METHODS

SAMPLING STATIONS

There are basically two types of coral reefs in the Florida Reef Tract: the smaller patch reefs that grow on the White Banks, and the larger bank reefs that form the offshore coral reef barrier (Jaap 1984). There are few patch reefs occurring inshore of Hawk Channel, a deeper (- 10 m deep) channel that runs N-S in the Key Largo area and E-W in the middle and southern Keys. Thus, Hawk Channel can be loosely considered a dividing-line between inshore non-reef areas and offshore reef areas. Transects were selected to run from shore locations where potential sources of nutrients occurred (e.g. major developments, landfills, as well as from natural sources (mangrove forests) or inter-key passes between Florida Bay and the Reef Tract), to the offshore coral reefs. Water and sediment samples were collected at a series of stations along each transect, where a minimum of four stations were selected to include both inshore and offshore sides of Hawk Channel. Transects in Biscayne National Park (6 stations) and off Long Key (13 stations) were sampled during both high and low tides, to include conditions of minimal and maximal, respectively, offshore excursion of Florida Bay water and shore-derived nutrient inputs, as well as during both dry (tourist) and rainy seasons. The transect in the Key Largo area (35 stations) was sampled once during the summer season, and a smaller more limited sampling (13 stations) during the winter season in association with collecting sediment samples (see below). Looe Key (7 stations) and associated areas were sampled only during the spring and summer

seasons.

WATER COLUMN SAMPLING

Water samples were collected with Niskin bottles deployed from a small boat, or by SCUBA divers. Samples were taken 1 m below the surface and 1 m above the bottom unless water depth was less than 3 m, in which case a single sample was taken at mid-depth. Water samples were transferred to conditioned polyethylene bottles and kept on ice in the dark until returned to the laboratory for analysis. In the laboratory, 10 ml subsamples of unfiltered water were analyzed for total N and P by the persulfate method of D'Elia et al. (1977). The remaining water was filtered through GF/F glassfiber filters to collect particulates for chlorophyll analysis. Filters were stored frozen until extraction with 90 % acetone. Samples were sonicated on ice to break up algal cells and extracted at 4°C overnight before measurement with a Turner Model 112 fluorometer (Parsons et al. 1984b). Subsamples of the filtered water were analyzed for nitrate plus nitrite, and phosphate by standard Technicon autoanalyzer techniques for seawater, and for ammonium by a high-sensitivity modification of the Slawyk and MacIssac technique (Szmant et al. 1990).

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SEDIMENT SAMPLES

Two types of cores were taken at each station: short 10 cm cores taken with corers made from 60 ml syringes, and longer 30 cm cores taken with 9-cm diameter aluminum corers. Ten ml sections of the small cores were extruded into pre-weighed aluminum dishes and dried for measurement of sediment porosity. Dried sediments were ground with an agate mortar and pestle, and analyzed for nitrogen with a Carlo Erba elemental analyzer, or for phosphorus after dissolving weighed subsamples in hydrochloric acid followed by persulfate digestion. The larger cores were extruded in 1.5 cm intervals, and pore-waters extracted from the sediments by filtration. Porewaters were analyzed for dissolved nutrients by the same methods used for the water column samples.

SEFCAR SAMPLES

Additional water samples were collected during SEFCAR hydrographic research cruises by Drs. Clarke and Lee, University of Miami, during shallow CTD casts (to 200 m) on transects from the outer reefs to the middle of the Florida Current (range: Carysfort Reef to Marquesas), and analyzed for nutrients. These samples were kept frozen until returned to the University of Miami for analysis of total and dissolved nutrients as described above. Chlorophyll analyses of these samples were done by Dr. Clarke.

RESULTS AND DISCUSSION

WATER COLUMN NUTRIENTS AND CHLOROPHYLL

Biscayne National Park

The transect sampled within BNP went from Adam's Key in Caesar's Creek to offshore of Pacific Reef (Figure 19). A station on the Biscayne Bay side of Caesar's Creek was sampled only once on March 3, 1990. Most of the N in the water column was as organic and particulate N; very little NH, was measured (usually below our limits of detection of about 0.05 μM) and the NO, concentrations were generally below 0.3 μM. As expected, N nutrient concentrations were higher in Caesar's Creek than more offshore. Total N concentrations differed greatly between calm and stormy days. During storms, when sediments were resuspended into the water column making the water quite turbid (January and March sampling dates), concentrations in some samples exceeded 40 μM (Figure 20). In fact, total N concentrations in the March samples were highest for the offshore stations which were the shallowest and most disturbed by the storm. During calmer days, concentrations of total N were in the 8 to 12 μM range typical for oligotrophic reef waters (Szmant-Froelich 1983; D'Elia and Webb 1990).

Map of the portion of Biscayne National Park studied showing sampling stations (indicated by triangles).

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Dissolved inorganic nutrients were not affected by the storms indicating that most of the storm-suspended N was organic in form.

Reactive PO, concentrations were generally very low, below 0.1 μM, with organic PO, concentrations somewhat higher, around 0.25 μM except during storms when values as high as 1 μM were measured (Figure 20). Phosphorus concentrations tended to follow the opposite pattern to N, with the highest concentrations usually found in the offshore samples (Figure 20): on four of the six sampling trips the highest organic or reactive PO, concentration was measured in samples taken from Pacific Reef.

Chlorophyll a concentrations were always low, even when organic nutrient concentrations were somewhat higher during storms. They were almost always lower than 0.25 μg/l, a value typical for oligotrophic open ocean waters (Parsons et al. 1984a). Thus, any increased turbidity or color of the water noticed during low tides and storms was not due to plankton blooms, but most likely to resuspended sediments or tannins originating from the mangrove forests on the barrier islands and Biscayne Bay.

Key Largo

The four transects in the Key Largo area covered from the northern to the southern ends of the Pennekamp Coral Reef State Park and Key Largo National Marine Sanctuary, the area with the best coral reef development within the Florida Keys (Jaap 1984; Figure 21). The latter is due to the fact that there are no major passes between Florida Bay and the Reef Tract in this area. Trends in the Key Largo area were somewhat similar to those in BNP. Inorganic and organic N and P concentrations were higher in the canal and inshore stations, with the highest concentrations of both measured in samples from marinas and developed canals, and in Largo Sound (Figure 21). Some of these samples were notable for their elevated concentrations of NH, NO, and total PO,. An area with especially high organic and inorganic P concentrations was that off the Ocean Reef development at the northern end of Key Largo, indicating a potential source due to human sewage. Chlorophyll a concentrations were also higher in these inshore samples, as might be expected. Both nutrient and chlorophyll concentrations, however, had decreased to normal oligotrophic levels by the time Hawk Channel was reached (concentrations similar to those of BNP) (Figure 21). NH, concentrations were below detection throughout the Hawk Channel-tooffshore stations with the exception of samples taken from ‘Algal Reef', which were somewhat elevated in both NO, and NH, but not PO; however, the Algal Reef samples were taken from under-hangs of corals rather than from the water column, and thus elevated nutrient concentrations were to be expected (Szmant-Froelich 1983). PO, concentrations were generally below 0.08 μM, but again, higher concentrations of P were encountered sporadically in samples from the outer reefs.