18-year variability of ultraviolet radiation penetration in the mid-latitude coastal waters of the western boundary Pacific

The 18-year time-series shows in situ ultraviolet radiation (UVR) and photosynthetically active radiation (PAR) diffuse attenuation coefficient K-d(lambda) have recurrent seasonal variability of high/low attenuation during summer/winter months, respectively, dependent on variability in water column stratification and concentrations of bio-optical properties. The mid-latitude coastal survey station displayed significant seasonality of the mixed layer depth (MLD) between 12 and 82 m which modified the distribution of chlorophyll a (4.6-24.9 mg m(-2)) and absorption of colored dissolved organic matter [a(CDOM)(320 nm) 0.043-134 m(-1)]. The median K-d(320 nm) displayed significant seasonality at 0.19 -0.74 m(-1) (C.V. = 44.1%) and seasonal variability within the euphotic layer [Z(10%)(320 nm) = 7-20%]. High attenuation of UVR with relatively moderate attenuation of PAR was consistently observed during the summer months when increased concentrations of terrestrially derived CDOM coupled with a shallow MLD were present. The winter season showed the opposite of low UVR and PAR attenuation due to a relatively deeper MLD coupled with low concentrations of bio-optical properties. Although the long term K-d(lambda) did not vary significantly during the time-series, analysis of the interannual variability suggests there are positive and negative phases following the Pacific Decadal Oscillation (PDO) vis-a-vis variability in bio-optical properties (p < 0.001). (C) 2015 Elsevier Ltd. All rights reserved.