Release of Trace Metals by Sewage Sludge and the Subsequent Uptake by Members of a Turtle Grass Mangrove Ecosystem John R. Montgomery* and Mary T. Price Harbor Branch Foundation, Inc., RR 1, Box 196, Ft. Pierce, Fla. 33450 This research was initiated to determine the rates of uptake, by a Thalassia testudinum Konig/mangrove ecosystem, over a 125-day period, of Cd, Cr, Cu, Ni, Pb, and Zn. The metals were leached from sewage sludge by flowing seawater (8.4 L min-l) and subsequently traversed the model system. T h e largest net uptake of metals occurred in the “fouling organisms” where Cd, Pb, and Zn uptake closely paralleled the net loss of metals from the sewage sludge. Thalassia leaves showed a net uptake for Cr, Pb, Ni, and Zn. The urchin L y techinus uariegatus (Lamarck), a grazer of Thalassia leaves, also demonstrated a net uptake of Cu, Cr, Pb, Zn, and Ni. The sea cucumber Holothuria mexicana (Ludwig) showed a net uptake of Cr, Cu, Pb, and Zn. Net uptake of metals was shown by the roots of the red mangrove Rhizophora mangle Linne. Because of insufficient sample mass, no consistent metal uptake was found for the clam Codakia orbicularis (Linne), oyster Crassostrea rhizophorae (Guilding), or the snail Nerita tessellata (Gmelin). T h e results indicate that the dumping of sewage sludge in coastal tropical waters can lead to the uptake and concentration of toxic trace metals by members of a turtle grass community.
The deposition of sewage sludge in the marine environment may adversely affect the resident organisms by the release of toxic materials into water in forms that may be incorporated into the organisms. The mass dumping of treated sewage sludge into oceans and shallow inshore areas is occurring in a number of areas. In the New York Bight, an area receiving large quantities of solid waste for almost a century, Carmody, Pearce, and Yasso ( 1 ) determined that there was a definite increase of Cr, Cu, Pb, Ni, and Zn in the sediment due to the dumping of sewage sludge. The uptake of metals by marine organisms and concentration within the food web have been demonstrated numerous times ( 2 - 7 ) . Schroeder (8) demonstrated, using radioactive isotopes, that Thalassia t e s t u d i n u m Konig would concentrate cobalt and manganese in the leaves rather than the root structure (excluding rhizomes) and that the incorporation of cations in Thalassia t e s t u d i n u m was primarily accomplished through the leaves. Very little has been published regarding the rates of release and subsequent uptake of potentially toxic trace metals from sewage sludge into complex tropical ecosystems. The necessity for this type of controlled field experiments was explained by Menzel (9). Our objectives in this research were to use a flow-through system of sufficient size and complexity so as to nearly duplicate a tropical marine ecosystem and also allow repeatability over varying seasons. This system would allow us to determine the rates of uptake of Cu, Cd, Cr, Zn, Ni, and P b which were leached from sewage sludge by seawater. The community investigated was comprised of turtle grass ( T h a lassia t e s t u d i u m Konig) with its associated benthic infauna, the sea urchin (Lytechinus uariegatus (Lamarck)), the sea cucumber (Holothurea mexicana (Ludwig)), attached “fouling organisms”, plankton, and the red mangrove ( R h i zophorae mangle Linne). This ecosystem was chosen for three reasons: (a) the system comprised a tightly bound community with strong interaction 546
Environmental Science & Technology
between the members, (b) the important role of the seagrasdmangrove ecosystem in the overall tropical near-shore environment, (c) the possibility of the active concentration of these trace metals in the higher trophic levels with passage of the materials through the food webs.
Methods Construction of Ecosystem Tanks. The seawater system and tanks were located at Mayaguez, Puerto Rico (18” 15’ N, 67” 12’ W). Two wooden, fiberglass coated tanks (9.2 m X 0.9 m X 1.1m) were constructed and housed in an open-sided, roofed (translucent plastic) structure. Seawater System. Each tank was filled to a depth of 0.5 m with calcareous beach sand with an additional 0.1 m of sand over the last 3 m of tank. The seawater for the system was pumped through a prefilter and gravity fed to the two seawater tanks. The flow rate in each tank was 8.2 to 8.7 L min-1. The total water volume per tank was 3.1 X lo3 L. Sampling. A t 1,5,25,50,85,and 125 days after activated sludge was added to the experimental tank samples of organisms, sediment, sludge, and water were taken frqm both tanks. A presample was taken prior to addition of any sludge; in this study it is referred to as day 0. The initial stocking quantity or density for the biological organisms used ranged from 52 to 57 for the urchins and Holothurians, 230 mangrove seedlings, and from 106 to 180 for the snails, clams, and oysters. Exact physical data on stocking density are shown in Montgomery et al. (10). The organisms were allowed to equilibrate for 2 weeks in the flowing seawater system. The turtle grass (Thalassia t e s t u d i n u m ) density was 400 erect shoots m+. Sewage Sludge. The sludge was allowed to settle and the overlying liquid decanted. The sludge (0.7 m:3) was added to a depth of 7.6 cm over the sediment, at the incurrent end of the experimental tank. Sample Analysis. All reagents were reagent ACS grade. All glassware was cleaned with detergent and rinsed with water (Milli-Q System). Samples for trace metal analysis were digested in previously refluxed, covered beakers. Only nonmetallic instruments and materials were used. Biological and Sediment Samples. National Bureau of Standards (NBS) bovine liver and orchard leaves were analyzed using the same methods as the regular samples. The number of organisms sampled per day of experiment is shown in ref 10. The samples were defrosted after collection, dissected, dried a t 105 “C, and homogenized with a ceramic mortar and pestle. A wet chemical digestion procedure was used for all biological samples (10). The experiments were replicated. Experiment I began on March 11,1975 and ended July 15,1975; experiment I1 began on December 12,1975 and ended April 26,1976. Seawater. Seawater samples were collected in 2.5- or 4-L glass bottles (reagent acid bottles). Samples were immediately filtered through precleaned 142 mm diameter, 0.45 mm pore size filters, then frozen. Analysis of Trace Metals. Extraction and preconcentration procedures using chelating resins ( I O ) were used. Flame atomic absorption spectrophotometry with background cor-
0013-936X/79/0913-0546$01.00/0
@ 1979 American Chemical Society
Table 1. Results in pg g-’ Dry Weight for Determination of Metals in NBS Bovine Liver and Orchard Leaf Standards a Zn NI Cd cu Cr Pb Experiment I, Bovine Liver MDC X
SD
N
0.3 0.56 0.26 9
3.4
8.9 177 20.2 10
0.5
