intertidal mud flats, and subtidal areas 

 were converted into about 79 acres (32 

 ha) of subtidal habitat by the develop- 

 ment (Trent et al. 1976). 



Phytoplankton production, oyster 

 production, benthic macroin vertebrates, 

 fish, and crustacean abundance were 

 studied in an open bay area, the bulk- 

 headed canal area, and in adjacent nat- 

 ural marsh area. Primary production of 

 phytoplankton was higher in canal than 

 marsh areas, and production in both 

 areas was much higher than in the bay 

 (Corliss and Trent 1971). Oyster set- 

 ting was 14 times greater in the natural 

 marsh than in a canal area. The faster 

 growth and lower annual mortality rates 

 in the natural marsh were also reported 

 by Moore and Trent (1971). Benthic 

 macroin vertebrates were numerically 

 slightly more abundant and volumetri- 

 cally over twice as abundant in the 

 marsh than in the canals. The lowest 

 abundance was in the bay. However, 

 when individual phyla were considered, 

 numeric and volumetric abundance var- 

 ied by area (Gilmore and Trent 1974). 

 More finfishes and crustaceans were 

 caught in the marsh than in the canals 

 and catches were much higher in both 

 areas than in the bay. Brown shrimp 

 ( Penaeus aztecus ), white shrimp (P. 

 setiferus), and spot ( Leiostomus XJLD. - 

 thurus )~ were most abundant in marsh; 

 and largescale menaden ( Brevoortia pat- 

 ron us ) .Atlantic croaker ( Micropo gonias 

 undulatus ) and bay anchovy ( Anchoa 

 mitchilli )~ were most abundant in canals 

 (Trent et al. 1972). These six species 

 comprised 89% of the' total catch. Mock 

 (1966) compared penaeid shrimp produc- 

 tion in a bulkheaded and natural area 

 in another area of the Galveston Bay 

 system. He found greater shrimp pro- 

 duction in the natural habitat. 



Numerous physical differences be- 

 tween the altered and unaltered marsh 

 areas were noted. Substrates in the 

 canal areas had a higher silt and clay 

 content than the marsh, and the amount 

 of organic detrital materials in marsh 

 substrate was twice that found in the 

 canals (Trent et al. 1972). Average 

 temperature, salinity, total alkalinity, 

 and pH were similar between the marsh 

 and canal areas. The averaoe dissolved 



organic nitrogen was highest in the 

 marsh and may have been due to cattle 

 grazing near the marsh. Average total 

 phosphorous was highest in the canals 

 of the housing development, but was 

 variable across time. Average levels of 

 dissolved oxygen and surface turbidity 

 were lowest in the canals, and dissolved 

 oxygen levels dropped to extremely low 

 levels at sampling stations farthest from 

 the bay during the summer months. 



In general, productivity was high- 

 er in the marsh than in canal areas and 

 lowest in the open bay. Plankton blooms 

 followed by low levels of dissolved oxy- 

 gen, high nutrient levels, fish kills, 

 and depressed oyster, benthic macroin- 

 vertebrate and shrimp production in the 

 summer months indicated the presence 

 of eutrophic conditions in canal areas of 

 the housing development. Moore and 

 Trent (1971) noted that eutrophic con- 

 ditions probably develop more rapidly in 

 housing development canals than in nat- 

 ural marsh areas because of high nutri- 

 ent levels, increased phytoplankton pro- 

 duction, and a reduction in water circu- 

 lation and exchange. 



Reduced productivity in bulkhead- 

 ed canals may not be directly attribut- 

 able to bulkheads, but rather to the in- 

 creased human usage of the area and 

 the removal of marsh habitat. Human 

 use of bulkheaded and filled areas is 

 generally increased in terms of housing 

 and boating. 



From a biological standpoint, bulk- 

 heading in this coastal region alters 

 existing communities and may eliminate 

 some species entirely. The energy base 

 of the community changes considerably 

 with the elimination of marsh grasses. 

 There are no satisfactory alternative 

 structures for the creation of new real 

 estate. However, existing land may be 

 protected from erosion by the use of 

 revetments or by planting vegetation. 

 When placing bulkheads or seawalls, it 

 is desirable to locate them as far upland 

 as possible, preferably above mean high 

 water. 



CASE HISTORY - 

 COASTAL REGION 

 MEXICO 



CAUSEWAYS IN 

 3 - GULF OF 



115 



