Phytoplankton growth and microzooplankton grazing dynamics across vertical environmental gradients determined by transplantin situdilution experiments.
The Costa Rica Dome (CRD) represents a classic case of the bloom-forming capacity of small phytoplankton. Unlike other upwelling systems, autotrophic biomass in the CRD is dominated by picocyanobacteria and small eukaryotes that outcompete larger diatoms and reach extremely high biomass levels. We investigated responses of the subsurface phytoplankton community of the CRD to changes associated with vertical displacement of water masses, couplingin situtransplanted dilution experiments with flow cytometry and epifluorescence microscopy to assess group-specific dynamics. Growth rates ofSynechococcus(SYN) and photosynthetic picoeukaryotes (PEUK) were positively correlated with light (Rpearson_SYN= 0.602 andRpearson_PEUK= 0.588,P< 0.001). Growth rates ofProchlorococcus(PRO), likely affected by photoinhibition, were not light correlated (Rpearson_PRO= 0.101,P= 0.601). Overall, grazing and growth rates were closely coupled in all picophytoplankton groups (Rspearman_PRO= 0.572,Rspearman_SYN= 0.588,Rspearman_PEUK= 0.624), and net growth rates remained close to zero. Conversely, the abundance and biomass of larger phytoplankton, mainly diatoms, increased more than 10-fold in shallower transplant incubations indicating that, in addition to trace-metal chemistry, light also plays a significant role in controlling microphytoplankton populations in the CRD.
Scripps Institution of Oceanography , 9500 Gilman Dr., La Jolla, CA 92093-0227 , USA.
Andrés
A
Gutiérrez-Rodríguez
Department of Oceanography , University of Hawaii at Manoa , Honolulu, Hi 96822 , USA.
Karen E
KE
Selph
Scripps Institution of Oceanography , 9500 Gilman Dr., La Jolla, CA 92093-0227 , USA.
Michael R
MR
Landry
Costa Rica Dome; community structure; growth and grazing dynamics; physical perturbation; phytoplankton