Seasonal effects on protozoan colonization of polyurethane foam (PF) units were investigated in.an oligotrophic lake, Mountain Lake, Virginia. PF units were placed into the lake's pelagic water to simulate barren habitat islands suitable for colonization by protozoa. The results were interpreted with respect to the MacArthur-Wilson equilibrium theory of island biogeography. Results showed seasonal changes can cause a substantial amount of instability in colonization curves. Within a season, changes in the lake's planktonic species pool were more prominent during the decay of the thermocline than during Fall or Spring overturn and Summer stratification. For different seasons, the relationship between changes in the lake's planktonic species pool and species accrual on PF units was positive, for total species and for ciliate species. For different seasons, the relationship between changes in colonization rate and species accrual was consistent with MacArthur-Wilson theory. In contrast, for different seasons, the relationship between changes in equilibrium number of species and species accrual was inconsistent with MacArthur-Wilson theory, and this places in question the interseasonal predictive value of colonization curves. For some situations where the MacArthur-Wilson equation for insular colonization did not describe adequately the colonization a modified equation was developed. The modified equation was used to account roughly for species which may rapidly colonize and persist in PF unit communities and described adequately more total species colonization curves than the unmodified equation. Moreover, the modified equation was a useful tool for the interpretation of PF unit colonization by protozoa. / Master of Science