with encrusting red algae (Choat and 

 Schiel 1982). Removal of the dominant 

 canopies of one species often allows the 

 invasion of another species. The result 

 depends partially on the season of the 

 year in which canopies are removed, as the 

 peaks of fertility for individual species 

 are different (Schiel 1981). 



Nova Scotia is another area which has 

 had large research programs in kelp bed 

 ecology. The perennial alga Laminaria 

 longicruris , may form extensive stands 

 from shallow depths to below 20 m (Mann 

 1972a). The extent of many of these 

 stands has been altered by sea urchins 

 ( Strongy 1 ocentrotus droebachiensis ) in 

 recent years (Breen and Mann 1976). The 

 biology and growth of I. longicruris have 

 been extensively studied. The growth 

 rates of this species can be limited 

 during part of the year by low nutrient 

 availability (Chapman and Craigie 1977). 

 Gerard and Mann (1979) found that the 

 morphology of plants was influenced by the 

 intensity of water motion, and was 

 different at exposed and sheltered sites. 

 Growth in the exposed population was lower 

 than in the sheltered one during 8 months 

 of the year due to low nutrient and light 

 levels. Gagne et al. (1982) reported that 

 the potentially limiting factors of light 

 and nutrients had different levels at 

 various sites. Growth rates of plants may 

 therefore be different at these sites. 

 Where nutrients are plentiful, maximum 

 growth can occur during summer when light 

 levels are high. In nutrient-limited 

 areas, plants tend to concentrate their 

 growth during winter when nutrient levels 

 are higher. 



Chapman (1984) examined the 

 reproduction, recruitment and mortality of 

 Laminaria longicruris and Laminaria 

 digitata in a series of innovative 

 experiments. By measuring sorus area and 

 microscopically examining sorus tissue, he 

 estimated the number of spores produced by 

 plants of both species. The recruitment 

 rate of each species was estimated by 

 placing ceramic bricks beneath each canopy 

 during each month of one year. At the end 

 of each month, bricks were brought into 

 the laboratory and placed under lights in 

 running seawater until plants were 

 visible. Natural recruitment rates were 

 monitored in the field. Chapman (1984) 



found that: (1) J., longicruris produced 

 about 9 x 10 9 spores/m 2 /yr and I. digitata 

 20 x 10 9 spores/m 2 /yr; (2) the recruit- 

 ment of microscopic plants was nearly 9 x 

 10 6 recruits/m 2 /yr for I. longicruris and 

 1 x 10 6 /m 2 /yr for J_. digitata ; (Tj~ the 

 chances of survival from microscopic to 

 visible size was 1 in 9 million for J_. 

 longicruris and 1 in 0.5 million for L_. 

 digitata ; and (4) once plants were 

 visible in the field, 1 in 4 survived to a 

 year for J., longicruris and 1 in 2 for J_. 

 digitata . The greatest mortality, 

 therefore, occurred between the time when 

 microscopic spores reached the substratum 

 and when sporophytes became visible. 

 Adult plants lived up to 25 months for i. 

 longicruris and 42 months for _L. digitata . 



In the British Isles and northern 

 Europe, several species of Laminaria are 

 abundant subtidally. Laminaria hyperborea 

 is perhaps the most prominent and 

 important of these species (Kain 1979). 

 Kain (1975, 1976) found that L. digitata , 

 which has a flexible stipe, was more 

 tolerant to wave action than was I. 

 hyperborea . In calmer subtidal sites, 

 however, U. hyperborea eventually appeared 

 to become dominant where both species had 

 recruited. In deeper water, this species 

 may compete with J^. saccharina . Much 

 research has been done on J_. hyperborea . 

 It extends from the low intertidal to 20+ 

 m in depth. Kain (1979) concluded that 

 its presence can be limited by available 

 substratum, grazing, and irradiance. The 

 ages of plants varied between populations, 

 but based on the presence of annual growth 

 rings, U hyperborea plants could live up 

 to 13 years (Kain 1963). 



For European Laminaria spp., the 

 maximum growth of blades occurs in late 

 spring. Adult sporophytes of I. 

 hyperborea , (those over one year old 

 LLuning 1969]) , usually stop growth during 

 June (Kain 1976). Luning (1979) stated 

 that adult sporophytes of i. digitata and 

 J^. saccharina continue to grow during the 

 second half of the year, but at a reduced 

 rate. He concluded that photoperiodism 

 may be important to the regulation of 

 seasonal growth for these species. 



Laminaria pal lida and Ecklonia maxima 

 both occur in shallow water in South 

 Africa, with L.. pal 1 ida dominant below ^ 8 



51 



