Bio-optical characteristics of Cariaco Basin (Caribbean Sea) waters

Bio-optical properties of marine waters of the Cariaco Basin (southeastern Caribbean Sea) were assessed monthly between 1995 and 2005 as part of the CARIACO Ocean Time Series program. Temporal changes in light quality and penetration were caused by seasonal variation in the concentration of three major optical constituents, namely phytoplankton, detrital particles, and colored dissolved organic matter (CDOM). All constituents showed higher absorption coefficients during the upwelling season (January–May) compared to the rainy season (June–November). Both the absorption coefficient due to CDOM (ag(440)) and due to phytoplankton (aph(440)) had similar contributions to total absorption of light during the upwelling season (aph(440)=0.062±0.042 m−1, ag(440)=0.065±0.047 m−1). In contrast, ag(440) dominated light absorption during the rainy season (aph(440)=0.017±0.011 m−1, ag(440)=0.057±0.031). This led to an overestimate in SeaWiFS-derived chlorophyll concentrations during the rainy season, of between 7% and 45%. The detrital component, ad(440), typically showed the smallest contribution (ad(440)=0.021±0.014 m−1 during upwelling and 0.007±0.001 m−1 during the rainy season). There was no clear relationship between the various optically active components in time. During the upwelling season the chlorophyll-specific absorption coefficient, a p h ⁎ (440), was nearly half the value observed during the rainy season due to changes in the package effect (explaining ∼40% of the variability) and in accessory pigment composition as a result of species succession (explaining ∼60% of the variability). The euphotic zone depth (depth of the 1% photosynthetic active radiation (PAR) level) was typically shallower during the upwelling season (36.7±12.3 m) than during the rainy season (47.9±13.5 m) due to the onset of a shallower and stronger phytoplankton bloom. During upwelling, the highest chlorophyll-a concentrations (Chl>1 mg m−3) were observed in the upper 25 m with primary production rates exceeding 1800 mgC m−2 d−1. During the rainy season, a deep chlorophyll maximum (DCM, concentrations between 0.2 and 0.8 mg m−3) was observed between 35 and 55 m, with low (<0.2 mg m−3) Chl concentrations above this depth and primary production values of ∼990 mgC m−2 d−1. The DCM occurred immediately above the seasonal thermocline and around the 1% PAR light level. During the upwelling season, no DCM was observed.