Integrating DNA sequencing and morphological identification enhances understanding of epiphytic diatom diversity and ecology

Eelgrass ( Zostera spp.) forms productive and biodiverse marine meadows throughout the world. There eelgrass provide living substrata for epiphytes, including diatoms, which contribute substantially to primary productivity. Despite their important role, there are many gaps in our understanding of the diversity and ecology of epiphytic diatoms. Here, we combined morphological identification by scanning electron microscopy and light microscopy with Illumina sequencing of the 18S rRNA and rbcL marker genes to survey the diatom community on eelgrass ( Zostera marina) from Galiano Island, British Columbia across a gradient of tissue age. We detected 76 genera of diatoms, 47 with both molecular and morphological methods. Using a spatially explicit morphological census, we observed a significant increase in density and a moderate increase in diversity across tissue age. We found that changes in community composition are driven by nestedness rather than turnover. The diatom community diversity and composition detected by molecular data did not differ by tissue age. We suggest that this is because molecular data surveyed a larger area (5 cm 2 vs. 200 µm 2 ), so rare taxa are more likely to be present. Overall, we found that integrating morphological and molecular datasets enhances detection of diversity and provides context for interpreting ecological patterns.