Katie Pelon, Jessica Lim, Katherine RM Mackey

Marine phytoplankton communities are typically studied in the context of a well-mixed ocean where environment selects among cosmopolitan species. However, certain marine microhabitats, including tide pools, sea ice, stratified water columns, marine snow, and kelp epiphytes, are structurally isolated from open-ocean dispersal by physical and chemical barriers, creating discrete "habitat islands" analogous to those described by island biogeography theory. We ask whether this structural isolation produces non-random, habitat-specific phytoplankton communities, and whether community composition differs predictably across habitat types. We conducted a systematic literature search and built a presence-absence matrix from 43 studies and 330 taxa. Biogeographic realm was assigned to each study using Marine Ecoregions of the World. Statistical meta-analyses included Kruskal-Wallis tests for species richness, PERMANOVA for community composition with realm as a covariate, indicator species analysis (IndVal), NMDS ordination, and species accumulation curves. All analyses were performed in Python.

Species richness differed significantly across habitat types (Kruskal-Wallis H=12.86, p=0.012), with tide pools and sea ice showing the highest mean richness. PERMANOVA confirmed significant compositional differences by habitat (pseudo-F=1.30, p=0.033). Marginal effects of both habitat and biogeographic realm were significant (realm p=0.030), though partial PERMANOVA revealed these signals are statistically confounded in the current dataset, motivating broader geographic sampling in the next stage of the study. Prochlorococcus was the strongest habitat indicator (IndVal=0.667 for stratified water column), consistent with its known role as an oligotrophic open-water specialist. Ice-associated diatoms including Fragilariopsis cylindrus emerged as strong sea ice indicators. These preliminary results from an ongoing study suggest that structural isolation may shape phytoplankton community assembly in marine habitat islands. Expansion of the dataset with 158 additional papers and addition of marine mammal and sea turtle epiphytes as a sixth habitat type (including obligate epizoic genera endemic to this group) will strengthen variance partitioning and enable cleaner separation of habitat versus biogeographic effects prior to poster presentation.

• Pelon, Katie. Aquarium Nature Journal. Environerd Studios, 2024.
• Pelon, Katie. Beach Nature Journal. Environerd Studios, 2024.
• Pelon, Katie. Coastal California Nature Journal. Environerd Studios, 2023.
• Pelon, Katie. The Case of the Abandoned Sea Otters. Environerd Studios, 2023.
• Pelon, Katie. Sea Everything: An Ocean Discovery Journal. Environerd Studios, 2021.