Phytoplankton composition, photosynthesis and primary production during different hydrographic conditions at the Northwest Iberian upwelling system

Abstract

The major hydrographic conditions along the NW Iberian Peninsula are upwelling and poleward current. Changes in prevailing winds create transitional hydrographic states such as down- welling, stratification and upwelling-downwelling relaxation. Four cruises were undertaken between 44.25 and 41.14° N during spring, summer and winter, when there was a clear demarcation of these hydrographic conditions. We studied the effects of hydrography on phytoplankton species composi- tion, photophysiology, light limitation and primary production (PP) derived from broad-band and spectral models. PP was significantly higher during upwelling (F3, 49 = 99.41, p < 0.001; mean 2595 ± 213 mgC m -2 d -1 ) compared with all other hydrographic conditions. The poleward current had the lowest PP (214 ± 22 mgC m -2 d -1 ). PP calculated using the spectral model (∑ PPPUR) was greater than PP from the broad-band model (∑ PPPAR), especially when southerly winds prevailed, and during stratified conditions when the chlorophyll maximum was deeper, irradiance levels were lower and most of the phytoplankton biomass was below the light limitation depth. During northerly winds, chl a was the single most important variable influencing the calculation of PP and the difference between ∑ PPPAR and ∑ PPPUR was small, since most of the phytoplankton biomass occurred above the light limitation depth, in saturating irradiance. Variations in hydrography were intrinsically linked to changes in phytoplankton species composition, photophysiology and PP. Over the whole region 89% of the variance in maximum photosynthetic rate Pm B was explained by daily mean spectral irradiance, biomass of flagellates and cyanobacteria and 75% of the variance in PP was explained by the along- shore Ekman transport component (Qy), cyanobacteria and diatom biomass. Qy alone explained 52% of the variance in PP over the shelf, which was also coupled with variations in Pm B and the biomass of diatoms and flagellates. At oceanic locations there was no relation between Qy and PP (r 2 = 7%), and PP co-varied with Pm B and the biomass of cyanobacteria.