Pacific Rocky Intertidal Monitoring: Trends and Synthesis

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Fucus (Northern Rockweed)

Fucus distichus (Linnaeus)

Kingdom Chromista, phylum Ochrophyta, class Phaeophyceae, order Fucales, family Fucaceae

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Description

This olive-brown thallus can reach up to 50 cm tall and 15-25 mm wide. Individuals in protected sites are often larger than those at exposed ones. Branches are flattened and dichotomously branched with a distinct midrib. Reproductive conceptacles are concentrated at branch tips (swollen when mature).

Habitat and Geographic Range

Fucus is common in the upper mid-intertidal zone, in exposed to protected outer and inner coast locals, from Alaska to Southern California (Lamb and Hanby 2005). In the Pacific Northwest and Alaska (particularly on sheltered shorelines), Fucus is often the dominant species of algae, forming a broad, distinctive band in the mid-intertidal.

Synonyms

Fucus gardneri

Similar species

Can be confused with the other common rockweeds: Hesperophycus californicus, Pelvetiopsis limitata, and Silvetia compressa. In California, Fucus has broader fronds and larger receptacles than the other species and, unlike H. californicus, can have irregularly spaced white hairs along the midrib rather than paired hairs and has less ruffled fronds. Fronds of Silvetia and Pelvetiopsis lack a midrib.

Natural History

Fucus forms broad, dense canopies in the mid intertidal zone and can extend well into the high zone, with plants becoming smaller and less dense at the upper edge of its tidal range. This fucoid is tolerant of a wide range of salinities, and occurs on the outer coast, on protected inland shores, and even in areas inundated by freshwater (O’Clair and Lindstrom 2000). Fucus canopies are important for providing protection from desiccation to a suite of other algae and invertebrates. Some grazers inhabiting the Fucus understory have been shown to facilitate the persistence of the rockweed by selectively grazing other algae that compete with Fucus for space. For example, the littorine, Littorina sitkana aids in the succession of Fucus by preferentially consuming more ephemeral algae like Ulva lactuca and Enteromorpha (Lubchenco 1983).

The life history of this algal species is diplontic, with a diploid thallus and gamete formation via meiosis (Searles 1980). When mature, receptacles (swollen, yellowish bumps) on the blade tips release gametes at low tide. Eggs are fertilized with the incoming tide, and the resulting zygotes secrete adhesive and attach to the substratum (O’Clair and Lindstrom 2000). Individuals are thought to live approximately 2-3 years at exposed sites, and approximately 4-5 years in protected areas (O’Clair and Lindstrom 2000).

It has been shown that desiccation, which affects this upper-intertidal species, can weaken Fucus thalli and thereby increase mortality from water motion via stipe breakage (Haring et al. 2002). However, Fucus is able to recover rapidly from desiccation when submerged; the same study showed that it is capable of recouping enough water within in 30 seconds to be able to withstand a dynamic load which broke experimentally desiccated stipes. In addition to desiccation, this alga is highly sensitive to oil contamination as shown by the documented dramatic population collapse following the Cosco Busan oil spill in 2007 (Cosco Busan Oil Spill Trustees 2012). However, it appears to be even more sensitive to heat, as was demonstrated by the increased Fucus mortality in hot water cleaned areas versus un-treated rocks following the Exxon Valdez oil spill in 1989 (De Vogelaere and Foster 1994). Despite high initial mortality rates following the Exxon Valdez spill, Fucus cover increased to match levels in reference areas by 1992; however, the uniform age structure of the cohort that recruited post-spill created an unstable population that precluded full recovery for more than seven years after the spill (Driskell et al. 2001).

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