ROSENTHAL, CLARKE, and DAYTON: ECOLOGY OF A STAND OF GIANT KELP 



50m T 



100 



ISOm 



0^ 50 



TRANSECT LINES AND FIXED 



QUADRATS WITHIN STUDY AREA 



Figure 1. — Location of the subtidal study area off Del Mar, 

 Calif. The drawing shows the layout of the transect lines and 

 the position of each fixed quadrat. 



depths between 14 and 20 m in the most seaward 

 Macrocystis stand directly off 8th Street in Del 

 Mar. These continuing studies were begun in 

 June 1967. 



Within the perimeter of the study area the sub- 

 stratum is composed primarily of sedimentary 

 mudstones and siltstones, coarse sand, and silt. 

 The sea floor is somewhat flat in appearance, 

 although low (<1.5 m) consolidated mounds and 

 shallow ledges break up the overall visual 

 uniformity of the bottom. 



The monthly mean surface water temperatures 

 recorded off the pier at Scripps Institution of 

 Oceanography, approximately 24 km south of the 

 study site, varied from a minimum of 10.1°C to a 

 maximum of 24.6°C between June 1967 and 

 December 1971 (Scripps Institution of Oceanog- 

 raphy, 1968-1971). The annual mean temperature 

 during this same period was 16.3°C. Since the 

 water column in this area is thermally stratified, 

 it is essential to consider bottom temperatures 



as well. To this end, 70 bottom temperatures were 

 recorded at a depth of 17 m in which the mean 

 temperature was 13.0°C with the minimum re- 

 corded temperature being 10.5°C, and the maxi- 

 mum being 16.0°C. 



A great deal of water movement is typical to 

 this environment (Inman and Brush, 1973) and 

 water transparency or underwater visibility is 

 greatly affected by the resulting sediment dis- 

 turbance. Over the 5.7-yr study, the underwater 

 visibility ranged from to 20 m, the average 

 visibility being about 3 to 4 m. However, on many 

 dives the underwater visibility was so reduced 

 that it was impossible to see along the bottom in 

 the vicinity of the kelp bed. 



METHODS 



The study was initiated in June 1967 after 

 several months of exploratory observations. All of 

 the in situ observations were made while scuba 

 diving and more than 300 h were spent under- 

 water in this location. Most observations were 

 made during daylight hours, however about 5 h 

 were spent in this location between 2000 and 2200 

 h. The observations reported in this paper span 

 more than 5 yr, from June 1967 through February 

 1973. 



Initially a 100-m transect consisting of 20 brass 

 stakes (75 cm in height) was placed along the sea 

 floor in June 1967. One stake was placed every 

 5 m, and the entire array was perpendicular to the 

 shoreline. During July 1967 the position of each 

 attached Macrocj's^/s plant growing within 1 m on 

 either side of the 100-m transect was recorded. 

 The condition (i.e., number of living stipes, ap- 

 pearance of the holdfast, etc.) of each plant was 

 noted at various times from the beginning of the 

 study until the plant disappeared. Also, juvenile 

 Macrocystis were recorded as they appeared along 

 the 2 m X 100 m strip. 



In the fall of 1967, the main transect was 

 extended 50 m shoreward, and during March 1968 

 three additional lines running parallel to the 

 coastline were added (Figure 1). Sectioned 25-m 

 polypropylene lines (0.60 cm in diameter) were 

 staked into the bottom alongside of the brass 

 stakes. The lines were sectioned so that fouling or 

 detachment of any part would not necessitate 

 replacement of the entire transect. All lines were 

 marked at meter intervals with tape, and the 

 brass stakes were numbered with line and plastic 



671 



