Determination of food web support and trophic position of the mummichog, Fundulus heteroclitus, in New Jersey smooth cordgrass (Spartina alterniflora), common reed (Phragmites australis), and restored salt marshes

The invasion of Phragmites australis into tidal marshes formerly dominated by Spartina alterniflora has resulted in considerable interest in the consequences of this invasion for the ecological functions of marsh habitat. We examined the provision of trophic support for a resident marsh fish, Fundulus heteroclitus, in marshes dominated by P. australis, by S. alterniflora, and in restored marshes, using multiple stable isotope analysis. We first evaluated our ability to distinguish among potential primary producers using the multiple stable isotope approach. Within a tidal creek system we found significant marsh and elevation effects on microalgal isotope values, and sufficient variability and overlap in primary producer isotope values to create some difficulty in identifying unique end members. The food webs supporting E heteroclitus production were examined using dual isotope plots. At both sites, the delta(13)C values of E heteroclitus were clustered over values for benthic microalgae (BMI) and approximately midway between delta(13)C values of Spartina and Phragmites. Based on comparisons of fish and primary producer delta(13)C, delta(15)N, and delta(34)S values, and consideration of E heteroclitus feeding habits, we conclude that BMI were a significant component of the food web supporting F heteroclitus in these brackish marshes, especially recently-hatched fish occupying pools on the marsh surface. A 2parts per thousand difference in delta(13)C between Fundulus occupying nearly adjacent Spartina and Phragmites marshes may be indicative of relatively less reliance on BMI and greater reliance on Phragmites production in Phragmites-dominated marshes, a conclusion consistent with the reduced BMI biomass found in Phragmites marshes. The mean delta(13)C value of F heteroclitus from restored marshes was intermediate between values of fish from naturally occurring Spartina marshes and areas invaded by Phragmites. We also examined the isotopic evidence for ontogenetic changes in the trophic position of larval and juvenile F heteroclitus. We found significant positive relationships between E heteroclitus WIN values and total length, reflective of an increase in trophic position as fish grow. F heteroclitus delta(15)N values indicate that these fish are feeding approximately two trophic levels above primary producers.