The equatorial Pacific Ocean: grazer-controlled phytoplankton populations in an iron-limited ecosystem. Limnology and Oceanography

• Main Authors: N. M. Price, B. A. Ahner, F. Ill M. Morel
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Published: 1994
• Subjects: Antarctic; Arctic; Chavez; Pacific; The Antarctic; Antarc*; Phytoplankton
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.589.3355; http://www.aslo.org/lo/toc/vol_39/issue_3/0520.pdf

Experiments were conducted in the equatorial Pacific Ocean to assess the role of Fe and grazing in regulating use of N03- by the phytoplankton community. Nitrate uptake rates in situ were slow because NH,+ concentrations were inhibitory and because phytoplankton biomass was kept low by grazing. When feeding of grazers was artificially suppressed, phytoplankton net growth rate increased, biomass accu-mulated, and NO,- was consumed. Rapid rates of Fe uptake [40 pmol Fe (g Chl a)- ’ h-l] decreased by an order of magnitude in l-2 d after Fe was added, demonstrating that these rates were under physiological regulation and were elevated in response to low Fe concentrations. Addition of Fe increased carbon uptake and the short-term N-specific NO,- uptake rate by 2-9 times. These physiological stimulations were confined to large phytoplankton (> 3 pm), which thus must have been Fe-limited in situ. N03- uptake rate and biomass of small phytoplankton were unaffected by Fe enrichment. The results thus suggest that the low biomass, N03--rich condition of the equatorial Pacific Ocean exists because low Fe concentrations limit use of N03- by large phytoplankton and favor growth of small phytoplankton that are grazed efficiently and use NH,+ preferentially. The equatorial Pacific is one of the regions of open ocean that is comparatively rich in nitrate and phosphate and low in chlorophyll; other such areas are in the Antarctic and sub-arctic Pacific. Several explanations for this un-usual condition have been offered. Two of these, grazing control and Fe limitation, were originally portrayed as antipodal hypotheses. We and others (Price et al. 1991; Chavez et al.