Iron Limitation and Uneven Grazing Pressure on Phytoplankton Co‐Lead the Seasonal Species Succession in the Ross Ice Shelf Polynya

Abstract

AbstractThe seasonal species succession of phytoplankton from Phaeocystis antarctica to diatoms in the Ross Ice Shelf Polynya (RISP) plays a fundamental role in food webs and ecosystem functioning. Previous studies have revealed that this seasonal succession is affected by differences in light, iron demand, aggregation, and subsequent sinking between these two taxa. This study further investigated the dominant process of species succession by analyzing the results of a bulk mixed‐layer ecosystem model. The mechanism of processes leading to the replacement by diatoms differs from that for triggering the initial P. antarctica bloom. Light triggered the spring bloom dominated by P. antarctica in iron‐rich surface water, which is resupplied by winter convective mixing from the lower layer pool. After reaching its maximum in mid‐November, the biomass of P. antarctica declined progressively due to the severe iron limitation. Seasonal species succession can be enhanced by a small amount of predation on P. antarctica by microzooplankton. The long‐term iron enrichment promotes the transition by supporting diatom growth. The balance between bottom‐up and top‐down processes plays a critical role in shaping the phytoplankton composition and promoting the seasonal species succession in the iron‐limited upper mixed layer of the RISP.