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

Abstract

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 chlorophy-a (Chl-a) biomass variability for both the active and relaxed phases of the local, winddriven 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 (3/4Chl-a) in terms of an overall "environmental condition" over a 50 km upwelling sensitive coastal strip, revisiting published and unpublished Chl-a ship (Cship = Chl-a + Phaeopigments) data.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 3/4Chl-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 3/4Chl-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 3/4Chl-a-values. Satellite chlorophyll (Csat) data obtained in clear skies (active upwelling only), showed a similar distribution to those of 3/4Chl-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. © 2006 Sociedad de Biología de Chile.