shell debris during periods of surge (Lees 

 and Carter 1972). Activity of the sea 

 urchin Centrostephanus coronatus is also 

 reduced in turbulent conditions, and 

 Lissner (1980) suggested that this urchin 

 may also be excluded from areas by high 

 water motion. Because sea urchin grazing 

 can significantly alter the distribution 

 of other kelp forest organisms (see 

 Chapter 3), the effects of water motion on 

 sea urchin distribution and activity can 

 have significant indirect effects on 

 community structure. 



In addition to affecting distribution 

 and behavior, water motion may alter the 

 orientation of sessile organisms. The 

 plane of the primary dichotomy of 

 Macrocystis (Neushul et al. 1967), the 

 branches of gorgonians, and the understory 

 kelp Eisenia arborea (Foster pers. obs.) 

 are commonly oriented perpendicular to the 

 most common swell direction, presumably an 

 adaptation to reduce mortality due to 

 water motion and/or to increase capture of 

 nutrients or planktonic food. Other 

 characteristics of seaweed morphology 

 related to water motion are discussed by 

 Neushul (1972). 



Water motion has numerous indirect 

 effects on kelp forests, and some of 

 these, such as nutrient uptake and changes 

 in turbidity, scour, and sedimentation, 

 are discussed with other abiotic factors. 

 Of particular importance are effects on 

 light caused by removal of overstory 

 canopies during storms. In exposed kelp 

 forests in central California, increased 

 light resulting from the removal of Macro - 

 cystis canopies by storms is correlated 

 with a three-fold or more increase in the 

 cover of understory plants (Foster 1982a). 

 Moreover, if understory kelps are sparse, 

 giant kelp removal can affect kelp 

 recruitment as well (Rosenthal et al. 

 1974, Pearse and Hines 1979, Reed and 

 Foster 1984). Many of the direct and 

 indirect effects of water motion are 

 illustrated in the community regulation 

 models shown in Figure 5. These models 

 illustrate that the relative importance of 

 particular factors can vary among kelp 



forests, that factors are often 

 correlated, and how they can interact to 

 affect the species composition and 

 distribution of organisms. 



> 



o 

 o 



cr 

 in 

 (- 

 < 



_i 

 m 



i 

 o 

 z 

 < 



CO 



substratum 



L-hard 

 moderate 



RELIEF 



WATER MOTION 



CLEAR WATER -"Macrocystis >30m DEEP 

 (HIGH LIGHT) 



'SEASONAL VARIATION IN Macrocystis 

 ABUNDANCE 



— ' Pterygophora MORTALITY LOWj , 



PERSISTENT CANOPY— <■ LOW LIGHT , 



_i» PERENNIAL ENCRUSTING AND 



PERENNIAL ENCRUST 

 ARTICULATED CORALLINE ALGAL 

 UNDERSTORY 



SEA 



URCHINS 



-» ABUNDANCE LOW DUE TO SEA OTTER 

 PREDATION-^-NO EFFECTS 



SUBSTRATUM 



l-SOFT ROCK 5 

 SAND 

 — LOW 

 RELIEF 



WATER MOTION 



TURBID WATER — Macrocystis. 12m DEEP 

 (LOW LIGHT) 



LARGE SEASONAL AND YEAR TO YEAR 

 VARIATION IN Macrocystis ABUNDANCE 



Pterygophora MORTALITY HIGH 

 (VARIABLE LIGHT) 



• 'ANNUAL' FLESHY RED ALGAL AND 

 Desmarestia UNDERSTORY WITH LARGE 

 SEASONAL AND YEAR TO YEAR 

 VARIATIONS IN ABUNDANCE 



MODERATE 



SEA 

 URCHINS 



-*■ MODERATE DENSITIES —"-REMOVAL OF 



SOME OVERSTORY PLANTS — » INCREASEO 

 UNDERSTORY ALGAL ABUNDANCE 



= Primary factors 



= Factor interactions 



= Regulatory pathways (how factors affect 

 organisms) 



Figure 5. Factors, factor interactions, 

 and regulatory pathways affecting the 

 algal associations in two giant kelp 

 forests in central California (see Chapter 

 3; from Foster 1982a). 



19 



