SOUTHEAST FLORIDA AND CARIBBEAN RECRUITMENT PROGRAM
(SEFCAR)
Principal Investigator:
Peter B. Ortner
Collaborating scientist(s):
S. Smith - U. Miami/RSMAS
T. Lee - U. Miami/RSMAS
G. Hitchcock - U. Miami/RSMAS
Objective:
1) Is the distribution of planktonic larval fish prey along
transport pathways connecting offshore spawning sites with local
neritic juvenile habitats controlled by meso- and submeso-scale
near surface circulation variability?
2) Is the distribution of invertebrate planktonic larval fish
predators along transport pathways connecting offshore spawning
sites with local neritic juvenile habitats controlled by meso- and
submeso-scale near surface circulation variability?
3) Is event-scale variability in local wind stress or Gulf Stream
position more significant in determining the relevant predation and
prey fields?
4) Are features remotely detectible by their surface manifestations
(OSCR data) closely correlated with variations in the abundance and
distribution of invertebrate prey, predators and potential trophic
competitors of ichthyoplankton?
Rationale:
Wind stress and horizontal current shear may result in either
organized patterns or randomized redistributions of zooplankton
abundance. Some members of the zooplankton have been shown to be
primary food resources for larval fish while others have been shown to be
either predators upon or competitors of larval fish. In general size
predominately determines whether certain species are ingestible or not, while
general taxonomic assignment [example: "Chaetognath" or "Medussae"] often in
combination with size determines potential invertebrate predation pressure. For
ecological questions of sufficient generality it is not always
necessary to determine the species of each and every individual.
Moreover, it is not always logistically feasible to do so.
Moreover it may be essential to sample biological parameters on the time and
space scales of the physical processes purportedly
influencing their distribution and abundance. To do this for each
and every species is impossible with present day technology.
Method:
Our long-term goal remains understanding the coupled biological and physical
mechanisms controlling the distribution of epizooplankton. We have argued
that linking physical and biological information into a dynamical
understanding requires collecting data on fully comparable time and space
scales. Our assumption derives from the general observation that, while
scales of biological and physical variation are often coupled the mechanisms
are less well understood due to their complex and highly non-linear properties.
In this pursuit we developed and used in the Arabian Sea and elsewhere an
integrated plankton sampler consisting of [OPC, multiple high-frequency
multiple acoustics, in-situ video, fluorometer, transmissometer and CTD
mounted upon an Aquashuttle.
Mesoscale eddies in this region typically move downstream at relatively slow
speeds (5-20km/dy) and can persist for up to 4 weeks in the middle Keys. The
submesoscale eddies move along the edge of the Florida Current relatively
rapidly (20-30km/dy). Given this speed, their smaller size and the subtlety
of the surface temperature, salinity or fluorescence signatures we saw in
1994 they can best be sampled by a fixed mooring array optimally within a
domain continually scanned by an Ocean Surface Current Radar (OSCR) or similar
system. An experiment of this type has been planned and preliminary data has
already been obtained using prototype biophysical instrumentation deployed at
a test mooring near the proposed experimental site. The advection of various
water masses with different biological signatures is readily apparent in the
data (figure 2).
Accomplishment:
We participated in two SEFCAR cruises utilizing the technology
developed in SABRE. On the first of these a freshwater plume was
discovered that proved to be relict Mississippi flood
waters (from reference). Mesoscale (50-100km) and sub-mesoscale
(5-20km) eddies are characteristic features of the narrow coastal regime
between the Florida Current and the Keys. The former have been well known
for some time and significantly affect local recruitment and transport
processes. The latter spin-off eddies became apparent only in May 1994 when
the Ocean Surface Current Radar (OSCR) was deployed concommitantly with a
pilot physical-biological sampling survey. One analysis suggests they are
intertial instabilities. Acoustic and optical data obtained at that time
with our prototype high speed integrated plankton sampler show that the
convergences and divergences in the velocity fields of the spin-off eddies
structure the particulate field resulting in enhanced coincident
phytoplankton and zooplankton grazer populations
(see figure 3). That features so transient have so substantial an effect
and that these transient features can dominate offshore-inshore exchange
during critical recruitment periods represents a fundamental change in our
understanding of biophysical interactions in this environment.
Key reference:
Ortner, P.,T. Lee, P. Milne, R. Zika, M. Clarke, G. Podesta, P.
Stewart, P. Tester, L. Atkinson, and W. Johnson, 1995, Mississippi River
flood waters that reached the Gulf Stream, Journal of Geophysical
Research, V100, #C7, pp 13,595-13,601
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