Field work began in June 2000 with the
deployment of a 7-mooring current and temperature array across the shelf
in the northern Keys off Key Largo. This array has been maintained
continuously from June 2000 to November 2002 resulting in a 2.5 year
time series of oceanographic variability. Data quality and quantity has
been excellent and data processing and analysis are continuing. The data
are used to investigate variability of oceanographic properties on tidal
to inter annual time scales and their influence on recruitment and coral
reef sustainability. An example of the time series is given in Fig. 1.
Temperature and current
time series from the deep outer reef station (24 m isobath) reveal that
large amplitude temperature variations occurred near bottom over several
periods of records that are visually correlated to onshore flow events.
These shoreward pulses of cold water are believed to be high in nutrient
concentration and represent a pulsed delivery of nutrients to the
fringing reefs from the Florida Current nutracline. These data are being
used to estimate onshore nutrient flux. Results to date indicate that
nutrient flux to the reef tract from the Florida Current is a transient
process that takes place in the spring and early summer
in a nearbottom layer.
Current and temperature time series show high frequency bursts of cold
nearbottom intrusions that appear to make a significant contribution to
the nutrient flux.
Figure 1: (a) Subtidal
(40hlp), rotated current components and water temperature at Mooring C,
located at the shelf edge near Molasses C-MAN station (Fig.1) for the
period October 2001 - April 2002. (b) Subtidal (40hlp), rotated current
components in the same coordinate system as noted in a). Note that the
cross shore current at 21m is more often onshore during this period from
April - November, 2002.
Spatial patterns and
seasonal variability of temperature, salinity and nutrient distributions
in the Florida Keys National Marine Sanctuary and surrounding regions
are determined on bi-monthly, multidisciplinary research cruises. These
surveys cover the entire south Florida coastal domain including the
Florida Keys, southwest Florida shelf and the Dry Tortugas. The survey
data are used together with the moored time series data to investigate
the interaction and influence of surrounding waters of Florida Bay, Gulf
of Mexico and the Florida Current on water properties of the Florida
Keys. An example is shown in Figure 2. Note that the cross shore current
is positive toward 130o and the along shore current is
positive downstream toward 40o.
A comprehensive, three-dimensional
hydrodynamic model of the coastal seas adjacent to the Florida Keys is
under development. The study goal is to link the shallow coastal regions
that encompass the Florida Keys to adjacent oceanic and shelf flows that
play an important role in the water circulation and exchange. The
modeling strategy is to follow a nested and downscaling approach, where
the regional model provides the link between coastal models of the
Florida Keys with a larger scale Atlantic Ocean and Gulf of Mexico
hydrodynamic model. The regional model has an intermediate resolution
(lying between the coarse resolution of the large scale model and the
fine resolution of the coastal scale models) and it provides appropriate
boundary conditions for the limited area models of the Florida Keys
region. This approach ensures that the coastal models receive inputs
from adjacent and remote sources, so that the calculated flows in the
coastal areas of interest are realistic. The regional hydrodynamic model
is an implementation of the HYCOM (Hybrid Coordinate Ocean Model,
http://oceanmodeling.rsmas.miami.edu/hycom/), a finite-difference
hybrid isopycnal/sigma/z-level model.
The regional model results establish the
link between the Florida Keys and the surrounding shelf and deep sea
flows. As seen in Figure 2, the core of the Loop Current approaches the
Florida Straits, influencing temperature values on the west Florida
Figure 2: East-west velocity (positive
toward east, upper panel) and temperature (lower panel) for the autumn
season during the 9th year of the regional South Florida
HYCOM (climatological simulation) along a north-south section at about
Implications for Larval Recruitment
As part of the NCORE
Program we have participated in an interdisciplinary effort to measure
and understand the recruit processes of reef fishes in the Upper Keys.
The varied time scales to circuit the different size eddies or coastal
countercurrents of the Keys provide the larval pathways and
opportunities for recruitment from both local and foreign sources. Su
Sponaugle and associates have made time series collections of larval
ichthyofauna in waters directly above Pickles Reef in the upper Keys
near our moored current arrays using nightly net tows during summer
months of 2000 and 2001.
During late July of 2000 a sharp shift in
larval assemblages from oceanic taxa to inner shelf/bay taxa was
observed. The shift to bay species appears to be related to discharge of
Florida Bay waters out of the channels in the middle Keys followed by
northeast transport of these waters to the reef study area by
entrainment with the Florida Current. Strong downstream currents
measured by the current meters indicate that the Florida Current was
located close to the outer shelf for most of July and August. The
northward movement of Florida Bay waters is also indicated in SeaWIFS
imagery that shows a turbidity plume extending northward from the middle
Keys passages at the time.
measurements of larval recruitment to the reef during the summer of 2001
showed an unusually high concentration of reef fish recruits on July 20.
Comparison of larval data to moored current time series data indicates
that this high concentration occurred during the northward passage of a
Florida Current frontal eddy over the outer shelf.