Seasonal Relationships Between Diatom Assemblages and Environmental Conditions Vary Between Epipelic- and Epipsammic-dominated Tidal Flat Diatom Communities in the Yaquina Estuary, Oregon, USA

Abstract

This research examined temporal and spatial patterns of microphytobenthos (MPB) and relationships with environmental variables in a Pacific Northwest, USA estuary with complex environmental gradients. Sediment composition, salinity, and nutrients varied between sites closer to the estuary mouth (MAR), and those further upstream (RIV) and seasonal patterns in these variables also differed between MAR and RIV sites. Average MPB biomass was 57.8 ± 22.9 μg chlorophyll cm−3 at MAR sites and 29.9 ± 13.3 μg chlorophyll cm−3 at RIV sites. Relationships between MPB biomass and environmental conditions depended on season. The MPB diatom assemblage was influenced primarily by location within the estuary and secondarily by season. Epipsammic taxa, including Catenula adhaerens, Planothidium delicatulum, and Opephora spp. 1, were most abundant at MAR sites, while highly motile taxa in the genera Navicula senso lato and Nitzschia were abundant at RIV sites. Seasonal shifts in diatom community structure were evident at RIV sites and were driven primarily by changes in dominant epipelic taxa. Relationships between the MPB diatom assemblage and environmental conditions varied between MAR and RIV sites. Physical variables, including distance from the estuary mouth, sediment composition, land use in the 100 m buffer, and to a lesser extent, salinity, were correlated to the MAR assemblage. In contrast for RIV sites, salinity and nutrients were correlated to the diatom assemblage. Despite the complexity inherent in elucidating diatom–environmental relationships, our results suggest that tidal flat diatoms can be used to assess the environmental conditions of estuaries.