Envlron. Sei. Technol. 1988, 20, 817-821
Relative Copper Binding Capacities of Dissolved Organic Compounds in a Coastal-Plain Estuary Avls D. Newell' and James G. Sanders The Academy of Natural Sciences, Benedict Estuarine Research Laboratory, Benedict, Maryland 20612
Naturally occurring organics in samples taken from the mesohaline portion of a Chesapeake Bay tributary were separated into four nominal molecular weight fractions and analyzed for their ability to bind copper by using an algal bioassay. The organics exhibited a relatively high capacity for copper, with the binding capacity directly proportional to dissolved organic carbon (DOC) concentrations. The relationship between binding capacity and DOC is similar in the Chesapeake Bay and in other estuaries and marine ecosystems. Such correlation is not found in freshwater ecosystems. The strong relationship between DOC concentrations and binding capacities in marine ecosystems may be due to the autochthonous origin of marine organics.
Introduction Naturally occurring organics in aquatic systems consist of highly variable mixtures of compounds from many sources. Aquatic organic material varies widely with season and location, originating from terrestrial sources such as plant decay and leaching from soils, or from autochthonous sources such as the decay of aquatic organisms. Within estuarine and coastal marine systems, autochthonous inputs outweigh terrestrial ones (1-3). Dissolved organics generally are composed of large, polydisperse molecules with multiple functional groups. Polysaccharides, amino acids, polypeptides, and humic and fulvic acids all occur dissolved in natural waters (4). Humic and fulvic acids are thought to persist, undergoing little change in their chemical composition. Other compounds are more rapidly degraded. The origin of persistent compounds may vary with ecosystem type. Harvey et al. (5) have proposed that marine humic and fulvic acids are formed from autochthonously produced fatty acids. Alternatively, freshwater humic and fulvic acids probably originate predominately from soil and soil microbes (6, 7). Without regard to their origin, the many functional groups present in these persistent organic molecules are able to form complexes with metal ions. These complexes can vary widely in their configuration, influenced by such factors as metal and organic concentrations and neighboring functional groups (8). Despite the difficulties in characterizing such metalorganic complexes, their existence is important biologically and ecologically because they help regulate the availability of essential and toxic trace metals (9-11). One area of research that has received considerable attention is organic regulation of the cupric ion, Cu2+. Copper is an essential element for biotic growth. At higher concentrations, however, it is toxic. The importance of organics in regulating copper bioavailability and toxicity has been extensively studied with algal cultures by using chemically defined culture media with additions of commercial chelating agents (11-13). The role of naturally occurring organic compounds is not well understood; however, they are considered an important regulator of toxicity to biotic communities (14-16).
* Address correspondence to this author at Northrop Services, Inc., 200 SW 35th Street, Corvallis, OR 97333. 0013-936X/86/0920-0817$01.50/0
The greatest potential for harm from man's activities for marine systems is within the estuary, which serves as a physical and chemical buffer between man's activities on the shoreline and the open ocean. It is necessary that we establish the importance of organics in alleviating potential metal toxicity within these ecosystems. In this paper, we examine and characterize naturally occurring estuarine organics and their potential for sequestering copper.
Materials and Methods Study Site and Sampling Procedures. Samples were taken from three stations on the Patuxent River, the sixth largest tributary to the Chesapeake Bay (Figure 1). The only large industry along the river is the Chalk Point Power Station, located approximately 2 km upstream from the intermediate station. The stations for this study were spread along the river at approximately equal salinity intervals, with the upstream sample containing a salinity of 5%0and the highest salinity at the mouth of the Patuxent River, 13%0. Samples were collected in acid-washed, 20-L polyethylene containers, submerged below the water surface from the bow of a 5-m open fiberglass boat as it slowly moved upstream. Filtration and Analysis. Water from each site was filtered through acid-washed glass-fiber filters and Nuclepore membrane filters (pore size of 0.4 pm) held in an all-plastic filtration apparatus. Samples were filtered through the glass-fiber filters immediately upon returning from the field and were stored at 4 "C until used. Membrane filtration was completed just prior to ultrafiltration. Amicon holofiber fiiters were used to separate water into four nominal molecular weight fractions: >lo0 OOO, 100OOO-10OOO, 1OOOo-lOOO and
