4, Section 4.3.2.1). Nutrients needed for 

 growth are also frequently high in the 

 nearshore waters where these plants occur 

 (see Chapter 2, Section 2.5). Finally, 

 the community is always submerged in 

 waters of near-constant salinity. Thus, 

 unlike other productive communities such 

 as estuarine sea grass beds or mangrove 

 forests, plants in the kelp forest have 

 relatively little non-photosynthetic 

 support tissue and do not use much of the 

 energy produced for osmoregulation to 

 adapt to periodic emergence-submergence. 



The final source of primary 

 production in giant kelp forests is phyto- 

 plankton. To our knowledge, phytoplankton 

 production within a forest has never been 

 estimated. Evidence from other studies, 

 however, suggests it is small relative to 

 the seaweeds. Piatt (1971) estimated 

 phytoplankton production at about 200 g 

 C/m 2 /yr in a bay in Nova Scotia, where the 

 Laminaria beds averaged about 1500 g 

 C/m 2 /yr (Mann 1973). Clendenning' s 

 (1971b) estimates for phytoplankton 

 production in southern California coastal 

 waters are similar. Shading no doubt 

 would reduce these values under a surface 

 canopy. Phytoplankton production, at 

 least around areas of kelp, is low 

 compared to the macroalgae, but is in the 

 range of values given by Ryther (1969) as 

 typical of phytoplankton in nutrient-rich 

 upwelling areas unshaded by a surface 

 seaweed canopy. 



3.6.4 Energy Flow - Food Webs 



The net primary production of 

 seaweeds in a kelp forest is available to 

 consumers in three forms: (1) living 

 tissue on attached plants, (2) drift in 

 the form of whole plants or detached 

 pieces, and (3) dissolved organic matter 

 exuded by attached and drifting plants. 

 Detritus is very small pieces of drift 

 algae, and particulate organic matter 

 (POM) is even smaller pieces. Both these 

 subcategories may be derived from attached 

 plants or from the breakdown of drift. 

 The fate of these forms, plus within- 

 forest phytoplankton production and 

 imported sources of energy (plankton and 

 drift seaweed from other areas), is 

 illustrated in the generalized kelp forest 

 food web of Figure 11. 



Gerard (1976) determined the general 

 disposition of Macrocystis productivity 

 (attached plants and drift) in the Point 

 Cabrillo kelp forest. She estimated that 

 of the yearly production (excluding 

 grazing, detritus, and dissolved organic 

 matter losses from attached plants), 70% 

 entered the consumer assemblage as drift. 

 Of this, an estimated 40% was utilized 

 within the forest, and 50% was transported 

 out. This latter figure is of interest as 

 this exported production may end up on 

 nearby shores where it is an important 

 source of energy for beach invertebrates 

 and, ultimately, for shore birds (Yaninek 

 1980, North 1971b; see Chapter 4, Section 

 4.6). Detached Macrocystis and other 

 macroalgae may also drift offshore along 

 the bottom where they presumably serve as 

 food (North 1971b) and habitat (Cailliet 

 and Lea 1977) for deep-water organisms 

 living where light is insufficient for in 

 situ photoautotrophic productivity. 

 Plants may drift offshore on the surface 

 as kelp rafts that provide habitat for 

 juvenile and some adult fishes (Mitchell 

 and Hunter 1970). 



Gerard (1976) also estimated that 

 only ^ 3%-6% of the total Macrocystis 

 production was consumed directly by 

 animals grazing on attached plants. 

 Direct detrital loss was not measured, 

 although it may be an important form of 

 primary production entering the community, 

 particularly from senescent fronds. Esti- 

 mates of dissolved organic matter (DOM) 

 produced by large seaweeds are highly 

 variable, and may partly be an artifact of 

 handling and laboratory technique 

 (Fankboner and de Burgh 1977). 



Little is known about the precise 

 fate of drift algae in giant kelp forests, 

 other than it can be eaten without 

 decomposition by large herbivores like sea 

 urchins and abalone (see Section 4.4.3). 

 The experiments by Bedford and Moore 

 (1984) in Laminaria saccharina beds in 

 Scotland show that much of the drift is 

 not decomposed by microbes but, instead, 

 eaten by small detritivores (echinoderms, 

 polychaetes, and amphipods). Decomposi- 

 tion by microbes increased when these 

 detritivores (that would otherwise crop 

 rotting tissue or repeatedly remove 

 healthy tissue preventing microbial 

 colonization) were excluded. In contrast 



39 



