Project Title

1.1. Physical Oceanographic Studies
 

The goal of the physical oceanography component of the NCORE study is to quantify the upwelling nutrient flux to the Florida Keys coral reefs and to understand the controlling physical processes, using an observational and numerical approach. The specific objectives are: 1) To determine the spatial and temporal variability of the near-bottom flux of nutrients from the Florida Current and to quantify the rates and pathways of nutrient inputs to coral reef communities. 2) To identify and describe the primary physical processes regulating this nutrient flux. 3) To provide input to the nutrient budget. 4) To quantify the temporal and spatial characteristics of water movements and temperature and salinity variability in Pennekamp Park. 5) To develop a regional numerical model around the Florida Keys, which includes the coastal seas around south Florida and provides the link between models near the reef tract and large scale ambient flows.
 

Observations

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.

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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 130^o and the along shore current is positive downstream toward 40^o.  

Numerical modeling

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 shelf.

Figure 2: East-west velocity (positive toward east, upper panel) and temperature (lower panel) for the autumn season during the 9^th year of the regional South Florida HYCOM (climatological simulation) along a north-south section at about 83 ⁰W.

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.

Time series 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.