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Larval Transport and Dispersal in the Coastal Ocean and Consequences for Population Connectivity

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Larval Transport and Dispersal in the Coastal Ocean and Consequences for Population Connectivity MARI N E P O P ULATIO N C O nn E C TI V ITY Larval Transport and Dispersal in the Coastal Ocean and Consequences for Population Connectivity BY JESÚS PI N E D A , J O N ATHA N A . H A R E , A nd S U Sp O N AUGLE MA N Y M ARI N E S P E C IES have small, pelagic early life stages. For those spe- cies, knowledge of population connectivity requires understanding the origin and trajectories of dispersing eggs and larvae among subpopulations. Researchers have used various terms to describe the movement of eggs and larvae in the marine envi- ronment, including larval dispersal, dispersion, drift, export, retention, and larval transport. Though these terms are intuitive and relevant for understanding the spatial dynamics of populations, some may be nonoperational (i.e., not measur- able), and the variety of descriptors and approaches used makes studies difficult to compare. Furthermore, the assumptions that underlie some of these concepts are rarely identified and tested. Here, we describe two phenomenologi- cally relevant concepts, larval transport and larval dispersal. These concepts have corresponding operational definitions, are relevant to understanding population connectivity, and have a long history in the literature, although they are sometimes confused and used interchangeably. After defin- ing and discussing larval transport and dispersal, we consider the relative importance of planktonic processes to the overall understanding and measurement of popula- tion connectivity. The ideas considered in this contribution are applicable to most benthic and pelagic species that undergo transforma- tions among life stages. In this review, however, we focus on coastal and nearshore benthic invertebrates and fishes. 22 Oceanography Vol. 20, No. 3 Larval transport is defined as the hori- a certain distance, herein referred to invasive species, and other phenomena zontal translocation of a larva between as dispersal distance. Larval transport (Cowen et al., 2006, this issue; Levin, points x1,y1 and x2,y2, where x and y are is an important component of larval 2006). By this definition, if the exchange horizontal axes, say, perpendicular and dispersal, and broad dispersal requires is measured at the time of settlement, parallel to the coastline. In larval trans- significant larval transport. Restricted connectivity is essentially larval dispersal port, only the spatial dimensions mat- dispersal, however, does not imply little from one population to another (e.g., ter. Although this definition ignores larval transport (Figure 1). Further, pro- Webster et al., 2002). Not all settlers will the vertical axis (z) for simplicity, this cesses and factors associated with the survive, however, and survival may be dimension is critical for larval transport end of larval transport (i.e., settlement) influenced by larval experience. Thus, because larvae can modify their hori- also influence dispersal, including settle- connectivity is frequently measured at zontal distribution by swimming verti- ment behavior, distribution of suitable some point after settlement, once set- cally, thereby encountering different settlement sites, and refuge availability tlers survive to enter, or recruit to, the currents (Nelson, 1912; Crisp, 1976). To (Figure 2). Similarly, because spawning juvenile population. Functionally, how- transfer from point x1,y1 to point x2,y2, a initiates larval dispersal, spawning time ever, this point is somewhat arbitrary larva can swim horizontally and may be and location are important, as are factors and differs among taxa. A more precise transported by diffusive and advective influencing spawning, including season demographic milestone is reproduction. processes (Scheltema, 1986). Defined as and synchronicity of spawning, age and If settlers die without reproducing, dis- the translocation of a larva between two condition of spawners, and fertiliza- persal is of questionable importance to points, larval transport appears decep- tion success. In addition to the spatial population growth or spread of invasive tively simple. However, the wide range dimensions inherent in larval transport, species. In this contribution we differen- of larval behaviors and physical mecha- larval dispersal involves a survival prob- tiate between population connectivity, nisms, together with their variability at ability, and thus food availability and measured at the time of settlement, and multiple scales, makes larval transport predation are important. The highest reproductive population connectivity, exceedingly difficult to measure. The mortality in marine populations occurs defined as the dispersal of individu- temporal and spatial scales of variability are enormous (Scheltema, 1986), even when considering a single physical trans- port mechanism (see Box 1). The fundamental challenge in population In contrast, larval dispersal refers to connectivity studies is to determine the the spread of larvae from a spawning source populations of settling larvae and source to a settlement site. This defini- tion is consistent with the terrestrial lit- the settlement sites of dispersing larvae. erature (natal dispersal in Clobert et al., 2001; Begon et al., 2006) that describes seed dispersal as the probability den- during the early life stages, so mortal- als among subpopulations that survive sity function of the number of seeds ity plays a large, but understudied, to reproduce. Reproductive population versus distance from the adult source role in larval dispersal. connectivity encompasses larval dis- (i.e., the dispersal kernel) (Nathan and Population connectivity has been persal but is also influenced by post- Muller-Landau, 2000; see Gerrodette, defined as the exchange of individuals settlement mortality (e.g., Hunt and 1981, for a rare marine example). Using among geographically separated subpop- Scheibling, 1997; Doherty et al., 2004), the dispersal kernel, dispersal can be ulations (see Cowen et al., this issue) and growth, and condition from settlement viewed as a probability that a released is thought to be a key process for popu- to successful reproduction. By the defini- zygote will make it to settlement over lation replenishment, genetics, spread of tion above, although dispersal of larvae Oceanography September 2007 23 that do not survive to reproduce can play (e.g., Roughgarden et al., 1988; Siegel et this and similar observations, combined a role in population and community al., 2003). An increasing number of stud- with recent modeling and genetic studies ecology, their contributions to reproduc- ies, however, conclude that a significant (Cowen et al., 2000; Gerlach et al., 2007) tive population connectivity are minimal amount of self-recruitment occurs in (Figures 1 and 2). marine populations (Jones et al., 2005; JESÚS PINEDA ([email protected]) is Almany et al., 2007). These conclusions Associate Scientist, Department of Biology, LARVAL TRANSPORT are not in and of themselves surprising: Woods Hole Oceanographic Institution, Reconsideration of the a population is defined as a self-sustain- Woods Hole, MA, USA. JONATHAN Scales of Larval Transport ing component of a species, and thus A. HARE is Research Marine Scientist, The term larval transport brings to self-recruitment is a defining attribute National Oceanic and Atmospheric mind small, passive larvae being moved of a population (Sinclair, 1988). What is Administration, National Marine Fisheries throughout the ocean by meso- and surprising is the relatively small spatial Service, Northeast Fisheries Service Center, large-scale physical processes (Johnson, scales over which self-recruitment has Narragansett Laboratory, Narragansett, 1939). This view has become a para- been observed. For example, despite a RI, USA. SU SpONAUGLE is Associate digm—larvae are released, trans- planktonic stage of 9–12 days, approxi- Professor, Marine Biology and Fisheries ported by mesoscale processes, mixed mately 30% of settling panda clown- Division, Rosenstiel School of Marine and in a larval pool, and then randomly fish self-recruited to an area of 0.5 km2 Atmospheric Science, University of Miami, recruited to juvenile or adult habitat (Jones et al., 2005). The implication of Miami, FL, USA. BOX 1. VARIABILITY IN SpATIAL And TEmpORAL ScALES OF LARVAL TRANSPORT The movement of larvae in internal bores is an example of the variety of Robertson, 1985). Thus, temporal scales relevant for understanding lar- spatial and temporal scales involved in larval transport. Larval accumula- val transport by internal tidal bores range from seconds to years. Other tion at surface-propagating convergences is critical for effective transport temporal scales important to internal tidal bore larval transport that are in internal bore warm fronts, and the time scales of these convergences not depicted here include fortnightly periodicity (~ 14.4 days), and the are from a few seconds to a few hours. On the other hand, water-col- periodicity of coastally trapped waves (a few weeks; Pineda and López, umn stratification, a seasonal phenomenon, modulates the energy of 2002). In the literature, larval transport generally encompasses horizontal internal bores and therefore also impacts larval transport (Pineda and distances ranging from tens to hundreds of kilometers, a usage we follow López, 2002). At even larger scales, stratification and internal bores are in this contribution. modulated by El Niño, an interannual phenomenon (Zimmerman and 24 Oceanography Vol. 20, No. 3 and the constrained nearshore larval dis- flows mainly because of the
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