Contamination by trace metals and their trophic transfer to the biota in a Mediterranean coastal system affected by gull guano

Transitional environments are vulnerable interface systems, ecologically connected with adjacent systems by several biotic or abiotic flows. The coastal system of the Marinello ponds (Tyrrhenian Sea, Italy) is affected by a colony of yellow-legged gulls Larus michahellis (Naumann, 1840), resident on the cliff beside the ponds. To investigate the role of the gull colony as a potential source of allochthonous non-essential trace metals (As, Cd, Pb and total mercury, THg) and the consequent metal trophic transfer to the biota in the ponds, we collected guano, surface sediment and biota from 3 ponds with different levels of avian input. The highest concentrations were observed in guano and surface sediment of the seabird-influenced pond, suggesting that trace metals have undergone biotransport mediated by seabirds. To test for biomagnification, the trophic pathways of the ponds were first identified by δ13C and δ15 N, revealing the existence of important differences in accordance with the importance of guano input. Trophic transfer along the food chains was metal-specific. Only THg was found to biomagnify based on significant positive linear regression of logTHg versus δ15 N in organisms. In contrast, As, Cd and Pd significantly biodiluted through the food chains. Regression line slopes were homogeneous among ponds, sug- gesting that metal transfer along food chains occurs independently of the background contamination levels. Our findings provide new insights about the role of seabirds in contaminant dynamics in transitional environments in light of the high abundance of seabirds and the current lack of knowledge about seabird-mediated ecological processes in the Mediterranean area.