Particulate silica and Si recycling in the surface waters of the Eastern Equatorial Pacific

The distributions of biogenic and lithogenic silica concentrations and net silica production rates in the upper 120 m of the Eastern Equatorial Pacific (EEP) were examined in December 2004, on two transects situated at 110°W (4°N to 3°S) and along the equator (110°W to 140°W). Lithogenic silica (lSiO2) was generally <10 nmol Si l−1 with maximum concentrations reaching 25 nmol l−1 in surface waters. These low concentrations confirm low atmospheric inputs of particulate Si, consistent with reported low inputs of wind-borne material in the EEP. In spite of active upwelling of silicic acid-rich waters the biogenic silica (bSiO2) concentrations were generally low, falling between 100 and 180 nmol Si l−1 in the upper 50 m and decreasing to less than 50 nmol Si l−1 below ∼90 m. Estimates of net bSiO2 production rates revealed that the rate of production exceeded that of dissolution in the upper euphotic layer (0-40 m) along 110°W with net production extending somewhat deeper (60-100 m) to the west along the equator. Net production rates in the surface layer were low, ranging between 5 and 40 nmol Si l−1 d−1, consistent with previous observations that diatoms are small contributors to autotrophic biomass in the EEP. Net silica dissolution predominated in the lower euphotic layer (40-120 m), indicating active Si recycling which diminished the strength of the silica pump in this region.