Guano-Derived Nutrient Subsidies Drive Food Web Structure in Coastal Ponds

A stable isotope study was carried out seasonally in three coastal ponds (Marinello system, Italy) affected by different gull guano input to investigate the effect of nutrient subsidies on food web structure and dynamics. A marked N-15 enrichment occurred in the pond receiving the highest guano input, indicating that gull-derived fertilization (guanotrophication) had a strong localised effect and flowed across trophic levels. The main food web response to guanotrophication was an overall erosion of the benthic pathway in favour of the planktonic. Subsidized primary consumers, mostly deposit feeders, switched their diet according to organic matter source availability. Secondary consumers and, in particular, fish from the guanotrophic pond, acted as couplers of planktonic and benthic pathways and showed an omnivorous trophic behaviour. Food web structure showed substantial variability among ponds and a marked seasonality in the subsidized one: an overall simplification was evident only in summer when guano input maximises its trophic effects, while higher trophic diversity and complexity resulted when guano input was low to moderate.