seagrasses. ParticvJaily, within Corpus Christi and Redfish Bays, seagtasses have the highest 6"C values (-3 to - 

 13 %o) in the ecosystem as reported by Fry and Parker (1979). High 6"C of -10 %o were also observed recendy 

 for seagrasses of Laguna Madre in south Texas (Street et al. 1997). Additionally, macroalgae is known to occur 

 in bay bottoms and along the jetties at Aransas Pass, but we have not sampled this source. As they returned 

 towards marine waters through Rincon Bayou mouth shrimp may also increase their feeding on Spartina detritus 

 direcdy or through predation on detritivores. Therefore, as subadult brown shrimp feed offshore, their 6''C 

 should progressively converge on the 6"C value characteristic of offshore environment, close to -18 %o (Fry 

 1981). 



Temporal 6"C Variation: Importance for Tissue Turnover 



Tissue tiunover rates are important to know to interpret temporal 6"C variation. There was about a 3.5 %o 

 decrease in 6"C values from the up marsh to the Nueces River site (Fig.3) indicating a high tissue turnover rate 

 for brown shrimp as they migrate. This isotopic 6"C variation occurred over a distance of less than 5 km and 

 within a period of 9 weeks. From one feeding habitat to another the "old carbon" of shrimp tissue is 

 progressively diluted due to 1) growth of new tissue using "new carbon" and 2) metabolic loss due to tissue turn 

 over (Anderson et al. 1987). Therefore, after a variation in food, shrimp 6"C will change isotopically as rapidly 

 as tissue turnover rate will allow (Fry 1982). In the present study, the 6"C decrease of migratory brown shrimp 

 is consistent with a high tissue turnover rate, which support the hypothesis of a high growth rate in the nursery 

 habitat. This suggestion is consistent with previous results based on experimental observations showing that 

 posdarval shrimp can increase in weight by a factor of 4 within a week or less at 25°C (Zein-Eldin & Aldrich 

 1965). A 14%o variation for o"C of posdarval brown shrimp has been observed after a 3.9 fold increase in 

 weight after a change in food source, indicating a high tissue turnover rate (Fry & Arnold 1982). From feeding 

 experiments Gleason (1 986) showed that the half-life of the initial tissue carbon of Penaeus a^ecus fed with plant 

 and animal diets was reached before the first doubling of weight. Finally, juvenile shrimp (initially 6"C:-18.6 

 %o) in an experimental feeding pond with feed at 6'^C: -22.9 %o for 8 weeks attained an eqtiilibrium 6"C at - 

 21.3 %o after 3 weeks and an increase in weight of 300 % (Parker et al. 1988). High tissue turnover rate for 

 young shrimp can be related with behaviour and feeding activity. In fact, small juveniles of Penaeus semisculatus 

 were active and fed both day and night and are thought to feed continuously and to digest most of their food 

 within only one hour (Heales et al 1996). It is likely that the variation in carbon isotope values of brown shrimp 

 in the Rincon Bayou Marsh are a result of changed food sources and rapid growth in this nursery habitat. 



Food Sources Determination from 6'*N Values 



Although only part of the shrimp collected were measured for 6'*N, a dual isotope approach is useful for 

 identifying food sources. In up and down Rincon Bayou marsh, O'^N for brown shrimp showed a high 

 variability both between sampling periods and between individuals (Table 2). This is consistent with a higher 

 range in o'^N values for sources in this habitat (i.e., from -0.7 to 7.6%o) and suggest a high diversity of nitrogen 

 sources for shrimps (i.e., Spartina spartinea, benthic diatoms, blue green algae). Lower o'^Nvalues that were 

 observed for some shrimps in Rincon Bayou marsh could be explained by a depletion in '*N during nitrogen 

 assimilation (Mako et al 1982). In fact, these authors observed a mean 6'^N fractionation of -0.3%o for the 

 amphipod Amphithoe vaUda fed with fresh and detrital algae. However, this hypothesis is unlikely for Penaeus 

 species because feeding experiments demonstrated a trophic enrichment in '^N of 2.4%o for Penaeus vannamei 

 (Parker et al. 1988). These lower 6'*N values for brown shrimp may partly result from a preferential 

 assimilation of specific chemical components of plant tissues (Mako et al 1982) and/or by the utilisation of '^N- 

 depleted blue green algae (Table 1). Therefore, in the present study, 6'*N was not as valuable as 6"C to 

 characterize food sources of brown shrimp. This result is consistent with the suggestion of Fry & Sherr (1984) 

 that o'^N is not as discriminating as 6"C for food sources determination in coastal ecosystems. However, at 

 the Nueces River mean 6'^N for shrimp (i.e., 8.2, 10.1 and 1 1.7 %o) are consistent with a significant utilization 

 of the terrestrial C3 plants Fraxinus sp (i.e., 6.6 %o) and/or Salix sp (8.3 ±1.1 %o) when taking into account the 

 mean trophic enrichment for 6'^N (i.e., 3.5 %o given by Minagawa & Wada 1984). In this riverine habitat, where 

 terrestrially-derived organic matter dominated, 6'*N values confirmed 6"C results. 



Appendix E ♦ E-7 



