Creating a balanced marine portfolio: Relating correlated dispersal fluctuations to oceanographic features to maximize metapopulation growth

Background/Question/Methods

How can we take advantage of known oceanographic features to obtain a
balanced marine "portfolio" when siting reserves?  Most sessile or
benthic marine species have larvae which disperse with ocean currents
during development before recruiting to their adult location.  The
high variability of ocean currents means that the probability of
dispersing from one point to another fluctuates and these fluctuations
can be shown mathematically to reduce the long-run metapopulation
growth rate.  Just as in a financial portfolio, the reduction in
growth rate is greater if the fluctuations covary.  Larvae can become
entrained by submesoscale eddies and mesoscale gyres which span tens
to hundreds of kilometers and persist for days to months, and so
marine dispersal probabilities do tend to covary over extended spatial
areas.  Ideally, marine reserves should extend over regions with
uncorrelated or anticorrelated fluctuations in dispersal.  We
simulated dispersal for Stegastes partitus larvae in the Florida Keys
from 2004 to 2008 to generate a population model and applied
hierarchical clustering algorithms to find clusters of highly variable
routes which covary.  We related these clusters, representing
subpopulations, to known oceanographic features.

Results/Conclusions

Mesoscale eddies translating from the Lower to the Middle Keys are
affected by wind forcing and topographic features such as the
Pourtales Terrace that create a convergence zone.  We find that
dispersal fluctuations north of this convergence zone are only weakly
correlated with fluctuations south of it.  Furthermore, within the
northern region, dispersal fluctuations from the Upper to Middle Keys
are anticorrelated with dispersal fluctuations within the Upper Keys.
Modulations of the meandering of the Florida Current (FC) generate
frontal spin-off cyclonic eddies that may enhance self-recruitment
among the Upper Keys (presumably when the FC moves toward the Upper
Keys) or translocation of larvae from the Upper to Middle Keys (when
the FC approaches the Middle Keys).

Convergence zones seem likely to
create decoupled fluctuation regions, and their locations are relatively
dependable.  And at least in this case, gyres slowing down on the
Pourtales Terrace and producing the convergence are sufficiently
frequent that they are likely to have a similar influence on species
with a variety of pelagic larval traits.  We therefore suggest that
reserves be placed to straddle expected convergence zones, so that
dispersal fluctuations on one side can be balanced by uncorrelated
fluctuations on the other side.