Sawgrass Biogeochemistry

Enrichment factors (EF) were calculated to measure the magnitude of element uptake in sawgrass relative to element bioavailability. This technique has also been used to assess possible anthropogenic vs. geogenic (natural) additions to plant chemistry when plants have been employed as air quality biomonitors (Wiersma and others,1992; Gough and others, 1994). The calculation takes the form:

EF =	[X] in sawgrass/[Al] in sawgrass [X] in sediment/[Al] in sediment

where [X] and [Al] are the concentrations of the element of concern and of Al, respectively, in plant material, relative to their concentrations in sediment. This procedure normalizes the data with respect to a geochemical reference (in this case Al). Aluminum is used because it is a conservative, lithogenic element and because in neutral substrates (pH near 7.0) it is relatively unavailable for plant uptake. Because sawgrass has a relatively shallow rhizomatous, branching root system (Steward and Ornes, 1975), we used the average for the concentration of elements in the top 30 cm of core material (where the majority of the root biomass is found) for the calculation of the EF. Assuming that the plant material has been properly cleaned before analysis, the EF is a measure of the relative uptake of an element from its substrate–a sort of "bioavailability" assessment–in the top 30 cm of sediment.

Figures 11A, 11B, and 11C present EF bar plots for selected major and trace elements for sawgrass leaf and root material at six core sites in Taylor Slough. The sites, with paired bars (for leaf (L) and root (R) material), are arranged roughly north/south along the Slough (fig. 2). Root material was not sampled at the core TS7 site and core TS9 lacks EF data for Cu because of a large number of Cu concentration values in sediment that were below the LLD of 2 ppm (table 6).

In general, except for Fe, the EF values in root were lower than in leaf tissue. In many instances this difference was substantial–greater than an order of magnitude. These EF values indicate that sawgrass tissue has a greater tendency to accumulate elements relative to the substrate within which it is growing. The high EF values may indicate a physiological compensation mechanism that allows for element uptake and accumulation by sawgrass in low nutrient sediments (Gunderson, 1994).

Figures 11A, 11B, and 11C also show a general north/south trend with higher EF values in the north (the core site TS1 does, however, show somewhat smaller EF values for most of the elements, especially in root tissue). Of particular interest in this study are the Hg values that also follow this trend. These data suggest a general process, regional in scope, that governs the uptake of elements in Taylor Slough. Even the elements that do not show a regional trend of decreasing element concentration in sediment from north/south (Mg, Na, Hg, and Zn, table 6) show strong decreasing north/south EF trends. For example, the concentration of Na in sediment increases north/south as the sites approach Florida Bay–a reflection of a subsurface salinity gradient. Nevertheless, figure 11B shows a strong north/south EF decrease.

Further, core sediment texture and composition does not show a north/south component. There appears to be no relation between bulk density and organic matter content on a regional scale (figs. 2, 7, and 12). In figure 12 the cores do appear distinct in composition, however, and each core does express a unique plot. For example, core TS1 (furthest north) appears intermediate in both bulk density and organic matter whereas core TS9 (south-central part of the Slough) has high bulk density but low organic matter. The two sites furthest south (core TS15 and TS16) have low bulk density and high organic matter–but so does core TS7 which is located at the north end of the Slough. How these data relate to the proportion of marl vs. organic carbon awaits examination.

Water Chemistry

Table 9 lists the analyses for the concentration of elements in surface water collected at six sites in Taylor Slough. Little interpretation of these data is offered at this time. The samples are generally arranged north/south (fig. 2). In general, the data are regionally very uniform with the possible exception of Ba, Co, Mn, Ni, and Sr. All total Hg analyses were below the 0.2 µg L^-1 detection limit for the atomic absorption spectrometry method used. This detection limit is two orders of magnitude above the values reported by Hurley and others (1998) for surface water in the canals that feed the Water Conservation Areas further north.

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