and Lindley 1980) indicated that seasonal 

 and site differences in kelp ( Laminaria 

 spp.) growth in these regions are related 

 to the availability of inorganic nitrogen. 

 The only other giant kelp forest plant so 

 far investigated that may be nutrient- 

 limited in the field is Gel idium robustum 

 (formerly G. cartilagineum ) , an agar- 

 producing red alga (see Chapter 4). Tseng 

 and Sweeney (1946) determined that this 

 plant u = ses dissolved C0 2 rather than HC0 3 " 



0.4m/s 



or CO, 



The plant is most abundant at 



wave-exposed sites in shallow water (< 12 

 m) where dissolved C0 2 is more common. 

 This suggests that carbon availability may 

 limit the distribution both within (with 

 depth) and between (exposed vs. sheltered) 

 sites. Barilotti (1980) indicated that 

 light and grazing may also affect G. 

 robustum distribution. 



2.6. WATER MOTION 



Currents and surge produced by wind, 

 tides, or waves have numerous direct and 

 indirect effects on kelp forest communi- 

 ties. Currents are unidirectional flows 

 (but the direction can change within 

 hours), while surge moves back and forth 

 over the bottom, as well as up and down 

 above the bottom (Figure 4). Current 

 speeds in kelp forests are highly 

 variable, but in the range of near to 15 

 cm/sec in the few kelp forests studied 

 (Wheeler 1980b, Bray 1981, Jackson 1983). 

 Neushul et al . (1967) measured speeds of 

 40 cm/sec near Anacapa Island. The drag 

 at higher speeds can pull Macrocystis over 

 at angles up to 30 degrees from vertical, 

 and the entire surface canopy may submerge 

 as a result (Neushul et al. 1967). This 

 canopy submergence, along with that caused 

 by changes in tide, can alter canopy 

 extent as estimated with aerial 

 photographs. As a result of plant drag, 

 current speeds within kelp forests are 

 often two to three times lower than the 

 surrounding water (Jackson and Winant 

 1983), and if the forest is small, the 

 incoming current will diverge around it, 

 producing a "bow wake" similar to that of 

 a ship (Jackson 1983). The reduction of 

 surface waves by kelp plants is commonly 

 observed as "quiet water" inshore from 

 kelp forests (Darwin 1860), and 

 artificial, kelp-like tethered floats have 

 been used as breakwaters to reduce water 

 movement in harbors (Isaacs 1976). 



Figure 4. Water motion produced by swells 

 in shallow water. Note that the orbital 

 motion of water particles flattens toward 

 the bottom, eventually becoming entirely 

 horizontal to produce surge. In the 

 example shown, a 5-m excursion of water 

 was measured on the bottom at a depth of 

 6 m, when a 2-m wave moved past at a 

 velocity (C) of 8 m/sec, a wave length (L) 

 of 120 m, and a wave period (T) of 15 sec. 

 Orbital velocities at the surface are also 

 shown (re-drawn from Neushul 1972). 



Even at high speeds, currents have 

 not been observed to cause mortality of 

 adult kelp forest organisms. They do, 

 however, indirectly affect nutrient uptake 

 by plants (see 2.5 above), larval and 

 spore dispersal within and between kelp 

 forests, and the distribution of particles 

 that may serve as food for benthic animals 

 (Pequegnat 1964) and fish (Bray 1981). 



Surge speeds can be much higher than 

 currents, particularly surge generated by 

 long period swells associated with winter 

 or tropical storms. In central 

 California, 4-m high swells are typical in 

 winter and produce water speeds of over 1 

 m/sec on the bottom at kelp forest depths 

 (10 m). The force generated by water 

 moving this fast is equivalent to the 

 force produced by a wind speed of 126 mph 

 (56 m/sec; Charters et al. 1969). As 

 noted above (Section 2.2), such forces can 

 pull benthic organisms off the bottom and 

 fracture the bottom in the process. 

 Storm-associated surge is perhaps the most 

 important source of mortality for adult 

 Macrocystis in California (ZoBell 1971, 

 Rosenthal et al. 1974, Gerard 1976, Foster 

 1982a, Reed and Foster 1984, Dayton et al . 

 1984) and at sites studied in Argentina 

 (Barrales and Lobban 1975). Plants are 



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