Interannual variation and spatial distribution of decapod larvae in a region of persistent coastal upwelling

Abstract

The California Current System is characterized by strong coastal upwelling that drives high primary production with implications for life in the plankton. We investigated the role of environmental variability at local, regional, and basin-wide scales in determining decapod larval distributions across space and time over 8 yr (2004 to 2011). We focused on an area of the California Current off the coast of central California, USA, characterized by a strong upwelling jet, a retentive zone with high primary production, a buoyant bay outflow plume, and proximity to both open coast and bay habitats. While multiyear studies of holoplankton distributions have revealed strong effects of basin-scale environmental variability, we did not find that basin-scale environmental changes resulted in major shifts in meroplankton distributions. Instead, meroplankton distributions in the California Current remained consistent across years -the oceanographic environment affected meroplankton distributions along complex shoreline topography but not across the shelf. Chlorophyll fluorescence and variables associated with regional upwelling were most correlated with larval distributions, indicating an association between larval distributions and primary production. We also found that environmental variability did not explain much of the variability in the larval distributions (only 5 to 20%), indicating that larval behaviors and demographic variables mediated the role of physical forcing in determining larval distributions. Larval distributions provide us with clues to determine larval transport and survival, adding to our understanding of how marine populations are connected, identifying threats to their persistence, and informing effective marine conservation and resource management planning.

Nearshore Larval Retention and Cross-Shelf Migration of Benthic Crustaceans at an Upwelling Center

“…In this work and previous studies, we have repeatedly sampled larval assemblages during many cruises over multiple locations and years (Morgan et al, 2009b,c;Morgan and Fisher, 2010;Hameed et al, 2018). In this work, we focused on the time and place of maximum upwelling, showing that larvae were not transported farther offshore at this region of strong, persistent upwelling, contrary to widespread expectations.…”

“…Moreover, the ability of larvae to regulate transport could have a large impact on dispersal and population connectivity and has been debated for decades. We recently determined that larvae occurred close to shore in persistent, strong upwelling on the Pacific Coast of the USA-rather than lost offshore as was widely expected (Morgan et al, 2009b(Morgan et al, ,c, 2011Morgan and Fisher, 2010;Fisher et al, 2014;Hameed et al, 2018)-and that larval mortality in the nearshore retention zone was much lower than previous estimates of larval mortality anywhere (White et al, 2014). We then demonstrated that larval behaviors enabling larvae to remain close to shore throughout development also enable them to settle near natal populations (Drake et al, 2013;Hameed et al, 2016).…”

“…During the present study, we sampled larvae of 42 nearshore benthic crustaceans and 99; l98.7989% of all larvae were collected over the inner shelf, <11 km from shore along both transects. Larvae were most concentrated within a few kilometers of the shoreline, in the coastal boundary layer where offshore Ekman transport and alongshore currents are weak (Lentz and Chapman, 1989;Largier et al, 1993;Kaplan et al, 2005;Kirincich et al, 2005;Roughan et al, 2006;Morgan et al, 2009c;Morgan and Fisher, 2010;Nickols et al, 2012Nickols et al, , 2013Hameed et al, 2018). Not only was this characteristic coastal-boundary-layer circulation (Pettigrew and Murray, 1986;Shanks, 1995a;Largier, 2002Largier, , 2003 persistent, but it is expected to contribute to nearshore larval retention of diverse benthic crustaceans in other upwelling regions worldwide (Morgan, 2014).…”

“…The characteristic vertical circulation in wind-driven upwelling areas enables invertebrate larvae and other zooplankters to reduce transport by regulating depth in vertically sheared currents (reviewed by Peterson, 1998;Morgan, 2014), as they do in other places where vertical shear occurs for different reasons (Epifanio and Garvine, 2001;Queiroga and Blanton, 2005;Morgan, 2006, in press;Epifanio and Cohen, 2016). Most species of larvae and copepods in upwelling regimes complete development close to shore within a coastal boundary layer, where Ekman transport is weak and alongshore currents are slow (Peterson et al, 1979;Morgan et al, 2009a;Shanks and Shearman, 2009;Morgan and Fisher, 2010;Nickols et al, 2013;Fisher et al, 2014;Hameed et al, 2018). They remain below a shallow Ekman layer or undertake diel vertical migrations, ascending near the surface to forage at night when offshore flow slows (Peterson et al, 1979;Peterson, 1998;Batchelder et al, 2002;dos Santos et al, 2008;Morgan et al, 2009a;Shanks and Shearman, 2009;Morgan and Fisher, 2010;Miller and Morgan, 2013).…”

Shoreward swimming boosts modeled nearshore larval supply and pelagic connectivity in a coastal upwelling region

“…Nevertheless, benthic larvae in these regions are consistently retained near the coast, typically within a few kilometers of shore. These larval distributions are observed along the US west coast Hameed et al, 2018;Morgan and Fisher, 2010;Morgan et al, 2009aMorgan et al, , 2009bMorgan et al, , 2012Morgan, in press;Nickols et al, 2013;Shanks and Shearman, 2009), as well as upwelling regimes off Chile (Bonicelli et al, 2016;Poulin et al, 2002) and the Iberian Peninsula (Bartilotti et al, 2014;dos Santos et al, 2008;Domingues et al, 2012;MartaAlmeida et al, 2006). Larvae may avoid offshore transport by residing subsurface, where currents are weaker or onshore.…”