often torn loose in patches 

 when one loose individual e 

 others still attached. This 

 drag on the holdfasts of 

 plants, contributing to 

 detachment (Rosenthal et al 

 the substratum is very hard 

 firmly attached, surge may 

 long fronds, leaving the 

 small fronds that may grow 

 (Foster 1982a). 



during storms 

 ntangles with 

 increases the 

 the attached 

 additional 

 . 1974). If 

 and holdfasts 



remove only 

 holdfast and 



vegetati vely 



Differences in swell exposure 

 probably account for many of the 

 differences in canopy and plant density 

 fluctuations between southern and central 

 California. The east-west trend in the 

 coastline, protection provided by offshore 

 islands, and the distance from the 

 northerly source of most winter storms all 

 combine to make many southern California 

 kelp forests relatively protected from 

 large swells. Surface canopies in this 

 region typically vary in extent in a 

 three- to four-year cycle (North 1971b, 

 Rosenthal et al. 1974), probably related 

 to an increased susceptibility of older 

 and larger plants with deteriorating 

 holdfasts to removal by water motion. 

 Canopies around Santa Barbara are even 

 less variable, with occasional 

 catastrophic losses due to atypically 

 large swells (Ebeling et al. MS.) or warm 

 water (low nutrient) periods (North 

 1971b). There are exceptions, however; 

 large swells in winter 1982-83 removed 

 nearly 70% of the adult Macrocystis at 

 some sites in the Point Loma kelp forest 

 near San Diego (Dayton and Tegner 1984b), 

 and over 90% of the Macrocystis surface 

 canopy along the Palos Verdes Peninsula, 

 Los Angeles (Wilson and Togstad 1983). 



In contrast, most canopies in central 

 California undergo a regular seasonal 

 change with growth in spring and summer 

 leading to maximum development in early 

 fall, and then frond and plant loss during 

 late fall and winter storms (Miller and 

 Geibel 1973, Cowen et al. 1982, Foster 

 1982a, Kimura and Foster in press). In 

 addition to these seasonal changes, there 

 are year-to-year differences correlated 

 with the severity of winter swells (Foster 

 1982a). This was especially evident in 

 winter 1982-83, when swells over 7 m high 

 with a 21-sec period were recorded in 

 central California (Seymour 1983). Large 



swells along the entire California coast 

 during this period removed almost all 

 Macrocystis surface canopies (McPeak pers. 

 comm. ). 



If swells are too extreme, 

 Macrocystis may not be able to persist in 

 a given area. In the absence of biotic 

 factors such as competition (Santelices 

 and Ojeda 1984b), swells may determine the 

 shoreward depth limit of kelp forests 

 (North 1971b) because, for a given set of 

 swell characteristics, surge speed 

 increases as depth decreases. On a 

 geographic scale, increasing surge may be 

 the primary reason why M. pyrifera does 

 not occur in large stands north of Ano 

 Nuevo Island (near Santa Cruz) in central 

 California. Nereocystis luetkeana is 

 extremely resistant to breakage from water 

 drag (Koehl and Wainwright 1977), and is 

 the common canopy-forming kelp from Ano 

 Nuevo Island north into Alaska. This 

 change in surface canopy species may also 

 be related to differences in life history 

 characteristics, growth rates, and 

 susceptibility to grazing (see Chapters 3 

 and 4). 



Understory kelps seem generally more 

 resistant to removal by surge (Reed and 

 Foster 1984, Dayton et al. 1984), but 

 these kelps, along with foliose algae that 

 cover the bottom, may also be directly 

 removed by surge, particularly if the 

 substratum is soft rock (Foster 1982a). 



Swells can also alter fish 

 distribution (Quast 1971c), and can remove 

 attached or mobile benthic invertebrates, 

 especially those that project into the 

 water above the bottom. Hines (1982) 

 suggested that winter swells may be an 

 important source of mortality in kelp 

 forest spider crab populations. Cowen et 

 al. (1982) found that sea urchin 

 ( Stongylocentr otus franciscanus ) behavior 

 was modified during winter when storms 

 apparently caused animals to clump in 

 cracks and depressions, and Agegian et al . 

 (in prep.) suggest that mortality caused 

 by surge at some sites in central 

 California may restrict S_. franciscanus 

 distribution to deeper water or to areas 

 protected from high water motion in 

 shallow water. The white urchin, 

 Lytechinus anamesus , moves less, covers 

 itself with debris, and even burrows into 



18 



