Influence of Coastal Geomorphology and Watershed Characteristics on the Water Quality of Mangrove Estuaries in the Ten Thousand Islands - Whitewater Bay Complex.

Topical Area: Water Quality

Joseph N. Boyer and Ronald D. Jones, Southeast Environmental Research Program, Florida International University, Miami, FL

The Ten Thousand Islands - Whitewater Bay area (TTI-WWB) is located on the southwest coast of the Florida peninsula between Cape Sable and Cape Romano (Fig 1). It is a brackish complex which drains into the Southwest Florida Shelf region of the Gulf of Mexico. In this region there are >60,000 ha of mangrove forest made up of tall trees (up to 25 m); mainly red (Rhizophora mangle), black (Avicennia germinans) and white (Laguncularia racemosa) mangroves. The mangrove forest forms a continuous band that stretches about 15 km inland from the coast. The benthos of the embayments in this region is composed largely of calcareous muds overlaid with occasional seagrass beds and oyster bars.

A total of 47 fixed stations were situated within the TTI-WWB complex so as to provide a sampling gradient from freshwater to estuary throughout the many rivers and islands in the region (Fig. 1). Stations in WWB were sampled monthly beginning Oct. 1992; sampling in TTI began in Sept. 1994. Water quality parameters measured at each station included the dissolved inorganic nitrogen (DIN) components: nitrite (NO2), nitrate (NO3), and ammonium (NH4) as well as soluble reactive phosphate (SRP). Total concentrations of nitrogen (TN), organic nitrogen (TON), organic carbon (TOC) and phosphorus (TP) were also determined. The biological parameters measured included chlorophyll a (Chl a; µg l-1) and alkaline phosphatase activity (APA; µM hr-1). Field parameters determined at both the surface and bottom of the water column at each station included salinity (ppt), dissolved oxygen (DO; mg l-1), temperature (°C), and turbidity (NTU).

Large ranges in most measured parameters were the norm owing to the wide spatial and temporal sampling plan. Overall, the region was warm and hyposaline, with a median temperature of 26.6°C and a median salinity of 12.9 ppt. The median DO was 5.9 mg l-1, or ~77 % of saturation. Concentrations of DIN were a small fraction of the N pool (4%) with TON making up the bulk. NH4 was the dominant DIN species in almost all of the samples (57 % of DIN). SRP concentrations were very low (0.09 µM, ~3 ppb) and comprised only 10% of the TP pool (0.92 µM). Chl a concentrations were also low overall (2.7 µg l-1) but ranged from 0.12 to 45.1 µg l-1. The molar TN:TP ratio suggested a strong P limitation of the water column with the median being 61, but had a range from 10-157.

Estuaries within the TTI-WWB region are highly compartmentalized by local geomorphology, making it difficult to study nutrient biogeochemistry using standard schemes of estuarine ecology. In addition, the sources of both freshwater and nutrients are difficult to quantify, owing to the non-point source nature of runoff from the Everglades and the dendritic cross channels in the mangroves. To gain a better understanding of the spatial patterns of water quality in the region, we used a multivariate statistical technique to reduce the number of variables into a smaller set of independent, synthetic variables that captured the variance of the original data set.

Principal component analysis of the 16 water quality parameters collected monthly resulted in 5 principal components (PC) which explained 72.5% of the variance of the original parameters. The "Organic" component (PCI) was composed of APA, TON, and TOC; the "DIN" component (PCII) contained NO2-, NH4+, and NO3-; the "Phytoplankton" component (PCIII) was made up of TP, Chl a, and turbidity; DO and temperature were inversely related in PCIV; and salinity and SRP made up PCV.

A cluster analysis of the mean and SD of PC scores by site resulted in the spatial aggregation of the 47 stations into 6 distinct zones having robust similarities in water quality (Fig. 1). The first cluster was composed of 13 stations in the Shark, Harney, Broad, and Lostmans Rivers and was called the Mangrove River group. This cluster also included a sampling site just off the Faka Union Canal. The second cluster, called Whitewater Bay, was made up of the 8 stations located within that body of water. Twelve stations situated mostly in and around the northern coastal islands formed the Gulf Island group. The next cluster contained the northernmost 2 stations in the Blackwater River estuary (Blackwater). The Inland Waterway zone included 11 stations distributed throughout the inside passage as well as the Chatham River and our station off Everglades City. Finally, water quality characteristics of the Coot Bay site were sufficiently different so as to make it an outlier.

Marked differences in physical, chemical, and biological characteristics among zones were illustrated by this technique. The spatial trend that emerges is one with relatively high APA, TON, and TOC in the south declining northward along the coast. Conversely, salinity, TP, SRP, and turbidity increase along the northward gradient. No significant trend in DIN or Chl a was observed. The net effect on the TN:TP ratio is that very strong and persistent phosphorus limitation occurs in the Whitewater Bay region (median=105) which grades to a more balanced ratio in Blackwater River (median=17).

We believe these gradients are the result of coastal geomorphology and watershed characteristics in the region. The width of the mangrove forest is widest in the south (15 km) but grades to only 4 km wide in the northern TTI; this being a function of elevation and sediment type. Whitewater Bay is an semi-enclosed body of water with a relatively long residence time which receives overland freshwater input from the Everglades marsh. The relatively long water residence time may explain the very low P concentrations (from biological uptake), while the high evaporation rate would tend to concentrate dissolved organic matter (DOM). The Mangrove Rivers are directly connected to the Shark River Slough and therefore have a huge watershed relative to their volume.