2 A
. Bennett et al. J. Exp. Mar. Biol. Ecol. 242 1999 1 –20
concentrations of fucoxanthin and zeaxanthin decreased dramatically after 4 days to 0.1–0.3 mg g
21 21
dry sed. and 0.01–0.15 mg g dry sed. , respectively. Increased total phaeophorbide con-
centrations a biomarker for metazoan grazing over the first 12 days of the experiment in treatments with and without L. irrorata additions indicated that meiobenthos may have been
important in grazing-down microphytobenthos. High-Exposure snails from High-PAH field sites gained more weight during the initial phase of the experiment when microphytobenthic abundance
was decreasing but eventually lost more weight than did Low-Exposure snails from Low-PAH field sites by the end of the experiment. Although there was considerable variability in the snail
growth values, we speculate that the High-Exposure snails were more active in feeding during this experiment and, after the initial decline in microphytobenthos, became resource-limited throughout
the remainder of the experiment.
1999 Elsevier Science B.V. All rights reserved.
Keywords : PAH; Plant pigments; Microphytobenthos; Littorina irrorata; Salt marsh
1. Introduction
Studies over the last several decades have shown that the accumulation of polycyclic aromatic hydrocarbons PAHs in sediments may have profound effects on benthic
communities Griffiths et al., 1981; Wyndham, 1985; Bauer and Capone, 1985a,b; Bunch, 1987; Bauer et al., 1988. In areas subject to petroleum pollution, benthic
communities may be particularly susceptible to diesel fuel and its by-products because of the high PAH levels associated with it National Toxicology Program, 1986.
High-molecular-weight HMW PAHs are resistant to bacterial metabolism and slow to evaporate, increasing their residence time in aquatic environments Clark, 1989. Past
work has shown that PAHs can have both positive and negative effects on mi- crophytobenthic communities Bott et al., 1978; Farke et al., 1985; Plante-Cuny et al.,
1993. For example, studies have shown that the positive effects of PAHs on microphytobenthic production and abundance are caused by reduced grazing by
meiofauna and macrofauna Farke et al., 1985; Plante-Cuny et al., 1993; Carman et al., 1997. Changes in microphytobenthos abundances are likely to have significant effects
on food webs in coastal systems because of their important role as a food resource for many benthic invertebrates Levinton and Bianchi, 1981; Levinton et al., 1984; Miller et
al., 1996. Moreover, many of the benthic invertebrate communities in shallow productive coastal environments, such as salt marshes, provide an important trophic link
to commercial fisheries. Thus, any effect of PAHs on the benthic community has the potential to be manifested in higher trophic levels in these coastal systems.
Most PAHs in aquatic systems are associated with natural particulate and dissolved organic matter Wakeham and Farrington, 1980; Schwarzenbach et al., 1993; Mitra,
1997. PAHs are hydrophobic organic compounds containing two or more fused or linked benzene rings that may have toxic and or carcinogenic effects on the surrounding
biota. The major sources of PAHs are derived anthropogenically from pyrolysis and combustion processes and spillage of petroleum hydrocarbons Kennish, 1998, with a
small natural contribution from sediment diagenesis Wakeham and Farrington, 1980.
Land-margin ecosystems, such as salt marshes, are highly productive and have a wide
A . Bennett et al. J. Exp. Mar. Biol. Ecol. 242 1999 1 –20
3
diversity of primary producers i.e., vascular plants, microphytobenthos, phytoplankton, and macroalgae, resulting in a heterogeneous mixture of organic matter sources in
coastal sediments. Plant pigment biomarkers have been widely used to identify different sources of algal organic matter in aquatic systems and to differentiate trophic dynamics
in aquatic food webs Mantoura and Llewellyn, 1983; Bianchi et al., 1993, 1996. Chlorophyll-a is used to determine general algal biomass, while carotenoids are more
class specific Wright et al. 1991. For example, the carotenoid fucoxanthin is a biomarker for the presence of diatoms, while zeaxanthin is a biomarker for cyano-
bacteria Wright et al., 1991; Bianchi et al., 1996. Phaeopigments, such as phaeophytins and phaeophorbides, are the decay products of chlorophylls; phaeophorbides are
typically used as markers for metazoan grazing Schuman and Lorenzen, 1975; Welschmeyer and Lorenzen, 1985; Bianchi et al., 1988, 1995; Bennett et al., 1999.
Thus, plant pigments can be used to determine the composition and heterotrophic consumption of benthic algal sources in wetland ecosystems.
Louisiana wetlands represent 41 of the coastal wetlands in the US Turner and Gosselink, 1975; Turner, 1997; many of these wetlands have been chronically exposed
to petroleum-hydrocarbon pollution from oil exploration in the Gulf of Mexico Fang, 1990. In particular, coastal salt marshes of Pass Fourchon, LA, were subjected to
produced water discharge from the petrochemical pumping processes of Chevron until 1994, creating a dramatic PAH gradient over a very short distance ca. 1 km Rabalais
et al., 1991; Means and McMillin, 1993; Means, 1995. Carman et al. 1997 suggested that there was an enhancement of microphytobenthic abundance due to reduced
meiofaunal grazing in PAH-contaminated sediments from Pass Fourchon. However, to fully understand the complexity of parameters that control the abundance and com-
position of microphytobenthos in Pass Fourchon sediments, further work is needed on the interactive effects of contaminants, nutrients, and macrofaunal grazing.
In this study, the effects of PAH contamination and macrofaunal grazing on the abundance and composition of microphytobenthos in sediments were investigated using
a laboratory microcosm experiment. This laboratory experiment was conducted in conjunction with a 2-year field study in an effort to understand the effects of PAH
contamination on the salt marsh at Pass Fourchon, LA Bennett et al., 1999. Our overall hypothesis was that PAH concentrations in sediments influence the classes and relative
abundances of microphytobenthos, which affect the trophic and population dynamics of the epibenthic gastropod herbivore Littorina irrorata. Thus, the objectives for the
microcosm portion of the study were to determine the effects of PAH concentration and grazing by L
. irrorata on the abundance and composition of microphytobenthos in sediments from Pass Fourchon, LA, and to determine the effects of PAH concentration
on the somatic growth and food resources of L . irrorata.
2. Materials and methods
