the vicinity of San Onofre. If water 

 motion is great enough, the drag on these 

 and other kelps is sufficient to dislodge 

 them and their substrata from the bottom. 

 These plants plus cobble "anchors" may 

 re-establish elsewhere if conditions are 

 suitable, or be transported to the beach 

 or to deeper water (SCE 1982). 



In areas of extensive rocky reefs, 

 the hardness of the substratum is also 

 important; organisms growing on soft rock 

 such as mudstone may be dislodged because 

 drag on the plants, induced by water 

 movement, fractures the rock. Differences 

 in community composition in central 

 California may partially result from 

 differences in rock type (Foster 1982a). 



The major exceptions to this occur- 

 rence on rocky substrata are kelp forests 

 in the vicinity of Santa Barbara, where 

 Macrocystis commonly grows attached to 

 sediment ("Thompson 1959, Neushul 1971a, 

 North 1971b). Neushul (1971a) suggested 

 that young plants first establish on solid 

 surfaces such as worm tubes. As the 

 plants grow, anchorage is increased by 

 sediment partially covering, and accumu- 

 lating in, the holdfast. These holdfasts 

 can be over a meter in diameter, and 

 Barilotti (pers. comm. ) found that, once 

 holdfasts are established, they become the 

 primary site for subsequent new recruit- 

 ment. The large size of the holdfasts 

 probably results from the accumulation of 

 haptera from successive generations of 

 plants. 



Sediment affects giant kelp forests 

 in two other ways: large amounts of 

 shifting sediment can scour or bury 

 established populations in rocky areas 

 (Weaver 1977), and relatively small 

 amounts of sediment on, or falling on, the 

 bottom can reduce the survivorship of 

 microscopic life history stages. The 

 former has been observed by North (1971b), 

 who suggested that Macrocystis fronds are 

 particularly susceptible to damage if 

 buried, and by Foster et al. (1983), who 

 indicated that changes in sediment cover 

 may be responsible for some of the his- 

 torical changes in the areal extent of 

 kelp forests in the vicinity of San 

 Onofre. Johnson (1980) recounts obser- 

 vations, made in the late 1800 ' s on San 

 Miguel Island (near Anacapa Island, see 



Chapter 3), indicating that kelp forests 

 were destroyed by sand eroded from the 

 land. Grigg (1975) listed burial as an 

 important cause of mortality in Muricea 

 cal ifornica , a gorgonian coral commonly 

 found on reefs and in kelp forests south 

 of Point Conception. Burial can also kill 

 young hydrocorals ( Allopora cal ifornica ) 

 (Ostarello 1973) and other sessile animals 

 (Weaver 1977), and may kill slow-moving 

 invertebrates like the California cowry 

 (SCE 1979). Sediments can clog the 

 filter-feeding apparatus of many inverte- 

 brates, and may be partly responsible for 

 the generally higher abundances of filter 

 feeders such as Al lopora on vertical 

 surfaces where sedimentation is reduced 

 (Ostarello 1973). 



Scour can also be caused by the 

 blades of understory algae rubbing over 

 the bottom. Velimirov and Griffiths 

 (1979) described bare areas between 

 patches of Laminaria pal 1 i da produced by 

 blades sweeping the bottom. The effects 

 of this type of scour have not been 

 examined within giant kelp forests. 



A mosaic of sediment and rock patches 

 is common in many kelp forests, and this 

 pattern may change, particularly during 

 storms. Small patches of shifting sedi- 

 ment are one disturbance that kills 

 established organisms, creating new space 

 for re-colonization. This disturbance may 

 thus have important effects on composition 

 and diversity within certain kelp forests 

 (Rosenthal et al. 1974, Foster 1975a, 

 Grigg 1975). 



Laboratory experiments by Devinny and 

 Volse (1978) showed that even very small 

 amounts of sediment can greatly inhibit 

 the attachment and growth of Macrocystis 

 spores. This could have a significant 

 effect on adult distribution in the field. 

 Sedimentation rates, as measured with 

 sediment tubes, are also negatively 

 correlated with sporophyte recruitment 

 (Dean et al. 1983). It is highly probable 

 that the small stages of other algae and 

 invertebrates (such as gorgonian corals; 

 Grigg 1975) are also negatively affected 

 in this way. 



Macrocystis can grow while drifting, 

 and early descriptions of the plant sug- 

 gested it might exist in large unattached 



10 



