Topical Area: Water Quality

Brian D. Keller, The Nature Conservancy, Marathon, FL

Florida Bay Watch (FBW) is a volunteer-based program of The Nature Conservancy (TNC) in which trained volunteers collect seawater samples and environmental data using standard scientific methods; it is designed to augment and assist scientific studies conducted by universities, agencies, and other institutions. This poster presents results of a monitoring project of water-quality parameters in nearshore waters at fixed stations in Florida Bay and along the Florida Keys for the three-year period November 1994-October 1997. These nearshore data complement offshore data collected in Florida Bay and along the Keys by Florida International University (FIU); analytical work for this project was performed at FIU’s Southeast Environmental Research Program (SERP).

The nearshore water-quality stations were located at the homes and workplaces of FBW volunteers. Stations were distributed from Key Largo to Key West and included sites both bayside (facing Florida Bay or the Gulf of Mexico) and oceanside of the Florida Keys. The addition of new stations and the termination of others occurred over the three years; eight stations were active for the entire period. Sample sites include fabricated (residential canals and boat basins) and natural/unobstructed shorelines. Besides these obvious differences, sites varied in many aspects, including water depth, circulation and flushing rates, nearby vegetation, and type and number of adjacent On-Site Sewage Disposal Systems (OSDS). The volunteers sampled from docks, sea walls, or the shoreline; sampling for each station was done at the same location.

Florida Bay Watch volunteers were trained in basic water-quality sampling methods, which included instruction on filling out data forms, techniques for calibrating field equipment, and emphasis on careful handling of water samples to ensure data integrity. Periodic evaluations were conducted to ensure consistency, and all data went through a quality-control check to identify possible sampling errors. Volunteers were instructed to sample each week during a low tide. The following information was recorded on a standardized data form: date, time, tide, Beaufort number for wind and sea state, wind direction, current strength, current direction, Secchi depth, time of Secchi reading, sea-surface temperature, specific gravity, and rainfall in the previous 24 hours. In addition, volunteers collected and froze a water sample for analysis of total nitrogen (TN) and total phosphorus (TP). For determination of chlorophyll-a (Chl-a) concentrations, two 60 mL aliquots of seawater were drawn into a syringe and then squirted through a filter unit containing a Whatman glass microfiber filter, GF/S, 25 mm diameter. The filter paper was placed in a vial, closed within a brown opaque bottle, and frozen. Analyses of samples for nutrients and chlorophyll-a were conducted by Dr. Ron Jones's water quality laboratory at SERP, FIU.

Water samples and data forms were collected from the volunteers monthly and the samples were sent to the laboratory. Total nitrogen was measured using an Antek Model 7000 Nitrogen/Sulphur Analyzer, total phosphorus was measured by standard methods (EPA 365.1) using an Alpkem instrument (EPA A303-5050), and chlorophyll-a was measured using standard fluorometric methods (Standard Method 10200). The nutrient and chlorophyll data were sent to TNC where they were matched with the corresponding environmental data for each sampling of each station. All water-quality and environmental data are in a database managed by TNC.

The stations had variable histories. To examine possible patterns through time in concentrations of total nitrogen, total phosphorus, and chlorophyll-a, eight stations having three-year data sets for the period 1 November 1994-30 October 1997 were employed. Five of the stations were in the Upper Keys (UK; Mile Marker 106-80), two were in the Middle Keys (MK; MM 79-45), and one was in the Lower Keys (LK; MM 44-0). Five stations were at fabricated shorelines and three were at natural/unobstructed shorelines; seven were bayside of the Keys and one was oceanside. Monthly station means were then averaged for the dry and wet seasons each year, defined as November-April and May-September, respectively.

Small-scale variations in nearshore water-quality parameters were a striking feature of this data set, as reported in FBW Quarterly and Annual Reports. Values often fluctuated considerably both at a particular station from week to week and among nearby stations; these fluctuations sometimes were on a scale of one or two orders of magnitude.

Overall, concentrations of chlorophyll-a at these stations tended to decrease over the three-year period during the wet season (see below). There was little variation among years during the dry season; wet season concentrations exceeded dry season concentrations during 1995 and 1996, but were less during 1997. The concentration of total phosphorus exhibited an 85% increase during the 1997 wet season over averages for the two previous wet seasons. Dry season values tended to increase slightly through time. There was little difference between wet and dry season values except for the relatively high 1997 wet season TP concentration. The concentration of total nitrogen tended to increase through time during the wet season. By contrast, TN tended to decrease somewhat during the dry seasons; concentrations were consistently higher during the wet season than during the dry season.

Chlorophyll-a (m g/L) Total Phosphorus (m M) Total Nitrogen (m M)

To examine water-quality parameters for possible patterns across space, all stations were categorized by the three criteria noted above: region of the Keys, fabricated vs. natural shorelines, and oceanside vs. bayside of the Keys. Fabricated shorelines included various kinds of canals (e.g., dead-end, open-ended, aerated) and boat basins. Natural shorelines often included a dock from which samples were collected. A one-year period between November 1994 and October 1997 was selected for the most complete distribution of stations among the 12 possible spatial categories. This period was November 1996-October 1997, which had 18 stations distributed among nine of the 12 categories; no 12-month period included at least one station in all 12 categories. Dry and wet season averages of station means were examined in pair-wise combinations of the spatial categories because the three-factor array was not complete.

Chlorophyll-a (m g/L) Total Phosphorus (m M) Total Nitrogen (m M)

Chl-a did not vary in a consistent manner between the dry and wet season across the station types (see above). Values were four- to seven-fold greater along fabricated shorelines compared to natural shorelines in the Upper and Lower Keys, but were similar in the Middle Keys. Bayside concentrations were greater than oceanside concentrations. Comparing regions, often LK>UK>MK, but there were several exceptions to this pattern. TP also did not vary consistently between the dry and wet season. Fabricated shorelines had higher values than natural shorelines. As with Chl-a, bayside TP concentrations were greater than oceanside concentrations. There was no consistent pattern among regions. Unlike Chl-a and TP, TN concentrations were generally greater in the wet season than in the dry season, but differences between fabricated and natural shorelines were not consistent. Bayside concentrations generally exceeded oceanside concentrations, with one exception. In general, TN concentrations in the Upper and Lower Keys exceeded those in the Middle Keys.

These findings underscore the complexity of nearshore water-quality variation in the Keys, both through time and across space. They indicate that wet season nutrient concentrations have increased in recent years, while Chl-a concentrations have decreased; dry season values showed relatively little variation through time at the eight stations monitored for three years. In addition, higher Chl-a and TP concentrations were measured along fabricated shorelines than natural shorelines, all three parameters tended to be higher at bayside than oceanside stations, and TN was generally higher during the wet season than during the dry season. The Florida Bay Watch project of monitoring water quality in nearshore waters at fixed stations along the Florida Keys remains active, and another full year of data will soon be available. Characteristically, monitoring projects such as FBW gain exponentially in value with the accretion of information through time. Through the continued diligence and willingness of volunteers, TNC will be able to augment these preliminary findings with a substantial record of water-quality variation in the nearshore waters of the Florida Keys.