Fry: Sustenance of Farfantepenaeus aztecus in Louisiana coastal waters 
157 
20 
co 
■'t 
OO 
10 
Offshore Mid-shelf 
oligotrophic * hypoxic 
-15 
o 
CD 
to 
-25 
Figure 10 
Covariation of proventriculus 5 34 S and 5 13 C with d 15 N, from brown 
shrimp ( Farfantepenaeus aztecus ) samples collected from the Gulf 
of Mexico from 2006. Equations for top and bottom lines are respec- 
tively 5 34 S=-0.5(8 15 N)+18.8 with r 2 =0.36, and 5 13 C = 0.38(5 15 N)-25.32 
with r 2 =0.64. The relationship between 5 34 S and 5 13 C (not shown) 
was 5 13 C = 0.26(8 34 S)-18.17 with r 2 = 0.21. All isotope values have 
units of %c. 
10 16 
5 15 N 
Offshore phytoplankton productivity studies in Missis- 
sippi River plumes generally show 13 C-enriched values 
for particulate organic matter formed in inshore and 
mid-shelf regions (Fry and Wainright, 1991; Fry un- 
publ. data), so that the dual isotope tag of low S 13 C 
and high 5 15 N used to source riverine shrimp seemed 
largely confined to estuaries and was only rarely pres- 
ent offshore (Fig. 7). 
It is also possible that bays in adjacent Texas and 
other northern Gulf states supply some shrimp to the 
offshore Louisiana system. But those shrimp would 
likely have been offshore for extended periods of time 
and therefore would have been counted as residents 
in this study. Thus possible contributions from other 
states should have little effect on the estimates given 
above for bay and riverine contributions to Louisiana 
shrimp stocks. 
It also was evident that estuarine conditions prevail 
offshore in this river-influenced shelf ecosystem that is 
often considered an offshore estuary. Isotopic composi- 
tions of shrimp and proventriculus contents followed 
the same triple isotope gradients involving high § 13 C, 
high S 15 N, and low 8 34 S nearest the river, vs. low 8 13 C, 
low 8 15 N, and high 8 34 S offshore. These gradients were 
largely aligned with other offshore features associated 
with the river, notably finfish biomass (Moore et ah, 
1970) and hypoxia (Rabalais et ah, 2002). It is possible 
that some of these isotope gradients reflect normal 
depth-related onshore-offshore productivity gradients 
not associated with rivers, and this idea should be ad- 
dressed in future comparative work involving continen- 
tal shelf systems with little river influence. Initial data 
for the Texas shelf have shown some cases of onshore- 
