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In five summer cruises during the period 2000 -2004 , seventy -four box cores
were collected from eleven locations from the Mississippi Canyon (480 - 2750m ,
northern Gulf of Mexico ) , and an adjacent transect (336 -2920 ) to understand the
community structure and trophic function of amphipods and for measuring the
bioaccumulation of polycyclic aromatic hydrocarbons , (PAHs ) . Amphipods were
discovered to be an important component of the macrofauna of the Mississippi Canyon
(40 % of the total faunal abundance ) . Seventy two species , belonging to nineteen
families , were collected from the study area with 61 species from the canyon and only
38 species from the non -Canyon transect . The head of the canyon (480m ) was
dominated by dense mats (15 ,880 ind /m2 ) of a new amphipod (Ampelisca
mississippiana ) . The logarithm of the amphipod abundance decreased linearly with
depth . The species diversity (H` ) exhibited a parabolic pattern with a maximum at
1100m . The differences in amphipod abundances and biodiversities were correlated with
the variation in the amount of available organic matter . The depression in diversity in the
canyon head is thought to be competitive exclusion resulting from the dominance by A .mississippiana , but the high species richness is presumed to be a function of the
structural complexity of the canyon .
Annual secondary production of A . mississippiana was 6 .93 g dry wt m -2 , based
on size -frequency method and corresponding to an estimated univoltine generation from
a regression model . The production /biomass ratio (P /B ) was 3 .11 . Production of this
magnitude is comparable to shallow marine ampeliscids but are high for the depauperate
northern Gulf of Mexico .
The effect of the organic contaminants and the bioavailability to the amphipods
was determined through measuring the bioaccumulation of the PAHs . The distribution of
PAHs in sediments was different from the distribution in the organisms suggesting
preferential uptake /depuration or uptake from pore or bottom waters . The average
bioaccumulation factor (4 .36 ± 2 .55 ) and the biota sediment accumulation factor
(0 .24±0 .13 ) for the total PAHs by the ampeliscids were within the range reported for
other benthic invertebrates . The average bioaccumulation factors were highest for
dibenzothiophenes (up to 132 ) and alkylated PAHs and lowest for parent high molecular
weight PAHs . |
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