Spatial and Seasonal Biodiversity Variation in a Large Mediterranean Lagoon Using Environmental <scp>DNA</scp> Metabarcoding Through Sponge Tissue Collection

ABSTRACT Ecosystem monitoring is a fundamental tool to avert biodiversity loss, gathering valuable information that can be used to develop conservation policies, evaluate management outcomes, and guide science‐based decision‐making. The Mar Menor coastal lagoon (South‐East of Spain) has experienced episodes of eutrophication due to intensive agriculture and other human activities, causing mass mortalities of marine fauna. In this scenario, biodiversity monitoring is crucial to evaluate the status of fauna and flora and take appropriate measures. Here, our main objective was to assess faunal composition and its spatial and temporal structure associated with the pillars used to support the built recreational well‐being facilities along the Mar Menor. We capitalized on the many sea sponges that settle on these structures by collecting tissue samples for subsequent extraction of filtered environmental DNA (i.e., ‘natural sampler DNA’, nsDNA) in northern and southern areas of the lagoon, both in spring and summer. We metabarcoded all samples at the cytochrome oxidase subunit I gene ( COI ), and reliably identified 76 taxa belonging to nine different phyla, with annelids, poriferans, mollusks and cnidarians being the richest groups. We detected emblematic species of threatened status, such as pipefishes (genus Syngnathus ) and the piddock clam, Pholas dactylus , and others known to become invasive, such as the ascidians Styela canopus and Botrylloides niger , the barnacle Amphibalanus amphitrite , and the polychaetes Branchiomma bairdi and Branchiomma boholense . The use of common and widely distributed sponges as natural eDNA samplers allowed us to characterize both spatial and temporal diversity, further emphasizing the importance of this low‐cost approach to monitoring biological communities in shallow coastal ecosystems.