Circulation and Exchange Processes within Florida Bay Interior Basins

Thomas N. Lee and Elizabeth Williams

University of Miami, RSMAS, Miami, Fl

Elizabeth Johns and Ryan Smith

NOAA/AOML, Miami, Fl

Nelson Melo

Cooperative Institute for Marine and Atmospheric Studies, U. of Miami, Miami, FL

The goal of this study is to improve understanding of the circulation and exchange processes that regulate water properties and flushing rates of the interior basins of Florida Bay to aid evolution and application of predictive hydrodynamic models. At present it is not clear how proposed modifications in surface water delivery to Florida Bay will affect salinity variability within the Bay interior. However, it is generally agreed that large Bay salinity variations have significant impact on interior ecosystems, and possibly also with adjacent ecosystems of the shelf and Florida Keys National Marine Sanctuary (FKNMS) due to transport processes linking the regions. In April 2001 we began a new study to quantify the circulation and exchange rates influencing salinity variability in the eastern, central and western subregions of the Bay and their interactions with connecting regions.

Each basin is the focus of intense two month studies during wet and dry seasons. The observational program initially focused on Whipray Basin in the central region of the Bay and then most recently shifted to the Northeast Basin. Whipray was chosen for it is one of the most isolated basins, receives little fresh water runoff and develops hypersalinity conditions in the dry season with high turbidity levels and algal blooms. Our experimental approach was to directly measure the volume flows and salt transports into and out of the basins at the more significant flow connections, together with sea level variations and rapid basin surveys of temperature and salinity to directly compute the terms of the salt balance equation and estimate exchange rates. The shallow environments of Florida Bay required use of specialized equipment and techniques for moored time series measurements, small boat surveys and surface drifter trajectories. The experimental approach for Whipray is shown in Fig. 1. Sampling strategy consisted of a four mooring array deployed for two months over the dry and wet seasons in the flow passages, with bi-weekly ADCP transects, basin C/T/fluoresence surveys and drifter releases over 2-3 day periods for the season duration. A similar strategy was used for the Northeast Basin.

A total of 28 salinity surveys of Whipray basin were made over the dry and wet seasons from March 28 to Oct. 28, 2001. Salinity patterns show strong north/south salinity gradients in both seasons that reversed sign from positive in the dry season to negative in the wet season. Maximum salinities in the dry season reached 45 to 48 in the northwestern part of Whipray and even higher values near 50 were found on the bank in the Tin Can Channel region. During the wet season there appears to occur a general freshening over the entire basin, but with greater magnitude in the northern part where salinity’s dropped to 25-30. Intrusions of lower salinity waters appear to enter the basin in the northeast and east via Mc Cormick Cr and Twisty section and from the northwest through the Tin Can Channel region. Drifter trajectories indicate a general basin-wide northwestward movement in the dry season and west to southwest movement in the fall. Basin average salinity was computed for each survey and is shown in Figure 2. The average salinity increased at a rate of approximately 4/mo during the dry season when rainfall and river discharge were negligible and evaporation maximum. During the wet season the basin average salinity decreased at a maximum rate of about 11.5/mo due to the combined influences of local rainfall, upland discharges and exchange with adjacent basins. The rapid rate of increase of basin average salinity during the dry season provides a rough estimate for the time scale of renewal of basin waters on the order of 6 months, using salt balance techniques. A similar renewal time estimate was previously made by Nuttle et al. (2000) using historical data.

Volume transports were measured with shipboard ADCP transects across the eastern opening to Whipray basin adjacent to Twisty Channel and across the southern opening of the basin adjacent to Topsy Key (Fig. 1). A total of 25 sets of transects were made at Twisty showing volume transports ranging +127 to –149 m3/s (+ is inflow and – outflow) for the 3 month wet season, and +44 to – 34 m3/s at Topsy from 27 transect sets. Mean flows from these data were +28 m3/s at Twisty and near zero at Topsy giving a mean inflow to Whipray of about 28 m3/s. However, these means are not reliable due to the scarcity of data and large variability observed, showing the importance of having time series measurements.

Shipboard volume transports were found to be linearly correlated to moored current records, accounting for 90% of the current variance and are used to derive volume transport time series for Twisty and Topsy sections. Volume transport time series were also derived for Dump and Crocodile Channels using moored current records with the cross-sectional areas. These transport time series show significant tidal and subtidal variations. Tidal fluctuations are primarily semi-diurnal at the eastern and western passages, but surprisingly there is a stronger diurnal variation through the southern passage (Topsy). Subtidal flows and sea level variability appear to be primarily responding to local wind forcing. Flows through the passages tend to be highly dependent upon the east-west wind component. East winds (toward west) cause inflows through Crocodile and Twisty and outflows through Dump and Topsy sections. The opposite occurs for west winds. In addition, flow through the southern passage (Topsy) is also dependent on the north-south wind component, ie north winds can cause outflow and south wind inflow. Surface drifters showed a similar response to east-west winds.

Means of the volume transports time series for the wet season were small. Inflows of 1.4 and 2.8 m3/s occurred through Crocodile and Twisty sections, respectively and outflows of 5 and 1 m3/s through Topsy and Dump sections, respectively. A very rough estimate of mean flow over the broad western bank adjacent to Whipray suggests an additional outflow on the order of 5 m3/s could have occurred. Using published estimates of mean wet season evaporation (9 m3/s) and precipitation (13 m3/s) for Whipray surface area requires a mean addition of river inflow at the northern boundary of 3 m3/s for a water volume balance. Our initial estimates of salt and water volume balances suggest a residence time for Whipray Basin of 6 to 11 months, which is in general agreement with previous estimates. These rough estimates will continue to be refined for the Science Conference and recent results from the Northeast Basin will be presented.

Thomas Lee, University of Miami, RSMAS/MPO, 4600 Rickenbacker Causeway, Miami, Fl, 33149, Phone: 305-361-4046, Fax: 305-361-4696, , Question 1

Fig. 1. Whipray sampling strategy / Fig. 2. Whipray average salinity 2001