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Temporal and Spatial Scales of Variability in Bivalve Connectivity
National Science Foundation (2007 – 2010)

Larval connectivity, the extent to which subpopulations exchange larvae, has emerged as a fundamental concept within the diverse arenas of population ecology, biotic resource management, biodiversity conservation, invasive species control, and habitat restoration.  However, determining dispersal trajectories of larvae and their scales of variability remain a major challenge. Here we proposal an integration of prospective (ROMS/ADCIRC modeling) and retrospective (elemental fingerprinting) approaches to assess variability in larval connectivity and its demographic consequences for mytilid mussel populations in southern California. This project builds on our initial studies of mytilid connectivity to address in greater depth and with a more strongly coupled physical-biological approach, questions of variability and its causes.  We will address hypotheses concerning the spatial and temporal scales of connectivity for Mytilus californianus and Mytilus galloprovincialis, examining their consistency among sites and species. This will be accomplished through (a) larval outplanting at 18 locations in San Diego County several times a year to generate reference signatures for trace elemental fingerprinting, (b) collection of recruits and elemental analyses of their larval shells to determine sites and regions of origins, and (c) high-frequency data collection at 2 bay locations for M. galloprovincialis and 2 open coast locations for M. californianus to carry out weekly analysis of recruitment variability, its link to chemical signals and recruit origins, and for collection of demographic data (size-specific survivorship, growth and fecundity).  Through numerical dispersal simulations and subsequent comparisons to fingerprinting-based assessment of recruit origins, we will examine the roles of circulation, local vs remote forcing, bay-ocean interaction, and larval attributes (vertical swimming, release times, planktonic duration) in defining the variability of connectivity. Demographic data will be combined with connectivity data to model the population- and metapopulation-level fitness consequences of observed mytilid connectivity patterns.

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Connectivity of Bivalve Populations: Assessing Sources of Larval Recruits
2004-2007

NSF - OCE PROJECT SUMMARY
The early life history of most marine benthic invertebrate organisms involves a planktonic larval stage of development that acts as an agent for increased dispersal and gene flow between sessile or sedentary adult populations. This exchange of genetic information among populations that may otherwise be reproductively isolated is defined as connectivity for purposes of this proposal. There remains considerable debate as to the spatial scale and strength of the connections between populations. Because the egg and larval stages are microscopic, it is all but impossible to follow individual larvae, or to track them with conventional tags. Technological advances have facilitated the use of trace element analysis to identify origins and evaluate trajectories of some planktonic larvae. Spatial variability in environmental, trace elemental and isotopic characteristics of different coastal water masses is recorded in the geochemistry of biogenic hard parts (e.g., shells). Since shells are deposited throughout planktonic larval development, they effectively record changes in environmental characteristics of different habitats occupied by larvae through development. Analysis of larval shell retained by newly settled bivalves will provide information about their source locations.

We propose to use trace element fingerprinting methods to evaluate the spatial scale of dispersal and strength of connectivity among bivalve population on the Massachusetts and southern California coasts. Hypotheses will address (1) the relative contribution of remote larval sources versus local ones (self seeding), (2) the relationship between circulation-driven dispersal potential and realized connectivity among bivalve populations and (3) the roles of species spawning period, planktonic period, and spatial separation of sites in determining probabilities of larval exchange. Our approach involves laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) to resolve spatial changes in larval shell composition that reflect recruit origins and temporal patterns of larval transport. We will work with the clam Mya arenaria and the mussel Mytilus edulis in New England and the mussels Mytilus galloprovincialis and M. californianus in southern California. Population connectivities will be studied with two metapopulation approaches that estimate dispersal probabilities from hydrodynamic models. One involves habitat area as a proxy for fecundity and the other is a multiregional matrix model that uses a demographic framework to describe the dynamics of the metapopulation. We will test realized population connectivity determined from trace elemental analysis of recruit origins against a priori predictions based on the metapopulation models.

Broader impacts: The resulting information about source populations and connectivities will enhance understanding of metapopulation dynamics in commercially valuable bivalve species. Connectivity information applicable to the east and west coasts of the USA will facilitate conservation of coastal resources through the improved design of marine protected areas and fisheries regulations. Key educational elements include the training of undergraduate, graduate, and postdoctoral students to conduct interdisciplinary research that integrates coastal ocean physics, larval ecology and metapopulation theory. There will be a transfer of trace element fingerprinting technology (from fish) into the realm of invertebrate dispersal and to collaborators in Mexico.

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M. californianus recruit

LARVAL ECOLOGY MEETINGS, 2006:

B.J BECKER1, P.A. MCMILLAN2, L.A. LEVIN2, F.J. FODRIE3

1. Interdisciplinary Arts and Sciences, University of Washington, Tacoma, Tacoma, Washington, USA
2. Integrative Oceanography Division, Scripps Institution of Oceanography, La Jolla, California, USA
3. Dauphin Island Sea Lab, Dauphin Island, Alabama, USA

POPULATION CONNECTIVITY PATTERNS DIFFER IN CLOSELY RELATED COASTAL BIVALVE SPECIES

Despite the importance of small-scale larval dispersal, direct connectivity patterns of most marine invertebrates with planktonic larvae are difficult to measure. Here we present the first successful application of trace elemental fingerprinting to determine the sources of settled coastal invertebrates. We used in situ larval culturing to create a reference signature of larval shell chemistry for two species of mussels (Mytilus californianus and M. galloprovincialis) at multiple sites in San Diego County, California, USA, in May 2003 and May 2005. The artificially created, geographically- referenced signatures were compared to the prodissoconchs of “wild”-caught juvenile mussels to determine their natal origins and create connectivity models. Despite apparent similarities in their life-history parameters, the two mussel species exhibit different connectivity patterns. In 2003 the majority of settling M. californianus (88%) originated in northern San Diego Country, with high self-recruitment in the north (87%) and high larval importation in the south (91%). M. galloprovincialis recruits exhibited a mix of northern (45%), southern (47%) and bay (7%) sources. During May 2005, fewer mytilid settlers appeared to be from the North and more were from the bays, with M. californianus exhibiting a greater diversity of origins. This novel approach to elemental fingerprinting has broadened these powerful chemical techniques to species with wholly planktonic larval life history strategies. As the technological challenges to the study of connectivity are being met, our understanding of how populations are connected is likely to be challenged as well.

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LEVIN, L.A.1, RASMUSSEN, L., MCMILLAN, P.A. 1, BECKER, B.J.2, FODRIE, F.J.1

1 Integrative Oceanography Division, Scripps Institution of Oceanography,
La Jolla, CA 92093-0218.

2. Interdisciplinary Arts and Sciences , University of Washington,
Tacoma, Tacoma, WA

COMBINING FINGERPRINTING AND PHYSICS TO ASSESS PATTERNS AND MECHANISMS OF LARVAL CONNECTIVITY

While numerical models are increasingly used to study the dynamics of larval connectivity, few methods exist to validate their predictions. Trace elemental fingerprinting offers one such approach. We have simulated time-specific larval transport of mytilid mussels in southern California during May 2003 & May 2005 with a nested 3-D ROMS coastal circulation model, generating connectivity data for 10 coastal and bay mouth release sites. Life-history differences between Mytlilus californianus and Mytilus galloprovincialis were modeled by varying larval planktonic duration as well as quantities and sources of larval supply (using area cover of adults as a proxy). We compare model-generated connectivities to measured connectivity patterns (settler origins) generated by outplanting (in situ larval culture) plus trace elemental fingerprinting of larval shells on new recruits in May 2003 and May 2005. The model results suggest that major influences on larval connectivity in San Diego Country include relative larval supply at spawning sites, spawning times (and years), planktonic duration, and remote wind forcing. Vertical positioning of larvae in the water column has a large effect on transport direction and determines coherence of model and fingerprinting connectivities. By combining outplanting, fingerprinting, and numerical circulation approaches we gain a more detailed understanding of local connectivity patterns, their forcing mechanisms, and their spatial and temporal variability. This information can be applied to a range of management issues associated with MPAs, habitat restoration, invasive species, and aquaculture.

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