Changes in nutrient concentration induced by hydrological variability and its effect on light absorption by phytoplankton in the Alborán Sea (Western Mediterranean Sea)

Abstract

The effect of hydrologic variability on the chlorophyll a-specific absorption coefficient [ a ⁎( λ)] was investigated in the NW Alborán Sea during a cruise performed in spring. The study area was characterised by a strong stratification of the water column. The lower limit of the mixed layer coincided with the lower limit of the euphotic layer (EL), which was characterised by very low nitrate (0.23 ± 0.30 μg L − 1 ) and phosphate (0.09 ± 0.04 μg L − 1 ) concentrations. Accordingly, the chlorophyll a (Chl a) concentration in EL was low (0.59 ± 0.40 μg Chl a L − 1 ) indicating that phytoplankton growth was probably limited by nutrient availability. Contrastingly, nitrate concentration increased by 10-folds in the seawater layer located below EL (IL, until 0.1% surface irradiance depth level). In most of the stations, the vertical maximum of Chl a was located in IL (the Chl a concentration mean in the maximum was 2.05 ± 0.89 μg L − 1 ). The optical properties of the phytoplankton communities in EL and IL differed significantly. Thus, a ⁎(440) and a ⁎(675) decreased by about 40% in IL in comparison to EL. The parameter Q ⁎ a (675), which was used as a proxy for pigment packaging magnitude, also decreased in IL (0.62) with respect to EL (0.93), suggesting that the reduction of a ⁎( λ) in IL was due to the pigment packaging effect. In spite of the strong stratification of the water column, inter-station shifts of the ML thickness were produced. Thus, the deepening of ML observed at some stations dealt with higher salinity and nutrient concentration in the surface layer. The horizontal variability of a ⁎( λ) was apparently related to this hydrological variability as a negative correlation between a ⁎(675) and surface salinity was obtained ( r 2 = 0.53) in EL. The correlation between salinity and Q ⁎ a (675) was also negative ( r 2 = 0.31), which indicates that nitrate enrichment in EL dealt with an increase in Chl a packaging effect. These results demonstrate that it is possible to describe variability of a ⁎( λ) from changes in hydrological features of the water column.