Revisiting chlorophyll data along the coast in north-central Chile, considering multiscale environmental variability

Phytoplankton abundance in the surface mixed layer of the coastal ocean responds to environmental changes at various time scales. Here the warm, cold and neutral phases of three environmental cycles have been jointly considered to assess chlorophyll-a (Chl-a) biomass variability for both the active and relaxed phases of the local, wind-driven coastal upwelling: (i) the interannual ENSO cycle (ii) the annual (seasonal) cycle and (iii) the intraseasonal cycle associated with equatorially-sourced, ocean trapped-waves along the coast in northern Chile. The main goal of this study is to quantitatively assess the variability of the depth-integrated Chl-a biomass in the euphotic zone (fChl-a) in terms of an overall environmental condition over a 50 km upwelling sensitive coastal strip, revisiting published and unpublished Chl-a ship (C-ship = Chl-a + Phaeopigments) data. All possible environmental conditions combinations were further ranked into seven environmental indices ranging from 0 (absolutely cold) to 6 (absolutely warm). Out of 332 samples of fChl-a, 198/134 were obtained during active/relaxed upwelling conditions from which 24/38 and 30/36 samples were associated with the simultaneous occurrence of at least two cold/warm phases of the three environmental cycles (cold/''warm environmental conditions), respectively. Lower fChl-a values during cold and warm environmental conditions relative to the neutral ones reached statistical significance for both active and relaxed conditions (144/60 samples respectively). Higher turbulent mixing during cold environmental conditions and a deeper nutricline during warm ones would explain lower fChl-a-values. Satellite chlorophyll (C-sat) data obtained in clear skies (active upwelling only), showed a similar distribution to those of fChl-a when classified into the corresponding environmental indices. These results suggest that during neutral (transitional) environmental conditions, nutrient supply, mean light exposure and mixing thresholds, including biological interactions, could be more effective in producing a higher phytoplankton biomass, in spite of a larger dispersion.