Seasonal stresses shift optimal intertidal algal habitats

  • Dethier M
  • Williams S
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We studied how the growth, reproduction, and survival of a common intertidal rockweed (Fucus distichus) varied across its tidal elevation at 14 sites around San Juan Island, Washington, USA in spring–summer and fall-winter seasons. We also measured a suite of environmental factors including temperature, light, emersion time, slope, fetch, and herbivory. To interpret the response of Fucus we included measurements of phlorotannins and carbon stor- age compounds (mannitol, laminarin). Growth and repro- duction exhibited parallel patterns across tidal zones and sites. Tidal zone was a signiWcant source of variation for many Fucus response variables, whereas variation between sites was high but not generally a signiWcant factor explain- ing Fucus growth and physiology. Unexpectedly, the tidal zone in which Fucus achieved its highest growth and repro- duction switched between seasons. High zone thalli grew and reproduced better than Mid zone thalli in fall but not in spring. This result can be explained by diVerent combina- tions of factors inXuencing Fucus in each season. In spring, longer emersion times due to daytime low tides resulted in lower growth rates higher on the shore, likely due to carbon limitation. In fall during nighttime low tides, emersion and carbon limitation stresses were minimal. Overall, fall growth was lower than spring growth, but low fall light was not responsible. Instead, warmer average fall temperatures in the High zone apparently favored growth and reproduc- tion relative to the Mid zone. In contrast, Mid zone thalli were subjected to more intense herbivory and hydrody- namic stress associated with wave exposure and steep sub- strata during the fall. At least for some seaweeds, living in the presumably more stressful high zone can actually con- fer higher integrated performance.

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  • Megan N. Dethier

  • Susan L. Williams

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