Effects of finfish farms on pelagic protist communities in a semi-closed stratified embayment

Abstract

Coastal aquaculture operations for feed additive species results in the release of waste into the surrounding environment, with the potential for adverse environmental change. Ubiquitous pelagic protists are sensitive to environmental changes making them potential sentinels for detecting and monitoring impacts. This study used 18S rRNA high-throughput amplicon sequencing as a molecular tool to study the pelagic protist community, with the aim of evaluating their potential as bioindicators of aquaculture activity in a low-oxygen, highly stratified marine embayment. Sampling occurred at three different depths along a distance gradient from two leases and at three control sites. Our results showed that the diversity and composition of both phytoplankton and other protist communities were more strongly influenced by depth stratification than the aquaculture activity. Nonetheless, differential abundance and machine learning analyses revealed a suite of potential bioindicators for aquaculture activity; this included the phytoplankton taxa Chrysophyceae, Gymnodiniphycidae (Gyrodinium), Cryptomonadales and Ciliophora (Philasterides armatalis, Plagiopylida, and Strombidium). Among the other protists, ciliates were also more abundant in closer proximity to the leases in both surface and bottom samples. Overall, our findings indicated that the use of 18S rRNA sequencing of protist communities is a promising tool for identifying environmental changes from aquaculture in the water column.