Abstract
Understanding underwater optics in natural waters is essential in evaluating aquatic primary production and risk of UV exposure in aquatic habitats. Changing environmental conditions related with global climate change, which imply potential contrasting changes in underwater light climate further emphasize the need to gain insights into patterns related with underwater optics for more accurate future predictions. The present study evaluated penetration of solar radiation in six sub-Antarctic estuaries and fjords in Chilean North Patagonian region (39–44°S) and in an Antarctic bay (62°S). Based on vertical diffuse attenuation coefficients (Kd), derived from measurements with a submersible multichannel radiometer, average summer UV penetration depth (z1%) in these water bodies ranged 2–11 m for UV-B (313 nm), 4–27 m for UV-A (395 nm), and 7–30 m for PAR (euphotic zone). UV attenuation was strongest in the shallow Quempillén estuary, while Fildes Bay (Antarctica) exhibited the highest transparency. Optically non-homogeneous water layers and seasonal variation in transparency (lower in winter) characterized Comau Fjord and Puyuhuapi Channel. In general, multivariate analysis based on Kd values of UV and PAR wavelengths discriminated strongly Quempillén estuary and Puyuhuapi Channel from other study sites. Spatial (horizontal) variation within the estuary of Valdivia river reflected stronger attenuation in zones receiving river impact, while within Fildes Bay a lower spatial variation in water transparency could in general be related to closeness of glaciers, likely due to increased turbidity through ice-driven processes. Higher transparency and deeper UV-B penetration in proportion to UV-A/visible wavelengths observed in Fildes Bay suggests a higher risk for Antarctic ecosystems reflected by e.g. altered UV-B damage vs. photorepair under UV-A/PAR. Considering that damage repair processes often slow down under cool temperatures, adverse UV impact could be further exacerbated by cold temperatures in this location, together with episodes of ozone depletion. Overall, the results emphasize the marked spatial (horizontal and vertical) and temporal heterogeneity of optical characteristics, and challenges that these imply for estimations of underwater optics.
Highlights
The southeastern Pacific coast of the Chilean North Patagonia is characterized by a large and complex system of fjords and estuaries
Increasing knowledge on the oceanography in this area has been gained in recent years, gaps still remain in bio-optical characterization of these systems, which limits our knowledge on the factors related to, e.g. primary productivity and exposure to UV radiation of pelagic and benthic assemblages
In highly humic small lakes with high CDOM, penetration of UV-B wavelengths may be only few centimeters [18,19,20], while in low CDOM oceanic waters or clear oligotrophic lakes, where phytoplankton contributes more to the light attenuation, it can reach even dozens of meters [14, 21,22]
Summary
The southeastern Pacific coast of the Chilean North Patagonia is characterized by a large and complex system of fjords and estuaries This area coincides with the oceanographic transition between the sub-Antarctic and the cold-temperate zones, influenced by the Cape Horn Current and the Humboldt Current System, respectively. Regarding the biogeography, it is in the northern limit of the Magellan Province, with unique but still widely unexplored marine biodiversity [1,2]. The role of dissolved organic matter (CDOM) in governing the attenuation of UV radiation, in freshwaters, where the impact of catchment area is relatively stronger than in oceans, has been widely described [17]. Spatial and temporal heterogeneity of bio-optical properties has been recognized as one of the challenges in larger-scale estimations [24,25]
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