VERTICAL MIGRATION OF RHIZOSOLENIA MATS AND THEIR SIGNIFICANCE TO NO3- FLUXES IN THE CENTRAL NORTH PACIFIC GYRE

Rhizosolenia mat abundance, distribution and chemical composition were studied on two cruises in the central North Pacific gyre in order to determine large-scale distribution patterns and contribution to upward nitrogen (N) flux. These macroscopic diatom mats are composed of mult iple species of Rhizosolenia that exploit subsurface nitrate pools by vertically migrating below the euphotic zone. Although numerically dom inated by the small-diameter species, R.fallax (73-95% of total number s), mat biovolume was dominated by large-diameter (>50 mu m diameter) Rhizosolenia spp. (85-99% of total volume). Integrated mat abundance w as substantially higher when mats accumulated at the surface during ca lm weather (less than or equal to 80 mats m(-2)) than during windy per iods (less than or equal to 23.1 mats m(-2)), suggesting that many mat s are found below diver-accessible depths. Chemical composition data i ndicated that negatively buoyant mats were physiologically stressed co mpared to positively buoyant mats; negatively buoyant mats had signifi cantly higher carbon (C):N ratios and carbohydrate per mat, and lower protein:carbohydrate ratios and internal NO3- pools than positively bu oyant mats. These ratios suggest that N is a key determinant of buoyan cy behavior, and are consistent with vertical migration by mats to exp loit deep N pools. The maximum ascent rate of mats was 6.4 m h(-1) wit h no relationship to mat size or biovolume. Short-term O-2 evolution r evealed no significant photoinhibition; conversion to C fixation yield ed assimilation numbers of 4.7 and 7.3 mu g C mu g(-1) chl h(-1) in ne gatively buoyant and positively buoyant mats, respectively, although p hotosynthetic 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 mu mol N m(-2) day(-1) into the euphotic zone. Using the wide range of liter ature values for vertical diffusive transport, this represents <1-2000 % of the NO3- flux into the euphotic zone and the average equivalent o f 3-35% of the new NO3- consumed in the surface mixed layer.