Seasonal sea ice retreat is known to stimulate Southern Ocean phytoplankton blooms, but depth‐resolved observations of their evolution are scarce. Autonomous float measurements collected from 2015–2019 in the eastern Weddell Sea show that spring bloom initiation is closely linked to sea ice retreat timing. The appearance and persistence of a rare open‐ocean polynya over the Maud Rise seamount in 2017 led to an early bloom and high annual net community production. Widespread early ice retreat north of Maud Rise in 2017, however, had a similar effect, suggesting that the polynya most impacted the timing of bloom initiation. Still, higher productivity rates at Maud Rise relative to the surrounding region are observed in all years, likely supported by flow‐topography interactions. The longer growing season in 2017–2018 also allowed for separation of distinct spring and fall bloom signals, the latter of which was primarily subsurface and associated with mixed‐layer deepening. Plain Language Summary: Antarctic sea ice retreat each spring regulates the growth of tiny algae called phytoplankton, which form the base of marine food webs and absorb carbon dioxide from the atmosphere through photosynthesis. Massive holes in the Antarctic sea ice, called polynyas, are also thought to support phytoplankton growth. Measuring the biological activity in the sea ice zone is difficult, however, because these environments are hard to access. In this study, we analyze observations from robotic floats in the Weddell Sea, including two floats near the ice edge and two floats that sampled in an offshore polynya that formed over a seamount called Maud Rise. The data show that opening of the polynya in 2017 triggered an early and prolonged phytoplankton bloom associated with high carbon sequestration. However, a similarly early bloom was observed far to the north of Maud Rise, induced by a widespread early ice retreat in that year, suggesting that the polynya did not greatly impact the bloom intensity. Instead, circulation over Maud Rise leads to high biological productivity in all years, even those without polynyas. The lengthened growing season in 2017–2018 also permitted the isolation of a fall bloom, which occurred primarily below the sea surface. Key Points: Autonomous float observations are used to characterize the evolution and vertical structure of phytoplankton blooms in the Weddell SeaBloom duration and total carbon export were enhanced by widespread early ice retreat and Maud Rise polynya formation in 2017Early spring bloom initiation creates conditions for a distinguishable subsurface fall bloom associated with mixed‐layer deepening [ABSTRACT FROM AUTHOR]