hand, late-stage larvae of many species, including barnacles, crabs, and mussels, are more abundant in surf zones (Morgan et al 2017, Hagerty et al 2018, as was found herein.…”

“…For example, cyprids (last larval stage of barnacles) respond to downwelling in the laboratory by swimming up in the water column (DiBacco et al 2011). This behavior can help them concentrate in internal bore warm fronts that may transport them to shore, as other authors have found, where strong thermal stratification has been re la ted to a greater abundance of barnacle cyprids closer to shore (Hagerty et al 2018). It has also been suggested that earlier stages of larvae located throughout most of the water column avoid the surf zone by detecting turbulence and shear from breaking waves near the coast (Fuchs & Gerbi 2016, Morgan et al 2017, explaining their low numbers in our samples.…”

Reduced barnacle larval abundance and settlement in response to large‐scale oceanic disturbances: Temporal patterns, nearshore thermal stratification, and potential mechanisms

“…Larval abundance of early nauplii and C. fissus cyprids, were markedly lower during the 2015/2016 El Niño than following El Niño, and abundance of these barnacle larvae were also low during the Blob, although the varying time periods that these phenomena impacted (spring vs. fall) makes direct comparisons difficult. Nauplii larvae in our samples collected in 4-12 m water depths were early-stage (II-III), and the majority were likely produced by local C. fissus adults (Tapia and Pineda 2007;Hagerty et al 2018), which reproduce year round. A decrease in nauplii early stages during El Niño and the Blob suggests suppressed reproductive output in fall 2014 and 2015, that early-stage larvae were dying at higher rates, or that these larvae were rapidly advected out of the nearshore in 2014 and 2015, during the Blob and El Niño.…”

“…Further, as stratification is associated with flows that reverse with depth, and are used by larvae to control their horizontal movement (reviewed by Pineda and Reyns ), a decrease in stratification may limit the ability of larvae to control larval transport and cross‐shore distribution. Hagerty et al () found that larval proximity to the adult habitat was related to thermal stratification, with more constrained distributions near the adult habitat in more thermally stratified waters.…”

“…Further, larvae use vertically sheared flows to control their horizontal distribution (reviewed in Pineda and Reyns ), and these flows tend to occur more frequently in stratified waters. Thus, a decrease in water column stratification can result in a larval center of distribution farther away from the nearshore adult habitat (Hagerty et al ) and potentially less settlement.…”