Mullet Juveniles As Biological Vectors Contributing Marine-Derived Nutrients to the Freshwater Food Webs of Coastal Streams

In higher latitudes of the northern hemisphere, anadromous fishes can act as biological vectors, moving marine-derived nutrients into freshwater ecosystems during their reproductive migration. Much less information is available on the potential role of marine and estuarine-related fishes as biological vectors in subtropical latitudes. Here, we investigated whether juvenile mullets Mugil liza might transport marine nutrients into the freshwater food webs of coastal streams in southern Brazil (32°17'S and 52°15'W). To test this hypothesis, we analyzed the stable isotope ratios of carbon (δ13C) and nitrogen (δ15N) in 10 basal production sources and 10 consumer species, including juvenile mullets, at three sites (surf zone, lower stream reach, headwater reach) along a marine-freshwater gradient. Our results revealed a significant decreasing trend in δ15N and δ13C values from the sea to the freshwater sites, suggesting an importation and incorporation of marine-derived nutrients into the coastal stream. Mixing model showed a marked shift in assimilation from marine to freshwater production sources by juvenile mullets as they move from the surf zone into the coastal stream. Our mixing model also indicated that material of marine origin is assimilated by freshwater predators, particularly those found at the lower stream reach.