la Cruz in press, b; Ray in press). 



12.4 MAN-INDUCED MODIFICATIONS 



Damage from oil spills follows a 

 predictable pattern (Table 7) which may 

 require years to complete. It is impor- 

 tant to recognize that many of the most 

 severe responses, including tree death, 

 may not appear for months or even years 

 after the spill. 



In Florida, Chan (1977) reported that 

 red mangrove seedlings and black mangrove 

 pneumatophores were particularly sensitive 

 to an oil spill which occurred in the 

 Florida Keys. Lewis (1979a, 1980b) has 

 followed the long-term effects of a spill 

 of 150,000 liters (39,000 gal) of bunker C 

 and diesel oil in Tampa Bay. He observed 

 short-term (72-hour) mortality of inverte- 

 brates such as the gastropod Mel ongena 

 corona and the polychaete Laeonerei s 

 cul veri . Mortality of all three species 

 of mangroves began after three weeks and 

 continued for more than a year. Sub- 

 lethal damage included partial defoliation 

 of all species and necrosis of black 

 mangrove pneumatophores; death depended 

 upon the percentage of pneumatophores 

 affected. 



In addition to the damage from oil 

 spills, there are many adverse impacts on 

 mangrove forests from the process of oil 

 exploration and drilling (Table 8). This 

 type of damage can often be reduced 

 through careful management and monitoring 

 of drilling sites. 



Although little is known concerning 

 ways to prevent damage to mangroves once a 

 spill has occurred, protection of aerial 

 roots seems essential. Prop roots and 

 pneumatophores must be cleaned with com- 

 pounds which will not damage the plant 

 tissues. Dispersants commonly used to 

 combat oil spills are, in general, toxic 

 to vascular plants (Baker 1971). If pos- 

 sible, oil laden spray should not be 

 allowed to reach leaf surfaces. Damage 

 during clean-up (e.g., trampling, compac- 

 tion, bulldozing) may be more destructive 

 than the untreated effects of the oil 

 spill (de la Cruz in press, b). 



In south Florida, man has been re- 

 sponsible for modifications which, while 

 not killing mangroves outright, have al- 

 tered components of the mangrove ecosys- 

 tem. One of the most widespread changes 

 involves the alteration of freshwater 

 runoff. Much of the freshwater runoff of 

 the Florida Everglades has been diverted 

 elsewhere with the result that salinities 

 in the Everglades estuary are generally 

 higher than at the turn of the century. 

 Teas (1977) points out that drainage in 

 the Miami area has lowered the water table 

 as much as 2 m (6 ft). 



Interference with freshwater inflow 

 has extensive effects on estuaries (Odum 

 1970). Florida estuaries are no excep- 

 tion; the effects on fish and invertebrate 

 species along the edge of Biscayne and 

 Florida Bays have been striking. The 

 mismanagement of freshwater and its 

 effects on aquatic organisms have been 

 discussed by Tabb (1963); Idyll (1965a,b); 

 Tabb and Yokel (1968) and Idyll et al. 

 (1968). In addition, Estevez and Simon 

 (1975) have hypothesized that the impact 

 of the boring isopod, Sphaeroma terebrans , 

 may be more severe when freshwater flows 

 from the Everglades are altered. 



One generally unrecognized side 

 effect of lowered freshwater flow and salt 

 water intrusion has been the inland expan- 

 sion of mangrove forests in many areas of 

 south Florida. There is documented evi- 

 dence that the mangrove borders of 

 Biscayne Bay and much of the Everglades 

 estuary have expanded inland during the 

 past 30 to 40 years (Reark 1975; Teas 

 1979; Ball 1980). 



Sections of many mangrove forests in 

 south Florida have been replaced by filled 

 residential lots and navigation canals. 

 Although these canal systems have not been 

 studied extensively, there is some evi- 

 dence, mostly unpublished, that canals are 

 not as productive in terms of fishes and 

 invertebrates as the natural mangrove- 

 lined waterways which they replaced. 



81 



