CHAPTER 2 



THE ABIOTIC ENVIRONMENT 



Environmental factors involved in seaweed ecology — light, 

 nutrients, water motion, and temperature — have similar stratified 

 distributions. As a result, it is difficult to determine which are 

 the key factors. But it is easy to find a relationship of any one 

 with algal distribution. Jackson (1977). 



2.1. INTRODUCTION 



The existence of a giant kelp forest 

 depends upon physical and chemical condi- 

 tions that favor the reproduction and 

 growth of Macrocystis . With few excep- 

 tions, this abiotic environment includes a 

 hard substratum, water temperatures gener- 

 ally < 20 °C, bottom light intensities 

 equivalent to \% of surface irradiance or 

 greater, adequate nutrients, oceanic 

 salinities, and protection from extreme 

 water motion. 



Much of the Pacific coast of 

 California and Baja California, Mexico 

 (Figure 2) fulfills these criteria. 

 Nearshore waters to the north may be too 

 exposed to water motion, while those to 

 the south are probably too warm or low in 

 nutrients (see Sections 2.3 and 2.5). The 

 outer coast within the range of Macrocys - 

 tis is moderately exposed to oceanic 

 swells, salinity is relatively constant at 

 around 33 ppt, and surface temperatures 

 vary from a seasonal low of 8 °C around 

 Monterey to a seasonal high of around 24 

 C C in Baja California (Section 2.3). In 

 the southern part of this range, kelp 

 forests are particularly well developed in 

 cool, nutrient-rich upwelling areas. 

 Plants commonly occur at depths between 5 

 and 20 m. The community may develop on 

 almost any rocky bottom within these 

 limits; the absence of Macrocystis 

 commonly indicates an unstable bottom. 



Macrocystis is usually not found in 

 estuaries or very far inside protected 

 bays. The reasons for its absence in such 

 habitats have not been investigated, but 

 are probably related to factors such as 

 lack of rocky substrata, decreased light, 

 increased sedimentation, and reduced 

 salinity. North (1969) reported severe 

 damage to adult giant kelp transplants in 

 Newport Bay when salinity was lowered to 

 10 ppt during a storm. North (pers. 

 comm.) has also found that cultured 

 gametophytes do not survive at salinities 

 below 25 ppt. 



In the rest of this chapter we 

 discuss the abiotic environment in detail 

 for Macrocystis . The abiotic conditions 

 necessary for other organisms are also 

 described but, in most cases, very little 

 information is available. 



2.2. SUBSTRATUM AND SEDIMENTATION 



With few exceptions, the sessile 

 organisms associated with giant kelp for- 

 ests require a hard substratum for 

 attachment. If these plants and animals 

 do manage to attach and grow on sediment, 

 they are usually swept away in all but 

 very calm water. Extensive areas of 

 cobble and boulder occur between Oceanside 

 and Del Mar in southern California, and 

 adult Macrocystis in this region commonly 

 grow on these substrata, particularly in 



