A primary producer of organic carbon in the south Florida ecosystem is periphyton. Periphyton is an aggregate that includes algae, associated bacteria and, in the case of much of the cyanobacteria (blue-green algae) of the south Florida ecosystem, calcium carbonate. Often, periphyton in the south Florida ecosystem is attached to the aquatic plant utricularia and is present as a floating mat on the surface of the water.

The value of periphyton in the study of the geochemical cycling of metals in the environment is related to the ability of algae in the periphyton to accumulate metal ions from the water column and to integrate over time water column concentrations of metals and other dissolved species. A high level of bacterial activity in the periphyton of south Florida is anticipated due to labile organic carbon in the mats and the elevated temperatures of this subtropical area. The mats offer food and shelter to many organisms, both macroscopic and microscopic.

Results are reported for samples that were collected in March, July, August, and December 1995, and June, August, and December 1996. Parameters that were measured include total mercury, methyl mercury, phosphorus, total carbon, organic carbon, and total nitrogen in the periphyton, and water column parameters including pH and concentrations of major cations and anions, including alkalinity. In 1966, mats that were collected were frozen on site, brought back to the laboratory, and, before drying, sectioned into horizontal layers. The individual layers were analyzed.

One of the questions that must be resolved before differences in concentrations of total mercury, methyl mercury and other geochemical species can be evaluated between sites in the Florida ecosystem in the statistical variation in concentrations of species of interest at any one site among samples collected at the same location. To evaluate this variation, two sets of samples from sites in south Florida were collected on the circumference of a circle having a diameter of 2 meters. Triplicate samples were collected at 60° intervals. These samples were analyzed for total mercury, methyl mercury, phosphorus, organic carbon, inorganic carbon and total nitrogen. The variation among sample concentrations in material collected from site U-3 in WCA2A for organic carbon and total mercury ranged from 30.7 ± 0.2 percent for organic carbon, which was the least variable parameter, to 0.12 ± 0.07 µg g^-1 for total mercury, which was the most variable parameter. All concentration are reported on a dry weight basis. The analyses for methylmercury showed that the presence of methylmercury in the periphyton samples was patchy at the detection limits of the method that was used (0.01 µg g^-1 dry weight). Spatial variation suggests that the reporting of trends rather than absolute concentrations of constituents in the mats might be desirable.

If an average of the total mercury concentrations determined for samples collected at various times of the year is used to compare mercury concentrations from site to site, difference between sites are less clear than if concentrations of samples collected during the same field trip are compared. Trends in mercury concentrations in a north to south direction are more clear among samples collected at approximately the same time. With this caveat in mind, it appears that the tendency is for total mercury concentrations in the periphyton mats to decrease as samples are taken in a more southerly direction, or in a direction from a canal into the marsh.

Mercury concentrations in periphyton are correlated with phosphorus concentrations. The linear correlation coefficient (r) is 0.62 (n=120) for a comparison of total mercury concentrations in all of the whole periphyton samples that were analyzed with the phosphorus concentration in the same samples. If the samples that are compared are limited to whole periphyton material from WCA2A, the linear correlation coefficient between total mercury and total phosphorus is 0.76 (n=19). While the concentrations of phosphorus in the mats from a given site are generally larger early in the calendar year as compared with later in the calendar year, this is not necessarily true for the concentrations of total mercury.

Trends in chemical concentrations in periphyton mats fall into two categories: (1) between sites, and, (2) within sectioned mats. Total mercury concentrations in the mats are inversely correlated with the inorganic carbon concentrations in the mats. This observation is true for comparisons of mercury concentrations calculated on a whole sample basis and for mercury concentrations corrected for the inorganic carbon content in the sample. This suggests that the decreased concentration of mercury in the mat might be due to coating of sorption sites by calcium carbonate. The agreement between mercury and inorganic carbon concentrations is better for whole samples than it is for sectioned mats. Total mercury concentrations are positively correlated with organic carbon and nitrogen concentrations in the mats. It is known that mercury is readily sorbed by organic matter. The mat materials that were collected from the south Florida ecosystem were 13 to 39 percent organic carbon, much of which is algae that provides active sites for the partitioning of mercury from the water to the solid mat material. Concentrations of total mercury tend to correlate positively with methyl mercury concentrations in whole mat material. Within mats, the largest total mercury and methyl mercury concentrations tend to be localized and not necessarily correlated with the same geochemical parameters as total mercury in the whole mats. Factors that affect the distribution of mercury in the mats are being investigated.

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