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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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California Sea Grant (2007-2009)

Understanding connectivity to sustain and manage coastal resources (52-ANS-N)

Project Leader: Lisa A. Levin Co-PI : Linda Rasmussen Associate Investigator: Bruce Cornuelle

Project Summary

Objectives:
The overall project objective is to determine patterns of mussel connectivity in southern California to provide information relevant for invasive species, aquaculture, and marine reserve management. We will target open coast, outer bay and inner bay mytilid species. Goals are to (1) develop trace elemental fingerprinting approaches for the invasive mussel Musculista senhousia, (2) generate and compare Fall and Spring fingerprinting -based connectivity patterns for Mytilus. californianus, Mytilus. galloprovincialis and Musculista senhousia in southern California, and (3) analyze and compare ing connectivity matrices based on realized (elemental fingerprinting) and simulated (ROMS models) bay to-bay and bay-ocean larval exchange. We seek to test hypotheses about rates of self seeding, the role of passive circulation and behavior in larval transport, and to identify regional source and sink populations. We will assess temporal variability in connectivity patterns for Mytilus spp, using experiments conducted previously (in 03, 04, 05 and 06) and during this grant (07, 08), compare results to passive transport predictions corrected for larval supply, and synthesize the information to generate generalized conceptual models of open coast, inner and outer bay connectivity patterns in southern California.

Methodology:
Connectivity will be assessed through trace elemental fingerprinting. This approach assumes that trace elements in carbonate shell deposited by developing mussel larvae reflect the chemistry of the water masses they are in. Different bay and coastal waters impart different chemical signatures to mussel shell. To determine time- and space-specific larval shell signatures, developing larvae will be ' outplanted' (reared in situ) at 18 coastal and bay sites. We will then analyze the larval shell retained on newly recruited individuals and match these against outplant signatures to determine site or region of larval development for new settlers. This methodology has been developed in our laboratory recently for M. californianus and M. galloprovincialis; we will develop comparable techniques for the invasive Asian mussel M. senhausia. Connectivity (outplant/recruit) studies will be conducted over 2 years, focusing on the reproductive season of each species. This will involve moored larval outplants at ~18 locations in San Diego County with recruit collections approximately 2-3 weeks later. Larval shells of outplanted larvae and new recruits will be analyzed by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) to determine concentrations of metals (e.g., Pb, Cu, Ba, Mg, Mn, Sr) ratioed to a rare Ca isotope (48Ca). Larval elemental data will be analyzed by discriminant function analysis to create site- or region-specific algorithms that will serve as the reference map for assessing recruit origins. Trace elemental signatures of larval shells of new recruits will be compared against the reference signatures to determine their site or region of origin and generate connectivity matrices. These results will be compared to real time predictions of passive coastal dispersal and origin probabilities using a nested 3-D ROMS coastal circulation model for study of larval transport in southern California. The terrain following model, with a 600-m grid size, is forced by climate and winds and will simulate larval transport for the specific time period studied during outplanting and recruit collections, allowing a more detailed understanding of transport mechanisms.

Rationale:
An understanding of larval connectivity can provide fundamental information for invasive species monitoring and control, regulation of open aquaculture activities, the design of marine reserves, identification of settings that must be conserved because they are important propagule sources for other communities, and identification of sites that are unlikely to be self sustaining. In all cases our ability to know the movements of larvae is technology limited because larvae are too small (and dilute) to track directly. One new approach is trace elemental fingerprinting, which employs natural chemical tags in carbonate structures (ie otoliths, statoliths, and shells) to track movements and assess origins of larvae and juveniles. While most applications of fingerprinting have involved fishes, there is an urgent need to develop such methods for invertebrates, which have fundamental roles in the health, structure and commercial value of ecosystems. In California, mussels dominate biomass and structural on rocky shores and in some mud bottoms, are cultured for commercial harvest in over half of the state’s aquaculture facilities, and include several extremely abundant invasive species. Connectivity results obtained for mussels and with numerical modeling should be representative of numerous species with planktonic periods of 2-4 weeks that inhabit open coast or bay habitats.

Accomplishments:
For the past 3 years we have developed elemental fingerprinting methods for mussels, testing methods to outplant laboratory-spawned larvae in the coastal zone, to identify new recruits with PCR, to use LA-ICPMS to characterize larval shell elemental fingerprints, and to detect regional origins of newly settled larvae. Using these approaches we have observed unexpectedly different connectivity patterns for two Mytilus species. A ROMS 3-D primitive equation circulation model has been developed for the San Diego County coastline by L. Rasmussen, B. Cornuelle and J. Largier to test the coherence of larval trajectories with passive transport, and to explore the physical and biological parameters that may affect larval distribution. Results suggest importance of planktonic duration, population size at the release sites, and vertical position within the water column for determining patterns of connectivity. Continued model development has also shown the importance of remote wind forcing and small-scale topographic features. The proposed work will develop fingerprinting methods for Musculista senhousia, provide a connectivity comparison of 3 mussel species, extend a Mytilus connectivity time series, and further the integration of physical and biological approaches.

SELECTED PUBLICATIONS

Becker, B.J., Levin, L.A., Fodrie, F.J. and McMillan, P.A. Complex larval retention patterns in marine invertebrates. Proc. Nat. Acad. Sci. 104: 3267-3272. (2007)

Thorrold, S., Zacherl, D., Levin, L. Population connectivity and larval dispersal: Using geochemical signatures in calcified structures. Oceanography 20: 80-89 (2007). Link to PDF

Rasmussen, L., Cornuelle, B.D., Levin, L.A., Largier, J.L., DiLorenzo, E. Effects of small-scale features and local wind forcing on tracer dispersion and estimates of population connectivity in a regional scale circulation model. J. Geophys. Res. 114, C01012, doi:10.1029/2008JC004777 (2009)

Carson, H.S., Lopez-Duarte,, M.P., Wang, D. and Levin, L.A. Time series reveals how reproductive timing alters coastal connectivity. Current Biology 20: 1926-1931. (2010)

Carson, H.S., G. Cook, M. Paola López-Duarte and Lisa A. Levin. Evaluating the importance of demographic connectivity in a marine metapopulation. Ecology 92: 1972-84. (2011)

Fodrie, F. J., Becker B. J., Levin, L.A., Gruenthal K., and McMillan P. Connectivity clues from short-term variability in settlement and geochemical tags of mytilid mussels. J. of Sea Research 65: 141-150. (2011). Link to PDF

Lopez-Duarte, P.C., Carson, H.S., Cook, G.S., Fodrie, F. J., Becker, B.J., DiBacco, C. and Levin, L.A. What Controls Connectivity? An Empirical, Multi-species Approach. Integrative and Comparative Biology. doi:10.1093/icb/ics104 (2012)

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