Transport of marine-derived nutrients to subtropical freshwater food webs by juvenile mullets: a case study in southern Brazil

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 mullet juveniles might transport marine nutrients into the freshwater food webs of coastal streams in southern Brazil (32[degrees]17'S, 52[degrees]15'W). To test this hypothesis, we analyzed the stable isotope ratios of carbon ( delta 13C) and nitrogen ( delta 15N) in 10 basal production sources and 10 consumer species, including juvenile mullets, at 3 sites (surf zone, lower stream reach, headwater reach) along a marine-freshwater gradient. Our results revealed a significant decreasing trend in delta 15N and delta 13C values from the sea to the freshwater sites, suggesting an importation and incorporation of marine-derived nutrients into the coastal stream. A mixing model showed a marked shift in assimilation from marine to freshwater production sources by mullet Mugil liza juveniles as they moved from the surf zone into the coastal stream (lower stream and headwater reaches). Our mixing model also indicated that material of marine origin is assimilated by freshwater predators, particularly those found at the lower stream reach. Further experimental and field work is needed to evaluate the implications of marine-derived carbon and nitrogen transport by juvenile mullets for the structure and dynamics of food webs in coastal streams.