Abstract
Parasites are important members of ecosystems, and recent metabarcoding efforts have revealed an abundance of microbial eukaryotic parasites in copepods. However, a putative linkage to host community biomass and composition has not been addressed. We performed zooplankton tows monthly for 2 yr at five stations in the Oslofjord, Norway. The samples were cleared of larger, non‐copepod zooplankton and contained mainly copepods and are henceforth referred to as copepod community samples. Metabarcoding was used to detect microbial eukaryotic parasites in the copepod community samples. We also metabarcoded water samples from one of the stations. Copepods were identified and their biomasses were estimated at three of the stations for 1 yr. Calanus spp. dominated biomass in winter, whereas Temora longicornis and other smaller, short‐lived coastal copepods prevailed during summer. Seasonality in the copepod community was less apparent in the deep waters (200 m) where larger, long‐lived copepods abound. The alveolate parasites Hematodinium and Chromidina were detected at most stations throughout the year and occupied a large proportion of the total reads. Although the microbial eukaryotic community in the copepod community samples reflected the seasonal copepod dynamics, common alveolate parasites (e.g., Syndinium , Blastodinium , and apostome ciliates) appeared year‐round. Co‐occurrence analysis suggested several pair‐wise associations between taxa of microbial eukaryotes and copepods. Parasites are a universal component of copepods in this fjord system and seem well‐adapted to seasonal fluctuations in host community composition. We hypothesize that the parasites may have host ranges that incorporate perennial copepods to maintain their presence in seasonally fluctuating environments.