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

We investigated changes in UV attenuation and macrozooplankton community structure in a set of lakes along a deglaciation chronosequence in Glacier Bay Alaska. Terrestrial succession in the watersheds of these lakes results in increasing dissolved organic carbon (DOC) content over time. Due to the primary role of DOC in controlling UV attenuation in lakes, one would suspect a gradient in UV attenuation and potentially zooplankton community structure in lakes of different ages. Field measurements of UV in seven lakes of different ages revealed that UV attenuation depths (1% of surface irradiance at 320 nm) ranged from 0.6 m in the oldest lake in the set (90 yr old), to more than 14 m in the youngest lake (10 yr old). Zooplankton community structure also changed across lakes of different ages. Patterns of distribution and abundance of the zooplankton both among and within lakes were consistent 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 Ceriodaphnia quadrangula) in all but the oldest lake, and the absence of Bosmina longirostris in the four youngest lakes. Transplant experiments in which UV radiation was manipulated in situ revealed that all three of these "delayed colonizer" species perish within only a few days when exposed to UV levels found in the surface waters (0.5 m depth) of the youngest lake. The strong dependence 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 zooplankton communities following deglaciation.