Most benthic marine organisms settling in the intertidal zone have planktonic larvae that reside temporarily (days to weeks) in the water column before returning to shore to complete their life cycle. Therefore, the extent of settlement and recruitment of benthic organisms relies, in part, on successful larval transport. Larval transport, defined as a flux, or as the mean horizontal translocation of larvae between points along a specified one-dimensional axis per unit time (Pineda and Reyns, 2018), is a critical component of larval dispersal, defined as the spread of larvae from spawning to settlement sites (Pineda et al., 2007). Studying the mechanisms of larval transport is fundamental to conserve marine species, manage fisheries, improve modeling of population dynamics (Cowen and Sponaugle, 2009), and understand population connectivity.

The physical processes and biological mechanisms driving coastal larval transport have been extensively studied, and yet remain poorly understood in the nearshore (here defined as within 1 km from shore but excluding the surfzone; reviewed in Pineda and Reyns, 2018). This is partly because studies on intertidal species suggest that larval transport and dispersal can be episodic, and occur at smaller spatial scales than previously anticipated, with larvae often remaining within the nearshore close to settlement sites (Shanks et al., 2003; Tapia and Pineda, 2007; Morgan et al., 2009; Shanks and Shearman, 2009; Hagerty et al., 2018). The generally poor horizontal swimming capabilities of larvae (Chia et al., 1984) make them susceptible to being swept away due to the advective nature of the nearshore (Lentz and Fewings, 2012). However, ontogenetic changes in larval swimming efficiency (Wong et al., 2020) and larval behavioral responses may regulate dispersal and improve chances of returning to a settling site. These behaviors include altering vertical distribution through buoyancy control and vertical swimming (Daigle and Metaxas, 2011; DiBacco et al., 2011) that allow larvae to exploit vertically sheared flows and ultimately control horizontal transport (e.g., Thièbaut et al., 1992; Weidberg et al., 2019; Guillam et al., 2020). Moreover, larvae of benthic organisms also exhibit diel vertical distribution patterns (dos Santos et al., 2008; Bonicelli et al., 2016; Guillam et al., 2020), allowing them to evade visual predators during the day or reducing energy consumption by remaining in colder waters (e.g., Zaret and Suffern, 1976; Forward and Rittschof, 2000). Thus, the extent to which larvae can exploit onshore flow by altering their vertical position in the nearshore may have profound consequences on successful recruitment of benthic populations and should be characterized at fine temporal resolutions to be better understood.

The degree to which larvae regulate transport in the nearshore is complicated because hydrodynamic conditions can be unpredictable and are highly variable (Winant and Bratkovich, 1981; Bonicelli et al., 2016; Morgan et al., 2018). Physical processes in shallow coastal waters are affected by topographic features (e.g., Lerczak et al., 2003), surface and internal tides (e.g., Weidberg et al., 2019), wind-driven processes (e.g., Griffin and Middleton, 1992), surface gravity waves (e.g., Kumar and Feddersen, 2017), and other meso- and large-scale physical processes (e.g., Pineda et al., 2018) that impact the water column from scales of seconds to days to years, all of which have implications on larval transport and dispersal (Pineda et al., 2007). Recent findings in southern California indicate that increases in thermal stratification within the nearshore results in larval accumulation closer to shore, in a region just seaward of the surfzone (Hagerty et al., 2018). It is possible that higher stratification promotes the development of fronts, internal tidal bores, or other internal motions that aid onshore larval transport (Pineda, 1999; Shanks et al., 2003; Weidberg et al., 2019). For example, in shallow waters, these internal motions tend to result from tidal flows interacting with bathymetric features and can propagate toward shore along the thermocline (e.g., Winant and Olson, 1976). Furthermore, higher stratification correlates with vertical variability in two-way cross-shore flows (e.g., Winant and Bratkovich, 1981), potentially enhancing larval behavioral control of cross-shore transport (Hagerty et al., 2018), and promoting settlement. Moreover, stratification can also act as a barrier to vertical larval distribution, as some species maintain bottom distributions (sea scallops: Daigle and Metaxas, 2011), or remain below the thermocline (gastropod, bivalve and polychaete larvae: Lloyd et al., 2012) in stratified conditions. A number of studies have also documented the association of fish larvae and other zooplankton with the depth of the thermocline (e.g., Haney, 1988; Gray and Kingsford, 2003), suggesting that larvae exhibit behaviors and respond to water-column dynamics.

For barnacles in central Chile, vertical distribution in relation to the pycnocline depth varies by species and by larval stage, over several months (Bonicelli et al., 2016), but it is unclear how such distribution and abundance responds to high-frequency (<24 h), in addition to the monthly, variations in stratification. Understanding how larval distribution varies in response to stratification is particularly relevant in the nearshore, where internal waves and diurnal heating can drive changes to the water column stratification on daily, hourly, and shorter time scales (e.g., Winant, 1974; Sinnett et al., 2018; Grimes et al., 2020a). Barnacles have a typical marine invertebrate lifecycle, the early and final larval stages are easily distinguished, are very abundant, and knowledge about the vertical migration of their larvae can help to understand larval transport of other benthic species with pelagic larvae. In temperate eastern Pacific coastlines, barnacles tend to have six naupliar stages that develop offshore, and a final non-feeding cyprid larval stage which transports through the nearshore to reach intertidal juvenile and adult settlement habitats (Tapia and Pineda, 2007; Bonicelli et al., 2016; Hagerty et al., 2018). Barnacle larvae exhibit ontogenetic differences in vertical distribution (Tapia et al., 2010; Bonicelli et al., 2016; Hagerty et al., 2018), and may display changes in vertical position during diel cycles (dos Santos et al., 2007; Bonicelli et al., 2016). Changes in vertical distribution can also take place at small-spatial scales (100’s m) in response to frontal circulation (Pineda, 1999), and in laboratory conditions when downwelling flows are experienced (DiBacco et al., 2011).

This study aimed to provide insights on how behavior, vertical distribution, abundance, and onshore larval transport of barnacle larvae are driven by dynamic conditions at a shallow, nearshore site over a 24 h period. We focused on barnacle cyprids, the stage that must transport from the offshore larval pool to adult intertidal habitats to settle. We emphasized thermal stratification processes because previous studies revealed an association between stratification and (a) nearshore cross-shore larval distribution at cruise-to-cruise (∼days to few weeks) and seasonal scales (Hagerty et al., 2018), and (b) settlement at 2–3 day (Pineda and López, 2002) and interannual scales (Pineda et al., 2018). Here, we build on this work to determine how larval abundance and vertical larval distributions are impacted by shorter-scale (hourly) fluctuations in hydrodynamic and hydrographic conditions. We hypothesize that larval abundance in the nearshore is related to thermal stratification. We focused in the shallow nearshore zone, located in ∼4 m deep area just offshore of the surfzone, because this is an important region (Hagerty et al., 2018) connecting offshore larval development grounds to the surfzone and adult rocky intertidal habitat. Nearshore regions of many coastal areas remain understudied (but see Porri et al., 2006; Dudas et al., 2009b; Pfaff et al., 2015), yet quantifying the temporal scales at which nearshore hydrodynamic and hydrographic processes operate is critical for understanding larval transport of organisms residing in coastal and rocky intertidal habitats. In this study, we specifically addressed the following questions: (1) does nearshore larval abundance vary with thermal stratification, and if so, at what temporal scales? (2) Is onshore larval transport associated with thermal stratification? (3) What is the hourly and day-night variability in larval vertical distribution?

We observed a positive relationship between Chthamalus fissus larval concentration and thermal stratification at cruise-to-cruise scales. Furthermore, onshore larval transport was associated with stratification, with little transport when the water column was weakly stratified. These results support our growing understanding of the central importance of thermal stratification on nearshore larval dynamics, as increased thermal stratification distributes cyprids closer to shore (Hagerty et al., 2018), increases nearshore larval abundances (this study), enhances onshore larval transport (this study), and settlement in rocky intertidal habitats (Pineda and López, 2002; Pineda et al., 2018).

Clear patterns between C. fissus cyprid concentrations and currents were not evident. However, in some cases (e.g., Cruises 1, 2, 5) increased larval concentrations appeared to be associated with high-frequency alongshore current reversals. Other studies have noted that alongshore currents can influence larval transport in the nearshore (Wing et al., 2003; Dudas et al., 2009a; Liévana MacTavish et al., 2016). It is possible that strong northward currents promote downwelling (onshore) flow (e.g., Lentz and Fewings, 2012), causing an increase in larval supply. Additionally, in open coastlines, alongshore currents tend to be more energetic and more relevant than anticipated to larval transport by impacting the cross-shore currents (Pineda, 2000; Lentz and Fewings, 2012). Another possibility that warrants future study is that alongshore current reversals at our study site may generate fronts that accumulate larvae on hourly time scales.

Most of the variability in the currents we observed was due to high-frequency (less than 33 h) processes such as surface and internal tides. There was no evidence that the phasing of reversals matched changes in sea level associated with the surface tide. Instead, these reversals might be related to the internal tide, a process that can cause significant cross-shore flows (e.g., Winant and Olson, 1976). Surface tidal currents are weak in southern California, and the high-frequency currents are dominated by the internal tidal variability (Lerczak et al., 2003) which tend to be decoupled in phase from the surface tide. Larvae may aggregate at the same depth where the internal tide causes shoreward flows (e.g., Weidberg et al., 2019), suggesting that larvae have the potential to use these features to mediate transport. A key consideration, however, is that internal wave and tide events near our study area are rare in the nearshore when thermal stratification is weak (e.g., Sinnett and Feddersen, 2019, and N.R and J.P. unpublished data). Thus, stratification may play a key role in the shoreward transport of larvae, as increases in thermal stratification could lead to more energetic cross-shore currents and internal tides that propagate shoreward enhancing larval transport and retention in the nearshore (Pineda, 1999; Weidberg et al., 2019).

Indeed, during periods of greatest thermal stratification, we observed the highest larval concentrations, and onshore larval transport, underlying the importance of understanding how stratification varies in the nearshore. Our results further the findings from a previous study that observed cross-shore cyprid distributions closer to shore when waters were more stratified (Hagerty et al., 2018). Moreover, both stratification and C. fissus larval settlement decreased during the 2016 El Niño (Pineda et al., 2018), indicating that reductions in stratification can disrupt larval supply to intertidal habitats. Increases in stratification limits vertical mixing and promotes sheared flows (Winant and Bratkovich, 1981), including two-way horizontal flows (e.g., Hagerty et al., 2018; Guillam et al., 2020). In fact, we also observed greater vertical variation in the cross-shore currents when the water column was stratified, suggesting more sheared flows occurred during these conditions. Changes in larval vertical distribution (Lloyd et al., 2012; Hagerty et al., 2018; Weidberg et al., 2019) in response to sheared flows could allow larvae to better regulate their horizontal distribution, and their distance from shore (Shanks and Shearman, 2009; Pineda and Reyns, 2018). Our larval flux calculations, which take into account the vertical positioning of cyprids and the cross-shore currents they experienced at each depth, varied on hourly scales, but net estimated larval transport was strongest in the shoreward direction when stratification was greatest. While we did not identify the physical transport mechanisms at play in this study, many nearshore cross-shelf exchange processes are dependent on stratification including internal waves and tides (e.g., Sinnett et al., 2018), and transient rip currents (e.g., Grimes and Feddersen, 2021), and cross-shore flows have the potential to shut down when conditions are unstratified (Lentz, 2001).

From previous work, we know that C. fissus cross-shore distributions were closer to shore, near 4 m depth, when nearshore stratification increased, and this area where cyprid concentrations increased can be thought as a zone of larval accumulation (Hagerty et al., 2018). By continuously sampling at a 4m depth station in the current study, we hypothesize that in addition to larval patchiness, some of the between-cruise variations in larval concentration might be due to shifting of the zone of larval accumulation relative to the position of our fixed sampling station, as nearshore stratification changed spatially. For instance, the very high larval concentrations we observed when stratification was highest throughout the nearshore suggests that our 4 m deep sampling station was within the larval accumulation zone. In contrast, the larval accumulation zone would be further offshore when the nearshore was unstratified (e.g., during Cruise 4 low stratification conditions extended to the 8 m deep station when compared to the other cruises), and could also help explain why larval concentrations were lower during this time. The seafloor offshore of the surfzone in Bird Rock is relatively rough, with scattered ledges. However, there are no large protruding features, with smaller features distributed throughout. Thus, it is unlikely that bottom bathymetry has a large influence on the position of the zone of accumulation compared to stratification. We speculate that higher offshore stratification allows the thermocline to penetrate into the 4 m deep station, thereby enabling larval transport closer to shore. A follow-up study should test the hypothesis that spatial variation in stratification influences where larvae accumulate in the nearshore.

Regardless of the time of day, ∼40% of cyprids remained within the 2–3 m depth bin. This pattern is consistent with those found in other studies where cyprids of intertidal species remained slightly above the bottom. For instance, Tapia et al. (2010) found that cyprids were usually most abundant between 15 and 25 m depth at a station that was 30 m deep, Bonicelli et al. (2016) found cyprids also remained near the bottom at a 20 m site, and Hagerty et al. (2018) observed cyprids distributed at depths near the bottom at stations along a transect extending from 4 to 12 m deep. Such depth distributions might allow cyprids to use cool, deep internal bores to transport closer to shore before reaching the intertidal zone (Pineda, 1991; Shanks et al., 2014; Fernández-Aldecoa et al., 2019). It is possible that deep waters are preferable for the non-feeding cyprids because cooler waters extend the lifespan of their lipid reserves, providing them with more time to reach intertidal habitats and increase their chances of successful settlement (see Satuito et al., 1996). However, cyprids of some barnacle subtidal species may occur in surface waters (e.g., Le Fèvre and Bourget, 1991) and vertical distribution might relate to adult habitat.

The significant day-night difference in C. fissus distribution was driven by concentration changes in the bottom and surface depth bins. At night, cyprids migrated away from the bottom and increased their distribution near the surface. While C. fissus MDD also shallowed slightly at night, the MDD calculations mask changes that occur at the tails of the cyprid distribution (near-bottom and near-surface). Nonetheless, chthamalid cyprid distributions were also slightly shallower at night off the coast of Chile (Bonicelli et al., 2016) and Portugal (dos Santos et al., 2007). Since cyprids are non-feeding, this migration is not to track prey but could be to avoid visual predators (reviewed in Haney, 1988), which is a common response of meroplankton in deeper waters (Zaret and Suffern, 1976; Forward and Rittschof, 2000). It is unclear whether the diel changes in cyprid distribution that we observed have implications for nearshore larval transport as there were no day-night differences in larval flux. In coastal locations off Brittany (France), brittle star larvae that undergo a nocturnal upward migration in a stratified embayment experience retention, as shoreward currents are generated at night by the stratification dynamics (Guillam et al., 2020).

At our study site, thermal stratification might break down at night due to diurnal cooling (e.g., Grimes et al., 2020b), but we could not test for day-night differences in stratification because the larger number of day-time sampling hours during the higher stratification cruises may have introduced bias. Possible day-night variations in thermal stratification, coupled with the significant positive relationship we observed between larval concentrations and stratification may help explain why larval concentrations were significantly higher during the day than at night. However, characterizing variations in thermal stratification in the nearshore is complicated because it is modulated by many physical processes (e.g., surface and internal waves and tides, diurnal heating/cooling, seasons, El Niño), operating on a range of temporal scales (Sinnett et al., 2018; Sinnett and Feddersen, 2019; Grimes and Feddersen, 2021). Therefore, describing the dependencies of larval transport on stratification, a unifying factor in the dynamics of many physical processes, may be one way to simplify understanding the complex problem of benthic recruitment.

We observed dynamic nearshore larval distributions, changing at the scale of hours, possibly as a response to varying physical conditions in the water column in addition to larval patchiness. Larval distributions did not change on hourly scales in response to thermal stratification; however, cruises with higher stratification yielded higher larval concentrations and onshore larval transport. This suggests that cruise-to-cruise variability in stratification is more important than within-cruise variability, and provides insight as to the relevant temporal scales (days rather than hours) over which stratification operates to influence larval dynamics in the nearshore. While variation in thermal stratification did not alter larval vertical distribution on hourly scales, the high-frequency, depth-resolved sampling undertaken in this study made it possible to calculate larval flux during each cruise, and identify the relationship between onshore larval transport and thermal stratification. Although a specific mechanism was not resolved, our results suggest that nearshore thermal stratification is key for onshore larval transport, likely driven in part by the high-frequency cross-shore currents. Finally, this study showed that C. fissus cyprid larvae undertake diel changes in their vertical distribution, with larvae generally slightly shallower at night. Both behavior and physical processes appear to play an important role in facilitating accumulation of barnacle cyprids in shallow waters and successful onshore transport, with implications for larval supply and recruitment to intertidal habitats.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

GY contributed to field collections, sample processing, data analyses, and writing of the manuscript. NR and JP conceived of the study, and contributed to field collections, writing of the manuscript and funding. All authors contributed to manuscript revisions and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.