erosion, a situation that makes the re- 

 maining mangrove area increasingly vital 

 to the welfare of these birds. 



MUD FLATS (12) 



Habitats classified as flats in- 

 clude several types of environments in 

 the MDPR. However, 85% of the flats can 

 be identified as one of three types: (1) 

 unvegetated mudflats in estuarine areas, 

 (2) unvegetated organic matter/mud de- 

 posits in estuarine and brackish areas, 

 and (3) unvegetated mudflats in fresh- 

 water areas. The distribution of this 

 habitat is shown in Figure 29. 



Flats in the MDPR covered 5,159 

 ha (12,750 acres) in 1978 (Table 5). 

 Although the area of this habitat is 

 small, the habitat is valuable in sever- 

 al respects. Mudflat soils contain or- 

 ganisms which are involved in nutrient 

 recycling, and the community found in 

 this habitat supports higher level con- 

 sumers. There have apparently been no 

 studies of mudflats in the MDPR, and the 

 role of this habitat in the MDPR is 

 based on work done in other areas. 



The most conspicuous characteristic 

 of mudflats is the lack of macroscopic 

 vegetation, which gives the impression 

 that this habitat is relatively unpro- 

 ductive. Although no large vascular 

 plants are found on mudflats, numerous 

 forms of microalgae grow on and in the 

 sediments. Most of the algae are con- 

 centrated in the top 1 cm of sediment, 

 although living algae are found as deep 

 as 10 cm (A inches) (Peterson and 

 Peterson 1979) . Blue-green algae 

 ( Oscillatoria sp . , Microcoleus sp . , and 

 Spirulina sp.) and diatoms are the two 

 most common groups found on and in mud- 

 flat sediments. While the production of 

 other habitats may be higher, much of 

 the primary production on mudflats is 

 consumed by benthic invertebrates in the 

 mudflat sediments, and directly con- 

 verted into secondary production 

 (Peterson and Peterson 1979). 



Blue-green algae are nitrogen 

 fixers. Casselman (1979) measured 

 nitrogen-fixation rates in Louisiana 



mudflat soils in Barataria Bay and found 

 rates of 1.56 g N/m 2 /yr (13.9 lb/ac/yr) . 

 During March, the mudflat was the site 

 of more intense nitrogen fixing activity 

 than that of salt marsh soils in the 

 same area. Bacteria and fungi are also 

 abundant on mudflats. Members of these 

 two groups are pivotal in the process of 

 converting detritus, which is low in 

 nitrogen, to high quality protein-rich 

 organic matter that can be used by de- 

 posit feeders. Wolaver et al. (1980) 

 found that mudflats play an important 

 role in the nutrient exchange between 

 habitats in estuarine systems. As water 

 flows over the mudflat during tidal 

 flooding, nutrients are released from 

 the sediments to the overlying water to 

 be carried to other habitats and used by 

 organisms in the water column. The salt 

 marsh can apparently function as either 

 a source or a sink for nutrients, de- 

 pending upon the season (Wolaver et al. 

 1980). This is consistent with Cassel- 

 man's (1979) results that show peak 

 nitrogen fixing activity for different 

 areas (mudflat and marsh soil) during 

 different times of the year. 



Microalgae associated with mudflat 

 sediments are consumed by many species 

 of benthic invertebrates. Both suspen- 

 sion feeders and deposit feeders are 

 found in intertidal flats, and these 

 same animals can be divided into two 

 other groups: the infauna, burrowing 

 organisms which live in the sediment, 

 and the epifauna, organisms which live 

 on the surface of the mudflat. Taken 

 together, all of the benthic organisms 

 are an important food source for other 

 higher level consumers, particularly 

 birds . 



The oyster ( Crassostrea virginica ) 

 is the best-known and most common epi- 

 benthic suspension feeder found in the 

 mud flat habitat. Oysters are commer- 

 cially important in the MDPR, but they 

 are ecologically important also because 

 they build reefs in mudflat areas. 

 Oyster reefs help to stabilize sedi- 

 ments, retard coastal erosion, augment 

 sediment deposition, provide space for a 

 variety of epibenthic organisms and prey 

 for shorebirds and nekton, and they re- 

 cycle nutrients, especially nitrogen, in 



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