Desmids (sensu lato) are a group of morphologically diverse, freshwater algae, which have been used as biological indicators in some regions. In this study, we sampled three New England ponds from 2017 to 2019 and recorded their desmid floras. We then curated early- to mid-20th-century floristic data from the same ponds and compared them to our findings from 2017 to 2019 to document changes in desmid assemblages over the last 100 years. We assessed the past and current conservation value of the ponds using desmids as biological indicators. We documented a major decline in desmid diversity and conservation value of two of the ponds, but we documented remarkable diversity in Hawley Bog in western Massachusetts, with 66 taxa newly reported from the locality. Additionally, we documented the first record of Micrasterias brachyptera from New England. To aid future floristic efforts, we provided our species records and images in a public repository, the community-science database iNaturalist.