Abstract
Environmental DNA (eDNA) offers an efficient approach to biodiversity monitoring and biosecurity surveillance in coastal eco-systems. Understanding eDNA dynamics in tidal areas is critical for guiding sampling strategy design and interpreting molecular detection results. In a field study conducted at Ōpua, Bay of Islands, Aotearoa, New Zealand, an international marina with adynamic coastal environment, we investigated eDNA detection patterns across a local spatial and temporal gradient. Hourly samples were collected from six sampling stations around the marina, from artificial shore structures and a channel, over a12-h tidal cycle. Detected eukaryotic communities were assessed using eDNA metabarcoding analysis of the small-subunit ribosomal RNA (18S rRNA) and mitochondrial Cytochrome C Oxidase subunit I (COI) genes. This core community was screened for marine nonindigenous species (NIS) using an online metabarcoding data screening app (ExPAT). Community composition varied significantly based on sampling location and time, with most of the core community captured around mid-tide. The NIS community exhibited inconsistent detection for some species (e.g., the amphipod Monocorophium acherusicum) across space and time, whereas other NIS (e.g., the mussel Arcuatula senhousia) were consistently detected and behaved similar to the core community. Overall, species-specific eDNA detection patterns may be linked to the biology and movement of the organism. The results indicated slightly higher NIS detection from artificial structures at the shore stations and within 2 h of low tide. For marine biosecurity applications, users may want to prioritize sampling near-shore during low tide, while samples collected during mid-tide may provide more comprehensive insights into the core community for biomonitoring. Further field studies across multiple tidal cycles and marinas may guide the standardization of molecular biomonitoring and biosurveillance sampling strategies and support their broader integration into marine biosecurity programs.