|Mercury (Hg) is a harmful bioaccumulative heavy metal to which humans are
exposed primarily through fish consumption. Several consumption advisories for
mercury have been issued for fishes along the US Atlantic and Gulf coasts, including the
Mobile-Tensaw River Delta in coastal Alabama. Mercury cycling in estuaries is complex
and little is known about the extent of mercury bioaccumulation in the ecologically
diverse fishes in coastal areas such as the Mobile-Tensaw Delta, Alabama. Using
traditional tissue analysis techniques combined with otolith microchemistry, diet an
age and growth analysis, and bioenergetics modeling, I investigated seasonal and spatial
trends of mercury accumulation in largemouth bass, Micropterus salmoides, and southern
flounder, Paralichthys lethostigma, inhabiting the Mobile-Tensaw Delta.
?Age-normalized largemouth bass mercury tissue concentrations increased
significantly from downstream to upstream locations with little seasonal variation.
Largemouth bass collected at both upstream and downstream locations exceeded
common minimum consumption and no consumption advisory levels. Microchemistry of
largemouth bass otoliths indicated that individuals did not migrate between upstream and
downstream regions in the Mobile-Tensaw Delta. Diet analysis showed that largemouth
bass at downstream locations foraged on lower trophic levels than those upstream and
bioenergetics analysis confirmed that this difference was the primary factor responsible
for lower largemouth bass Hg accumulation downstream.
Southern flounder mercury tissue concentrations were uniform across the sample
area and were lower than those of largemouth bass. No southern flounder had Hg tissue
concentrations that exceeded common minimum consumption advisory levels.
Microchemistry analysis of southern flounder otoliths indicated a highly variable
migratory life history across salinity gradients. This migratory life history was likely
responsible for similar Hg tissue concentrations across the Mobile Delta for southern
flounder. Differences in Hg bioaccumulation between largemouth bass and southern
flounder were not the result of trophic differences between species but rather the result of
faster growth rates in southern flounder. For both largemouth bass and southern flounder
a negative relationship existed between salinity exposure and Hg tissue concentration,
although there was considerable variability in this relationship. Initial efforts to detect
mercury directly in the otoliths of both species as a way to measure lifetime Hg
accumulation trends were not successful.