CHAPTER 4 

 COMPARATIVE ROLES OF VASCULAR PLANTS AND ALGAL MATS 



Vascular plants are large and 

 conspicuous, while algal mats are 

 generally ignored. Yet investigations of 

 algal productivity and food value for 

 marsh consumers suggest a functional role 

 which is disproportionate to their size. 

 Although information about the relative 

 importance of these two primary producer 

 groups is limited, the data available 

 suggest that we should look much closer at 

 the algal mats and identify more clearly 

 how modifications of salt marshes affect 

 them. Interactions between vascular 

 plants and algal mats occur, and 

 management practices which favor one group 

 may hinder another. 



4.1 RELATIVE PRIMARY PRODUCTIVITY 



Both vascular plants and algal mats 

 contribute to the food production of a 

 salt marsh. Where productivity rates of 

 each producer group have been measured, it 

 is possible to assess their relative 

 importance to the ecosystem's food base. 

 However, such comparisons are not very 

 precise, because measurements of 

 productivity, and assumptions made in 

 calculating annual rates, differ for the 

 two groups of marsh plants. Only general 

 conclusions can be drawn from such data 

 bases. 



On the Atlantic coast of the United 

 States three studies have compared algal 

 and above-ground vascular plant 

 productivity. The proportion of annual 

 productivity contributed by algal mats 

 (mostly diatoms) was about 20% in Georgia 

 (Teal 1952), about 25? in Delaware 

 (Gallagher and Daiber 1974) and about 25? 

 in Massachusetts (Van Raalte et al. 1976). 

 From these results, it appears that algae 

 provide less food for marsh consumers than 

 vascular plants. Perhaps this is to be 

 expected for marshes where the vascular 

 plant canopy is tall and dense. 



The study of the algal and vascular plant 

 productivity at Tijuana Estuary suggests a 

 contrasting situation for hypersaline 

 marshes. Even the most conservative 

 estimates indicate that algae are about as 

 productive as the overstory plants (Zedler 

 1980). Proportions of total net carbon 

 fixed by algae fell within 40% to 60%. 

 The highest value was obtained in the low 

 elevation Jaumea-dominated habitat, where 

 both moisture and light were abundant. 

 Evidence suggests that these algal mats 

 are limited by light; productivity 

 declined in winter and productivity was 

 nearly always lower under dense compared 

 to open canopies. 



Generalizing the observations at 

 Tijuana Estuary to the hypersaline marshes 

 of southern California, a conceptual model 

 of comparative productivity emerges 

 (Figure 35). Soil salinity is seen as the 

 major controlling factor. Where soils are 

 hypersaline, vascular plant growth is 

 reduced, canopies are open, and 

 considerable light penetrates to the soil 

 surface. Algal mats develop and 

 contribute a major proportion of the marsh 

 primary productivity. Brackish soils 

 support denser vascular plant canopies, 

 which in turn intercept more light, and 

 soil algae are light limited. 



While this model has not been tested 

 by field experiment, our observations of 

 other marshes under brackish conditions 

 (at Los Penasquitos Lagoon and the San 

 Diego River) suggest the same patterns. 

 When canopies are densest, algal growth is 

 sparse. With mortality in the overstory, 

 algal mats are quick to develop. 



The algal productivity takes on 

 greater importance when the quality of the 

 food produced is examined and the impact 

 on food chains is explored. 



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