Drivers of <i>Perkinsus marinus</i> and <i>Haplosporidium nelsoni</i> prevalence and intensity in oyster reefs around Sapelo Island, Georgia

Parasites can strongly influence host populations, particularly when the host is an ecosystem engineer. Oysters are ecosystem engineers that support estuarine communities and fisheries but are susceptible to 2 protozoan parasites, <i>Perkinsus marinus</i> (causing Dermo) and <i>Haplosporidium nelsoni</i> (causing MSX). Although both parasites are known to be influenced by environmental conditions, fine-scale temporal and spatial patterns remain underexplored in southeastern US estuaries. We examined parasite prevalence and intensity biweekly from April to October 2023 across 4 intertidal reefs on Sapelo Island, Georgia, and analysed concurrent water quality data (temperature, salinity, dissolved oxygen, pH) to identify potential environmental drivers of parasite prevalence and intensity. Parasite prevalence was high overall, 88% of oysters were infected with at least 1 parasite, and 34% were co-infected. <i>Haplosporidium nelsoni</i> prevalence was consistently high across sites, while <i>P. marinus</i> prevalence showed greater spatiotemporal variability, increasing through late summer and fall. Models indicated a time-lagged effect of environmental conditions on <i>P. marinus</i> prevalence, specifically with temperature and dissolved oxygen. Prevalence of <i>H. nelsoni</i> remained high throughout the year among sites and was best explained by temperature variability, salinity, and dissolved oxygen. Intensity levels did not differ among sites for either parasite. Our results demonstrate that even at small spatial scales and over time, oyster-parasite dynamics are shaped by multiple, interacting environmental factors, with time-lagged responses particularly evident for <i>P. marinus</i>. Understanding these dynamics is essential for predicting disease impacts under changing environmental conditions and informing management, restoration, and aquaculture strategies.