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Fishery Bulletin 109(2) 
offshore and seasonal C isotope changes in shrimp (Fry 
et al., 1984), but N and S isotope changes have not yet 
been systematically investigated. Comparative work 
might also focus on other river-influenced shelf sys- 
tems and one recent study of the Thames River estuary 
(Leakey et al., 2008) has shown the same triple isotope 
riverine-offshore gradients observed in this study for 
the Mississippi River. Because of these similar results 
for the Thames River, and because the pattern of iso- 
tope signals is consistent with a riverine source with 
high S 15 N, it seems likely that the Mississippi River 
is forcing many of the isotope signals observed on the 
Louisiana shelf. 
The three regional shelf groups shown in Figures 
8 and 9 were identified by cluster analysis by using 
three proventriculus isotope variables and three mus- 
cle isotope variables. These six variables were used in 
concert for two reasons. First, the separated proven- 
triculus and muscle results each gave strong mid-shelf 
vs. offshore patterns (see significant differences among 
averages in Table 2), and therefore results could be 
legitimately combined for a stronger overall assess- 
ment. Second, the proventriculus and muscle samples 
show somewhat different aspects of shrimp biology and 
available diets, with proventriculus samples providing 
stronger local information and muscle samples provid- 
ing stronger time-integrated samples. On the negative 
side, the proventriculus samples are often the leftovers 
after digestion and can include inorganic sediment 
grains with pyritic sulfides (Howarth, 1979, 1984), 
whereas muscle samples are taken from shrimp that 
are mobile and may reflect diets from another place. 
Because there were both positive and negative aspects 
to using the separated proventriculus and muscle iso- 
tope data, the combined data (Table 2) were used to 
reach a balanced overall assessment in the cluster 
analysis. 
The offshore C isotopes showed a broad pattern of 
river influence across the inshore and middle shelf, 
consistent with wide dispersal of carbon from river- 
influenced planktonic primary producers. The riverine 
influence was expressed as higher S 13 C values in a mid- 
shelf maximum standing out against a background of 
lower 8 13 C values both in shallower bays and in deeper 
offshore waters (Tables 1 and 2, Fig. 9). Higher S 13 C 
values are found associated especially with high pro- 
ductivity and diatom blooms (Fry and Wainright, 1991; 
Fry, 1996) — conditions that regularly occur on the Loui- 
siana shelf that is affected by Mississippi River inputs 
(Rabalais et al., 1996; Dagg et al., 2007; Green et al., 
2008). The deeper shelf to the south and west had lower 
8 13 C consistent with lower primary productivity (Fry 
and Boyd, 2010). 
The offshore S isotopes were perhaps the most ex- 
pected results, showing a consistent onshore-offshore 
gradient in both proventriculus and muscle 8 34 S values 
(Table 2, Fig. 9). These gradients likely originate with 
river-induced organic carbon gradients in primary pro- 
ductivity that subsequently fuel benthic sulfate reduc- 
tion and sulfide production in underlying sediments. 
The exact mechanism of sulfide incorporation into ben- 
thic food webs is still unknown but is likely the use of 
sulfides by bacteria growing in bottom sediments. Hy- 
poxia may increase aspects of sulfide cycling, especially 
by decreasing the importance of oxygenic decomposition 
reactions while increasing the importance of anaerobic 
reactions such as sulfate reduction and sulfide produc- 
tion. Hypoxia also may decrease oxidation reactions 
that consume sulfide, and decreased infaunal activity 
and decreased bioirrigation in sediments may also oc- 
cur when bottom waters become hypoxic (Eldridge and 
Morse, 2008). In sum, hypoxic conditions may promote 
more anaerobic conditions, more sulfide production and 
accumulation, and stronger bacterial uptake of sulfides 
into benthic food webs. 
The N isotope results were quite surprising in the 
very high S 15 N values (up to 15.2%c) found for some 
proventriculus content samples in the mid-shelf hypoxic 
region — values that were higher than those for shrimp 
muscle. Ongoing studies show no large 15 N enrichment 
in particulate organic nitrogen samples collected in 
the water column in the offshore region, where values 
average 6-8%o (Wissel et al., 2005; Fry, unpubl. data). 
In the absence of a planktonic origin, the source of the 
high S 15 N values likely is in the benthos. Brown shrimp 
are benthic carnivores that consume polychaetes and 
meiofauna (McTigue and Zimmerman, 1998; Fry et al., 
2003), and offshore brown shrimp generally rely on a 
benthic food web with bacterial contributions. York et 
al. (2010) have speculated that nitrogen cycling in the 
benthos is leading to high S 15 N values of benthic bac- 
teria, perhaps with some bacterial use of 15 N-enriched 
ammonium left over from nitrification or annamox re- 
actions. Such processes are likely ubiquitous in shelf 
sediments, but details that are still to be elucidated 
could make these processes much more dominant in the 
low-oxygen mid-shelf hypoxic region. High S 15 N values 
were also found in inshore shrimp and proventriculus 
contents from the Bird’s Foot Delta region (Table 2), 
and the common denominator leading to these high 8 15 N 
values is likely eutrophic deposition of large amounts of 
organic phytodetritus to the benthos. 8 15 N values >15%c 
have also been observed in Mississippi River zebra mus- 
sels during summer, where high animal 8 15 N values 
have been correlated with low ammonium concentra- 
tions in the river (Fry and Allen, 2003). 
Whatever the mechanism underlying the high S 15 N 
values, it was evident that the proventriculus 8 15 N val- 
ues were often higher than those of offshore shrimp eat- 
ing these foods (Table 2). Previous work with estuarine 
brown shrimp has shown that brown shrimp normally 
have positive trophic enrichment factors (TEFs) aver- 
aging about 2.3%o higher than proventriculus contents 
8 15 N (Fry et al., 2003), and a similar average TEF of 
2.8%c was observed for the most offshore shrimp of the 
present study (Table 2). The observed opposite pattern 
of negative TEF values for some mid-shelf shrimp likely 
means that these shrimp have not spent the several 
weeks (that can be calculated from diet turnover dy- 
namics) (Fry and Arnold, 1982) that they would need 
