Contribution of Riverine Inputs to Brown Shrimp Diet 



6"C values also changed as shrimp moved near and into areas that are subject to freshwater inundation. In May 

 and June 1996, juvenile brown shrimp were collected at the Nueces River site up to Rincon Bayou marsh 

 (Fig.l). Other shrimp, (e.g. Penaeus merguiensis (Staples 1980) and Penaeus setiferus (Dall et al. 1990)), have also 

 been observed in lower salinities environments far upstream from the river mouth. As juvenile brown shrimp 

 occupied Nueces River, their O'^C became significandy more negative (-24 to -21 %o) compared with o"C 

 measured within Rincon Bayou marsh (Fig.3). This depletion in '^C indicates that a significant part of brown 

 shrimp carbon at Nueces River is derived from terrestrial detritus and/or riverine phytoplankton carried by 

 freshwater inflow, because Rincon Bayou marsh lacks a "C-depleted source (Fig.3). Unfortunately, 6"C for 

 riverine phj'toplankton could not be estimated in the present study. However, previous results showed 

 phytoplankton 6"C values of -44 to -47 %o in rivers (Rau 1978) and of -40 %o (Hedges et al. 1986) in a lake. 

 Similarly, 6"C of freshwater phytoplankton in the Charente River (France) ranged between -41.8 and -31.2 %o 

 (Riera & Richard 1996). Considering this higher depletion in "C for freshwater phytoplankton compared with 

 terrestrial C3 plants, a primar)' contribution of riverine phytoplankton to the diet of brown shrimp is tinlikely, 

 but cannot be excluded from these results. 



In fact, because the metabolic '^C-enrichment dviring assimilation is close to 1 %o (De Niro & Epstein 1978, 

 Rau et al. 1983), shrimp 6'^C values at Nueces River are consistent with a significant utilisation of terrestrial 

 C3 plants (-29 to -28 %o) as food source. Moreover, the similarity of 6"C values of terrestrial C3 plants {Salix 

 sp, Fraxinus sp) and POM in the Nueces River indicates that detritus firom C3 plants contribute predominandy 

 to river POM. Therefore, the resiJts of this study can explain the depletion in '^C for brown shrimp observed 

 in lower salinity bays that are flushed by freshwater inflow or are most influenced by river inputs along the 

 South Texas coast (Fry 1981). This result is consistent with the hypothesis that terrestrial organic inputs could 

 be incorporated into estuarine food weebs (Hackney & Haines 1980, Incze et aL 1982). In fact, previous results 

 showed the significant contribution of terrestrial detritus derived from C3 plants to the diet of oysters 

 {Crassostrea gigas) in the upper reaches of the Charente Estuary (Riera & Richard 1996), and in the middle 

 estuarine reaches as a high river discharge period occurred (Riera & Richard 1997). In addition, from o"C 

 analyses, Stephenson & Lyon (1982) reported that the bivalve Chione /tofrA^w^; inhabiting the Avon-Heathcote 

 Estuary (New Zealand) could utilise carbon of terrestrial origin depending upon its position in the estuary' and 

 local hydrolog)'. Freshwater inputs can be an important source of nutrition for juvenile brown shrimp in 

 habitats lacking salt marsh plants and benthic diatoms. Therefore, during periods of high river discharge, 

 riverine inputs may support a substantial part of the food webs in South Texas bays and elsewhere as well. 



Offshore Migration of Subadult Penaeus aztecus 



Subadult Penaeus a:(tecus that are migrating offshore have different diets than the subadults found in marshes. Aa 

 enrichment in "C for Penaeus a^ecm (-13.8 ±1.5 %o) was observed at a\ransas Pass at the end of July 1996 

 (Fig.3). Brown shrimp in Aransas Pass were likely returning towards the nearshore Gulf of Mexico because they 

 were sub-adidt size and offshore migration occurs in summer in Texas bays (Moffet 1970). Similar 6"C values, 

 between -12.6 and -14.6 %o, were also obser\'ed by Fr}' & Parker (1979) for brown shrimp collected offshore in 

 the Gulf of Mexico, but more negative 6''C were observed for other shrimps {Penaeus setiferus) from the same 

 area. This enrichment in "C in shrimp tissues is not Ukely to be a result of a metabolic effect as shrimp grow 

 due to a variation of carbon fractionation. In fact, the inter-individual 0"C variability among animals having a 

 similar food source does not usually exeed 2 %o for fishes and invertebrates, these differences being attributed 

 to size, age or sex (Fry & Parker 1979, Hughes & Sherr 1983). The 8 %o mean enrichment in "C in shrimp 

 tissues (Fig.3) between the marsh and pass locations in spring and summer may have other interpretations. 



At Aransas Pass, phj'toplankton was likely to be the main organic matter source for brown shrimps because 

 there are no seagrass meadows. However, considering the 6'^C of -22.7 %o for marine phytoplankton in the 

 Northern Gulf of Mexico given by Thayer et al. (1983), the enrichment in ''C due to metabolic fractionation 

 between phytoplankton and brown shrimps would be much more than 1 %o. Most likely, less negative 0"C for 

 brown shrimp, as they enter the offshore area, may be explained by a progressive enrichment in "C as shrimps 

 returned from Nueces River environments towards offshore waters (Fig.3), as su^ested by Fry & Parker 

 (1979). In fact, the shrimps collected in late July, at the end of the migration, exhibited 6"C typical of the 

 feeding habitats recendy encountered where they used "C -enriched food sources, e.g., marsh grass or 



E-6 ♦♦♦ Utilisation of Estuarine Organic Matter During Growth 

 and Migration by Juvenile Brown Shrimp 



