High concentrations of marine snow and diatom algal mats in the North Pacific Subtropical Gyre: Implications for carbon and nitrogen cycles in the oligotrophic ocean

A video plankton recorder (VPR) and a remotely operated vehicle (ROV) were utilized on three cruises in the oligotrophic North Pacific Subtropical Gyre (NPSG) between 1995 and 2002 to quantify the size and abundance of marine snow and Rhizosolenia diatom mats within the upper 305 m of the water column. Quantitative image analysis of video collected by the VPR and an ROV-mounted particle imaging system provides the first transect of marine snow size and abundance across the central North Pacific Gyre extending from 920 km NW of Oahu to 555 km off Southern California. Snow abundance in the upper 55 m was surprisingly high for this oligotrophic region, with peak values of 6.0–13.0×103 aggregates m−3 at the western- and easternmost stations. At stations located in the middle of the transect (farthest from HI and CA), upper water column snow abundance displayed values of ∼0.5–1.0×103 aggregates m−3. VPR and ROV imagery also provided in situ documentation of the presence of nitrogen-transporting, vertically migrating Rhizosolenia mats from the surface to >300 m with mat abundances ranging from 0 to 10 mats m−3. There was clear evidence that Rhizosolenia mats commonly reach sub-nutricline depths. The mats were noted to be a common feature in the North Pacific Gyre, with the lower salinity edge of the California Current appearing to be the easternmost extent of their oceanic distribution. Based on ROV observations at depth, flux by large (⩾1.5 cm) mats is revised upward 4.5-fold, yielding an average value of 40 μmol N m−2 d−1, a value equaling previous estimates that included much smaller mats visible only to towed optical systems. Our results suggest that the occurrence across a broad region of the NPSG of particulate organic matter production events represented by high concentrations of Rhizosolenia mats, associated mesozooplankton, and abundant detrital marine aggregates may represent significant stochastic components in the overall carbon, nitrogen, and silica budgets of the oligotrophic subtropical gyre. Likewise, their presence has important implications for the proposed climate-driven, ecosystem reorganization or domain shift occurring in the NPSG.