Biogenic silica cycling during summer phytoplankton blooms in the North Pacific subtropical gyre

Abstract

Biogenic silica (bSiO2) cycling, diatom abundance and floristics were examined within summer-period diatom blooms in the North Pacific Subtropical Gyre (NPSG) in 2008 and 2009. Hemiaulus hauckii was the most abundant diatom observed in an expansive (100,000km2) bloom near the subtropical front in the northeastern NPSG in 2008 and the small pennate diatom Mastogloia woodiana dominated a smaller (30,000km2) bloom sampled in 2009 in the gyre interior. In both blooms, the bSiO2 stock and production rates were up to an order of magnitude higher relative to non-bloom areas. Remnants of a bSiO2 export event was sampled in the H. hauckii bloom area where the export rate at 300m exceeded that at 150m, and was among the highest values recorded in the NPSG. The M. woodiana bloom was very active with specific bSiO2 production rates of 0.50–0.75d−1 and net bSiO2 production rates were among the highest observed in any subtropical-gyre diatom bloom to date. Net silica production rates in the euphotic zone were strongly positive within blooms and near zero outside of blooms, consistent with an important role for blooms in bSiO2 export. The difference in the areal extent of the H. hauckii and M. woodiana blooms was consistent with remote-sensing observations that blooms in the northeastern portion of the NPSG, near the subtropical front, are typically more extensive than those in the gyre interior near Hawaii Ocean Time-series station ALOHA. Initial estimates suggest that blooms in the northeast region produced 3–25 times more bSiO2 in 2008 and 2009, respectively, than did blooms in the gyre interior; and due to the large areal extent these blooms, their area-integrated production of bSiO2 is similar to intense diatom blooms coastal upwelling systems (e.g. Monterey Bay, Santa Barbara Channel) despite significantly lower production rates and standing stock.