Distribution of coccoid cyanobacteria and small eukaryotic phytoplankton in the upwelling ecosystem off the Oregon coast during 2001 and 2002

Patterns of spatial distribution of small-sized phytoplankton: coccoid cyanobacteria (Synechococcus, SYN) and small photosynthetic eukaryotes (PEUK) were investigated in the upwelling ecosystem off the coasts of Oregon and Northern California during nine cruises of the GLOBEC Northeast Pacific Long Term Oceanographic Program (LTOP), from March 2001 to December 2002. SYN cells were 1–2 μm, and most PEUK cells <5 μm, in size. We found a consistent pattern of lowest abundance of small-sized phytoplankton in shelf regions, despite high nutrient and chlorophyll concentrations in recently upwelled water. Seaward of the upwelling front, the region of transition between inshore vertical sigma-t surfaces characteristic of upwelling and offshore horizontal sigma-t surfaces characteristic of vertical stratification, there were high abundances of both SYN (5–58×104 cells ml−1) and of PEUK (1–8.6×104 cells ml−1) in the upper 50 m of the water column. Log-linear plots of SYN and PEUK abundances against nitrate+nitrite concentration showed a negative relationship between abundance and nutrient concentration. At chlorophyll-a concentrations of <∼2 μg l−1), small cells typically comprised most of the carbon biomass of the phytoplankton. The pattern of phytoplankton distribution found in this study suggests a dramatic spatial shift in the structure of pelagic food webs in the Oregon upwelling ecosystem, from shelf upwelling blooms dominated by large diatoms, to slope and basin food webs dominated by<5 μm-sized phototrophic cells. Why the abundances of small-sized phytoplankton, and especially of coccoid cyanobacteria, were low in high-nutrient, high-chlorophyll shelf waters remains to be explained.