Long-Distance Dispersal Potential in a Marine Macrophyte

Abstract

Plant populations have long been noted to migrate faster than predicted based on their life history and seed dispersal characteristics (i.e., Reid's paradox of rapid plant migration). Although precise mechanisms to account for such phenomena are not fully known for all plant species, a combination of theoretical and empirically driven mechanisms often resolves this paradox. Here, we couple a series of direct and indirect field and laboratory exercises on one marine macrophyte, Zostera marina L. (eelgrass), to measured distances between new patches and established beds in order to elucidate the long-distance dispersal and colonization potential of this marine seagrass.Detached, floating reproductive shoots with mature seeds were found to remain positively buoyant for up to 2 wk and retain mature seeds for up to 3 wk before release under laboratory conditions. Analysis of the detritus wrack along a remote shoreline found reproductive fragments with viable seeds up to 34 km from established, natural beds. Analysis of different regions of the Chesapeake Bay and coastal bays of the Delmarva Peninsula that once supported eelgrass populations, revealed natural patches at 13 sites ranging from 1 to 108 km from established populations. A combination of tidal currents and wind influences has the potential to move a passive particle at the surface (e.g., a floating reproductive fragment) up to 23 km in a 6-h tidal window suggesting that most unvegetated areas in this region that can support eelgrass are within the colonization potential envelope.We suggest that, when combined with earlier work on seed dispersal ecology of this species, eelgrass has strong qualities for high colonization potential of new habitat. The finding of natural patches at such great distances from established beds when studied in the context of the dispersal mechanism (currents and wind) make the dispersal distances of this species one of the highest for angiosperms, comparable in scale to mangroves and coconuts. This new understanding of the dispersal dynamics of eelgrass is critical in the context of seagrass restoration in areas distant from established beds, maintenance of existing populations threatened by anthropogenic inputs of sediments and nutrients, and examining metapopulation concepts in seagrass ecology.

Relationships between bed age, bed size, and genetic structure in Chesapeake Bay (Virginia, USA) eelgrass (Zostera marina L.)

“…It is also possible that bed size is a function of clonal growth, with seed recruitment playing a less important role, so that larger beds contain more ramets but not more genets. Alternatively, the genetic diversity of small beds could be relatively high because they are founded by multiple clones (Oostermeijer et al 1994), a hypothesis consistent with the observation that seeds are transported as sibling clusters attached to maternal reproductive shoots (Setchell 1929;Harwell and Orth 2002a).…”

“…Eelgrass (Zostera marina) is an interesting subject for studies of population genetic structure because of its great economic value (including its role as habitat for species of commercial interest) (Costanza et al 1997), broad distribution (den Hartog 1970;McRoy and Helfferich 1977), clonal and sexual reproductive strategies (Orth et al 1994;Ruckelshaus 1994;Ewanchuk and Williams 1996;Harwell 2000;Rhode 2002;Rhode and Duffy 2004), dispersal via both shoots and seeds (Harwell and Orth 2002a), and morphological diversity (Rhode 2002). In addition, seagrasses worldwide have declined precipitously in recent decades; they are now targets for conservation and restoration in many countries (McRoy 1996), including the United States, and restoration efforts are especially intense in Chesapeake Bay (Virginia, USA).…”

Hydroponic versus rooted growth of Zostera marina L. (Eelgrass)

“…Rafting of dislodged reproductive shoots (=fragments) of Z. marina is a commonly observed phenomenon in the late spring in Chesapeake Bay (Orth et al, 1994). Harwell and Orth (2002) reported that reproductive shoots of Z. marina had the potential to disperse up to 100 km from their source bed and indicated that this may take 2-3 weeks. Given the lack of information reported in the literature on survival duration of free-floating fragments or seedlings, a pilot experiment to determine whether or not hydroponic growth is feasible in eelgrass, Z. marina, was undertaken.…”

“…Fragmentation can be an effective propagation and dispersal mechanism in temperate Zostera marina L., tropical Halodule wrightii Ascherson and endangered Halophila johnsonii Eiseman (Hall, 2002;Harwell & Orth, 2002). Rafting of dislodged reproductive shoots (=fragments) of Z. marina is a commonly observed phenomenon in the late spring in Chesapeake Bay (Orth et al, 1994).…”

Temporal variability in potential connectivity of Vallisneria americana in the Chesapeake Bay

“…Results from a two-generation analysis of paternity indicate that, on average, pollen moves *1-20 m depending on local currents (Lloyd et al in prep). Pollen movement of \15 m has been documented for the ecologically similar species Zostera marina (Harwell and Orth 2002).…”