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

Abstract

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 estuarinerelated 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°17'S, 52°15'W). To test this hypothesis, we analyzed the stable isotope ratios of carbon (δ 13 C) and nitrogen (δ 15 N) 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 δ 15 N and δ 13 C 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.KEY WORDS: Brazil · Food web spatial subsidies · Freshwater piscivore · Mugil liza · Sea −land connectivity · SIAR mixing models

Trophic connectivity and basal food sources sustaining tropical aquatic consumers along a mangrove to ocean gradient

“…Due to its high motility, animal migration can substantially affect the interchange of nutrients (especially N and P) and energy between the estuary and the ocean (Deegan 1993, Oliveira et al 2014). Detritus exported to the ocean by physical forces (e.g., tides, freshwater discharge) usually have lower nutritional values and have undergone rapid sedimentation before reaching higher trophic levels in the food chain (Nixon, 1980).…”

Elemental turnover rates and trophic discrimination in juvenile Lebranche mullet Mugil liza under experimental conditions

“…In addition to providing experimental values for isotopic turnover rates in muscle tissues of a widely distributed marine species, these findings may help to refine isotopic mixing models being applied to assess the trophic position and food pathways that sustain mugilid species (Claudino et al ., ; Oliveira et al ., ). As previously shown by Oliveira et al .…”

“…Mugil liza were fed to apparent satiation four times a day (NRC, ). The ration was manufactured based on C 3 plants (rice and soybean) (Table ) in order to have an isotopic value (δ 13 C = −27·1‰ and δ 15 N = 1·0‰) sufficiently different from those of M. liza captured at the surf zone (δ 13 C = −17·7‰ and δ 15 N = 8·6‰; Oliveira et al ., ). This difference, greater than 7‰ for both δ 13 C and δ 15 N, is necessary for the proper characterization of the F TD and R IT (Seminoff et al ., ).…”

“…As previously shown by Oliveira et al . (), juvenile M. liza act as biological vectors carrying marine‐derived nutrients to the food web of coastal streams, where they are assimilated by freshwater piscivores, but the magnitude of this marine‐to‐freshwater transfer mediated by juvenile mullets remains unknown. The current determination of F TD and R IT parameters in muscle tissues of these juveniles will allow future inferences of residence time of this species within this freshwater system.…”

Coastal zone use and migratory behaviour of the southern population of Mugil liza in Brazil

“…Mugil liza eggs and larvae are neustonic, but before recruitment to the estuarine environment, juveniles under 30 mm L T temporarily occupy surf zones (Rodrigues et al ., ; Vieira, ). This surf zone life stage, which may last for over a year, gives juveniles under 30 mm L T a better opportunity to enter estuaries, where growth and survival may be improved (Castellini et al ., ; Oliveira et al ., , ; Rodrigues et al ., ; Vieira, ).…”