Abstract

During 2007–08 the U.S. Geological Survey, in cooperation with the U.S. Air Force, evaluated the concentration of polychlorinated biphenyls (PCBs) in aquatic invertebrates and fish from one site in the main body of Lake Worth, two sites in a small inlet in Lake Worth (upper and lower Woods Inlet), and one site in Meandering Road Creek in Fort Worth, Texas. The four sites sampled during 2007–08 were located at or near sites where surficial bed-sediment samples had been collected and analyzed for PCBs during previous U.S. Geological Survey studies so that PCB concentrations in aquatic invertebrates and fish and PCB concentrations in surficial bed-sediment samples could be compared. Stable nitrogen and carbon isotopes were used to help assess differences in the amount of these isotopes by species and sampling location. The sum of 15 PCB-congener concentrations was highest for aquatic invertebrates and fish from the upper Woods Inlet site and lowest for the same aquatic invertebrates and fish from Lake Worth site, where PCBs historically had not been detected in lake bed sediment. An increase in the ratio of the heavier nitrogen-15 (¹⁵N) isotope to the lighter nitrogen-14 (¹⁴N) isotope, referred to as enrichment of ¹⁵N, was highest in largemouth bass (representing the highest trophic level sampled) at all sites and lowest for true midge larvae inhabiting surficial bed sediment in the lake (representing the lowest trophic level sampled). Enrichment of ¹⁵N was less variable in largemouth bass and other fish from the highest trophic level compared with shorter lived, primary consumer invertebrates from lower trophic levels, such as true midge larvae, mayfly nymphs, and zooplankton. The delta carbon-13 (δ¹³C) values measured in true midge larvae collected at the Lake Worth and upper and lower Woods Inlet sites were more negative compared with the δ¹³C values measured for all other taxa, indicating true midge larvae were more depleted of carbon-13 (¹³C) compared with all other aquatic invertebrate and fish. The relative depletion of ¹³C might indicate the carbon sources consumed by true midge larvae are different from the carbon sources consumed by all other taxon that were sampled. Ratios of stable nitrogen isotopes nitrogen-15 to nitrogen-14 (δ¹⁵N) were similar between taxa from the Lake Worth site and Woods Inlet sites. The sum of 15 PCB-congener concentrations, however, was an order of magnitude higher in largemouth bass from the upper Woods Inlet site, indicating that PCB-congener concentrations in lake bed sediment likely controls biomagnification within the lake because of the similarities in trophic structure of the resident aquatic community. The biota at the Lake Worth reference site, where PCBs were not detected in the surficial sediment during previous studies, were less contaminated than the biota at sites where PCBs had been detected in the surficial sediment. The highest trophic-level consumers (as evidenced by the most ¹⁵N-enriched δ¹⁵N values) showed the maximum bioaccumulation.

Revised March 15, 2011

First posted January 20, 2011