Table 2.11. Primary Productivity in the Galveston Bay System (Data Sources in Parentheses). 
Average 
Estimated 
Estimated 
Primary 
Areal 
Annual 
Productivity 
Coverage 
Production 
Flora 
(g dry/m 2 /yr) 
(km 2 ) 
(metric tons) 
Phytoplankton (44,45,47) 
350 
1,425 
498,750 
Benthic microflora (44,47) 
500 
1,425 
712,500 
Submerged vegetation (1,15,48) 
2,600 
1 
2,600 
Freshwater marsh (1,12) 
820 
40 
32,800 
Salt-Brackish Marsh (12,43) 
1,100 
370 
407,000 
Woodlands/swamps (12,47) 
700 
500 
350,000 
roughly estimated in Table 2.11. Phytoplankton, benthic microflora, salt and brackish marshes, and 
woodlands and swamps each contribute roughly the same order of magnitude of organic materials 
to annual production. Fresh marshes produce an order of magnitude less, while seagrasses contrib¬ 
ute two orders of magnitude less production than the four main components. Some of the assump¬ 
tions made in constructing Table 2.11 need testing, such as productivity of phytoplankton and 
benthic microflora within Galveston Bay and presumption that such productivity occurs under the 
total bay surface of 1,425 km 2 (550 mi 2 ). Within the various habitats, the variation in productivity can 
be dramatic. For example, in the fresh marsh Sagittaria graminea produces 215 g dry/m 2 /year while 
Phragmites australis produces 2,984 g dry/m 2 /year (1), and in the salt marsh Batis maritima 
produces 425 g dry/m 2 /year while Spartina spp. produce 1,100 g dry/m 2 /year (43). The most 
productive component, the seagrasses, are the least abundant in this estuary. 
Most of the plant production is separated in space and time from the consumer community. In 
fact, some of that production may never reach the consumers due to inundated regimes and tissue 
storage. It has been estimated that woodlands, swamps and freshwater marshes export only 8 to 10 
percent of the annual aboveground production whereas the frequently inundation low salt marshes 
may export 30 to 45 percent annually (1,47). The low nutritional quality, refractory nature of much 
of the biomass, and resistance to direct grazing all increase from phytoplankton and algae through 
submerged aquatic vegetation to emergent vascular plants of the salt marsh and woodlands. Thus, 
the primary consumption of most of the plant biomass is only available along the detritus pathway. 
Although many organisms play major roles in breaking down this refractory material, they rarely 
directly assimilate the organic plant matter and, instead, utilize the surface microbial decomposers 
(47). 
Primary Consumption 
Less than 10 percent of emergent vegetation of these wetlands is consumed directly, and most of 
the grazers are insects (47). Ondatra zibethicus (muskrat) and Myocastor coypus (nutria) are other 
direct consumers. Submerged vegetation may be directly consumed by a small number of aquatic 
organisms (snails, fishes such as Lagodon rhomboides [pinfish]) as well as certain species of ducks. 
Phytoplankton are directly grazed by many zooplankters and planktivorous fishes, while benthic 
algae and epiphytes are utilized by snails, fiddler crabs and other organisms (47). The vast majority 
of primary consumers in the system are detritivores, species that directly or indirectly consume 
detrital particles and, lacking the necessary digestive enzymes, in reality utilize only the surface 
bacteria and fungi. This group includes many benthic organisms (bivalves, gastropods, crustaceans) 
and bottom feeding fishes and macroinvertebrates (47). 
The available evidence suggests that the phytoplankton-based branch of the food web may not 
be as important to the Galveston Bay system as is the emergent marsh-detritus branch, even though 
annual primary production may be similar for both groups. First, average phytoplankton densities 
38 
