Zooplankton as a Bioindicator of Marine Contamination for Filter-Feeding Basking Sharks, Fin Whales and Devil Rays at Caprera Canyon (Mediterranean Sea)

The Canyon of Caprera (Mediterranean Sea) hosts high biodiversity, including threatened filter-feeding vertebrates such as the basking shark and fin whale. Due to their elusive nature, mobility and conservation status, direct contaminant assessment in these species is often challenging. To address this limitation, the present study focuses on zooplankton, an essential prey and bioindicator, to indirectly assess the levels of persistent organic pollutants and trace elements (TEs) in their feeding grounds. This approach provides a noninvasive method to estimate potential pollutant exposure in vulnerable megafauna and to evaluate the associated risks of pollutant transfer through the food web. Zooplankton samples accumulated mean PCB and DDT levels of 31.3 ± 17.5 ng g−1 and 19.8 ± 15.3 ng g−1 dry weight (dw), respectively. Regarding TEs, zooplankton exhibited high levels of essential TEs (e.g., Fe 778 ± 640 mg kg−1 dw), while nonessential TEs were found in lower concentrations (e.g., Hg 0.1 ± 0.1 mg kg−1 dw). Potential daily pollutant intake by the filter-feeding species was estimated using published zooplankton consumption rates, highlighting a significant pathway for contaminant transfer. Zooplankton contaminant levels exceeded the Environmental Quality Standard for Hg and the Maximum Allowable Limits established for foodstuff for Cr, Cd, Cu, Zn, As, Pb, suggesting potential risk for the aquatic environment, with implications for zooplankton feeders. These results demonstrate the utility of zooplankton as a noninvasive proxy for contaminant exposure in threatened marine megafauna and highlight the importance of monitoring pollution in key Mediterranean habitats.