Ultraviolet radiation and zooplankton community structure following deglaciation in Glacier Bay, Alaska

We investigated changes in UV attenuation and macrozooplankton community st ructure in a set of lakes along a deglaciation chronosequence in Glacier Ba y Alaska. Terrestrial succession in the watersheds of these lakes results i n increasing dissolved organic carbon (DOC) content over time. Due to the p rimary role of DOC in controlling UV attenuation in lakes, one would suspec t a gradient in UV attenuation and potentially zooplankton community struct ure in lakes of different ages. Field measurements of UV in seven lakes of different ages revealed that UV attenuation depths (1% of surface irradianc e at 320 nm) ranged from 0.6 m in the oldest lake in the set (90 yr old), t o more than 14 m in the youngest lake (10 yr old). Zooplankton community st ructure also changed across lakes of different ages. Patterns of distributi on and abundance of the zooplankton both among and within lakes were consis tent with the hypothesis that UV influences zooplankton community structure , The major differences in species composition among lakes were the absence of two primarily epilimnetic species (Asplanchna priodonta and Ceriodaphni a quadrangula) in all but the oldest lake. and the absence of Bosmina longi rostris in the four youngest lakes. Transplant experiments in which UV radi ation was manipulated in.situ revealed that all three of these "delayed col onizer" species perish within only a. few days when exposed to UV levels fo und in the surface waters (0.5 m depth) of the youngest lake. The strong de pendence of UV radiation transparency on terrestrially derived DOC suggests : a linkage between development of terrestrial plant communities within the watershed, changes in lake hydrology, and the early succession of zooplank ton communities following deglaciation.