Nitrogen Fixation, Suspension Characteristics, and Chemical Composition of Rhizosolenia Mats in the Central North Pacific Gyre

Abstract

Rhizosolenia mats were hand-collected from the central North Pacific gyre in March-April 1989 to examine their suspension properties, composition, and potential for nitrogen fixation by symbionts. Six species of Rhizosolenia were found in 10 different mat combinations. Nitrogen fixation could not be demonstrated either in in situ incubations or under N-poor culture conditions. Measured C: N (7.4 ± 2.8 by atoms) and C: Chi (150 ± 100 by weight) ratios were characteristic of healthy high-light-adapted phytoplankton populations, and this suggests that mat formation is not a senescence-induced phenomenon. Mats accounted for 2–30% of the total chlorophyll a in the upper 10 m and 98% of the biogenic particulate silicate. Positive buoyancy was documented in the aggregates, with an average daytime ascent rate of 3.6 ± 1.8 m h⁻¹. Mats accumulated at the surface under low wind conditions. This study shows that macroscopic diatom associations need not sink. The vertical migration documented in one buoyant nonmotile marine dinoflagellate coupled with literature evidence on the physiology and distribution of large diatoms suggests that vertical migration could also occur in positively buoyant marine diatoms.

Keywords:

• Rhizosolenia
• diatoms
• nitrogen-fixation
• sinking
• phytoplankton
• buoyancy
• Pacific