The relation between trophic regime and phytoplankton composition and function in oceanic systems is well accepted in oceanography. However, the relative dynamics and carbon cycling contributions of different phytoplankton groups across gradients of ocean richness are not fully understood. In this work we investigated phytoplankton dynamics along two transects from the NW African coastal upwelling to open-ocean waters of the north Atlantic subtropical gyre. We adopted a pigment-based approach to characterize community structure and to quantify group-specific growth and grazing rates and associated carbon fluxes. Changes in pigment cell concentration during the incubation experiments due to photoadaptation were corrected to obtain reliable rates. The oceanic region was dominated by Prochlorococcus (PRO) (45±7% of total chlorophyll a) while diatoms dominated in upwelling waters (40±37%). Phytoplankton grew faster (γ=0.78±0.26d-1) and free of nutrient limitation (γ/γn=0.98±0.42) in the coastal upwelling region, with all groups growing at similar rates. In oceanic waters, the growth rate of bulk phytoplankton was lower (γ=0.52±0.16d-1) and nutrient limited (γ/γn=0.68±0.19d-1). Diatoms (0.80±0.39d-1) and Synechococcus (SYN) (0.72±0.25d-1) grew faster than Prymnesiophyceae (PRYMN) (0.62±0.26d-1) and PRO (0.46±0.18d-1). The growth rates of PRO and SYN were moderately nutrient limited (γ/γn=0.81 and 0.91, respectively), while the limitation for diatoms (γ/γn=0.71) and PRYMN (γ/γn=0.37) was more severe. Microzooplankton grazing rate was higher in upwelling (0.68±0.32d-1) than in oceanic waters (0.37±0.19d-1), but represented the main loss pathway for phytoplankton in both systems (m/γ=0.90±0.32 and 0.69±0.24, respectively). Carbon flux through phytoplankton, produced and grazed, increased from offshore to coastal (2 to 200γg CL-1d-1), with diatoms dominating the flux in the upwelling region (52%) while PRYMN (40%) and PRO (30%) dominated in the open ocean.
|Original language||English (US)|
|Number of pages||15|
|Journal||Deep-Sea Research Part I: Oceanographic Research Papers|
|State||Published - Nov 2011|
Bibliographical noteFunding Information:
This study was supported by the project COCA ( REN2000-1471-C02-01/MAR ) and EFLUBIO ( REN2002-04151-C02-01/MAR ). Financial support was provided by a Ph.D. fellowship from the Spanish Ministry of Education and Science to A.G.R. We thank B. Molí and G. Vila for their help with HPLC and flow cytometry sample analysis, respectively, and the staff of the UTM, the captain, the crew and other scientists onboard B/O Hespérides for their excellence support during the COCA cruises. We thank Moira Decima and Mike Landry for their insightful comments on a previous version of the manuscript.
- Carbon fluxes
- Microzooplankton grazing
- Nutrient limitation
- Phytoplankton growth
- Subtropical northeast Atlantic
ASJC Scopus subject areas
- Aquatic Science