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The growth, reproduction, and senescence patterns of the epiphytic marine alga Phaeosaccion collinsii were tracked over two consecutive seasons at its type locality of Little Nahant, Nahant, MA (USA). We investigated the potential and/or combined effects of temperature and ambient nutrient supply (NO3 - and PO4 3-) on the phenology of this ephemeral species in its natural environment by collecting microscopic and macroscopic P. collinsii specimens from blades of eelgrass (Zostera marina) in a shallow coastal subtidal zone. Our results suggest that temperature is a strong driver of the alga’s in situ cycle and that the optimal temperature for P. collinsii growth and reproduction may be between 5 and 8°C, a narrower temperature threshold than previous laboratory studies on this subject have suggested. Several large winter storms also allowed us to observe the effect of physical disturbance on the integrity of the eelgrass beds and the population of microscopic and macroscopic P. collinsii. This study contributes the first in situ information on the abiotic conditions necessary for the successful growth and development of P. collinsii and a greater understanding of the life cycle of this unique golden brown alga.