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

Abstract

We assessed the effects of large‐scale oceanic disturbances on the settlement rate of an abundant intertidal barnacle. Daily or weekly settlement, nearshore temperature, currents, and abundance of early‐stage barnacle nauplii and Chthamalus fissus cyprids were measured in La Jolla, California, from the inception of the large‐scale warm‐water anomaly known as the “Blob” in 2014, to 1 yr following the 2015/2016 El Niño. We also measured over 2 yr of weekly settlement rates during the 1997/1998 El Niño in La Jolla and in two Mexican sites. Settlement was low during the Blob and the 2015/2016 El Niño, but increased dramatically after its end, and rates remained high through spring 2017. Barnacle nauplii and C. fissus cyprid abundances were also low during the Blob and El Niño. Nearshore water column thermal stratification and variability of the high‐frequency cross‐shore currents, a measure of internal wave activity, declined during the Blob and El Niño, but increased at the end of El Niño. Similarly, settlement rates in the Mexican sites were low during the 1997/1998 El Niño. The reduction in settlement during the Blob and El Niño may be related to two factors: a decrease in water column stratification, which can influence internal bore larval transport and the cross‐shore distribution of larvae, and also to reduced early‐stage larval abundance, potentially related to lower reproductive output or high nauplii mortality during the disturbances.

Marine heat waves, climate change, and failed spawning by coastal invertebrates

“…By our measures, spawning (as inferred from embryo abundance) by these 43 coastal invertebrates in winter 2015 and 2016 when compared to 2014 ranged from a complete failure to vastly reduced output. Consistent with our observations, during the 2015 and 2016, MHWs in the Southern California Bight, fecundity, and larval production of barnacles were also significantly reduced (Pineda et al ).…”

Effects of ocean climate on spatiotemporal variation in sea urchin settlement and recruitment

“…Spatial variability in such patterns could explain the opposing climatic responses in larval settlement between Fort Bragg and our sites in southern California. El Niño events in southern California are associated with lower stratification and reduced internal waves that promote the onshore transport of larvae (Shanks 1983;Pineda 1994;Pineda et al 2018). In contrast, El Nino events in northern California are often associated with relaxed upwelling, downwelling Kelvin waves, and increased stratification (Chavez et al 2002).…”

Diversity and distribution of nearshore barnacle cyprids in southern California through the 2015–16 El Niño

“…During each cruise, we sampled the entire water column at five stations roughly perpendicular to the coast and within one km of shore. Pineda, Reyns & Lentz (2018) found that abundance and settlement of C. fissus barnacle larvae decreased, thermal stratification was lower, and high-frequency internal waves were less energetic at our sampling site during the 2015–16 El Niño compared to after this period. When stratification is high, internal motions such as internal tidal bores that have the potential to transport larvae are more energetic (Pineda, 1999; Pineda & López, 2002).…”

“…Analysis of 1,251 CTD profiles indicated that thermal stratification is an excellent proxy for density stratification (top-to-bottom temperature difference is correlated with top-to-bottom density difference: r = 0.987). Furthermore, given the relatively widespread deployment of temperature loggers on moorings, our calculations allow comparisons of thermal stratification to be made with other studies that have collected temperature profile data (Hagerty, Reyns & Pineda, 2018; Pineda, Reyns & Lentz, 2018). Salinity was not considered because stratification at our study location is mainly dependent on vertical differences in temperature (Hagerty, 2017).…”

“…This in turn may affect marine communities, especially planktonic organisms that do not swim effectively against currents and are thus subject to changes in ocean circulation (Ohman et al, 2018). Southern California experienced El Niño conditions from April 2015 to May 2016 (McClatchie et al, 2016), and barnacle larval abundance in nearshore waters off La Jolla, southern California was lower during El Niño than it was before or after the disturbance (Hagerty, Reyns & Pineda, 2018; Pineda, Reyns & Lentz, 2018). The 1982–83 El Niño also impacted southern California, and two barnacle species usually found farther south appeared offshore of San Diego: Megabalanus coccopoma and M. californicus (Newman & McConnaughey, 1987).…”