UC Irvine

Rhizosolenia real abundance, distribution and chemical composition were studied on two cruises in the central North Pacific eyre in order to determine large-scale distribution patterns and contribution to upward nitrogen (N) flux. These macroscopic diatom mats are composed of multiple species of Rhizosolenia that exploit subsurface nitrate pools by vertically migrating below the euphotic zone. Although numerically dominated by the small-diameter species. R.fallax (73-95% of total numbers), mat biovolume was dominated by large-diameter (>50 μm diameter) Rhizostdenia spp. (85- 99% of total volume). Integrated mat abundance was substantially higher when mats accumulated at the surface during calm weather (<80 mats m-2) than during windy periods (≤23.1 mats m-2), suggesting that many mats are found below diver-accessible depths. Chemical composition data indicated that negatively buoyant mats were physiologically stressed compared to positively buoyant mats; negatively buoyant mats had significantly higher carbon (C):N ratios and carbohydrate per mat, and lower protein:carbohydrate ratios and internal NO3 pools than positively buoyant mats. These ratios suggest that N is a key determinant of buoyancy behavior, and are consistent with vertical migration by mats to exploit deep N pools. The maximum ascent rate of mats was 6.4 m h-1 with no relationship to mat size or biovolume. Short-term O2 evolution revealed no significant photoinhibition; conversion to C fixation yielded assimilation numbers of 4.7 and 7.3 μg C μg-1 chl h-1 in negatively buoyant and positively buoyant mats, respectively, although photosynthetic parameters were not statistically different between the two buoyancy classes. Based on photosynthetic rates, ascent rates and estimated N uptake rates, we calculate that a complete migration cycle requires 3.6-5.4 days. When combined with two different estimates of average abundance, we estimate that mats could transport 3.9-40 μmol N m-2 day-1 into the euphotic zone. Using the wide range of literature values for vertical diffusive transport, this represents <1-2000% of the NO3 flux into the euphotic zone and the average equivalent of 3-35% of the new NO3 consumed in the surface mixed layer.