57 



some other physical factor or biological agent (Cottam, 1934; Milne and 

 Milne, 1951). Recently, Short (pers. coram.) has demonstrated that 

 Labarynthula was the biological cause of the wasting disease, but what 

 triggered the catastrophic decline in 1931-32 remains unclear. 



Rasmussen (1977) presented an analysis of the wasting disease that 

 has been widely accepted. He rejected all previous hypotheses 

 concerning the disease except the effect abnormally warm temperatures 

 which were elevated during the early 1930' s. Water temperatures were 

 not exceptionally warm in all areas during that period, but came after a 

 prolonged cool period. This warm period resulted in the elevation of 

 mean water temperatures by several ^'C that stressed eelgrass, making it 

 more susceptible to a pathogen. He explained the occurrence of the 

 disease one year later in Europe was because the warming period occurred 

 one year later there as well. 



Rasmussen acknowledged that Zostera can tolerate wide temperature 

 ranges throughout its geographical range, but suggested that eelgrass 

 populations are adapted to local temperature conditions and were 

 sensitive to these changes. He suggested that the survival of eelgrass 

 populations near streams and other sources of freshwater may have been 

 due to higher rates of germination in annual populations near these 

 sources or that the disease organism was stenohaline. 



The temperature hypothesis cause of the decline of 1931-32 has 

 been criticized for several reasons, and these are discussed below. 

 Past declines of eelgrass have also been reported, such as in 1894 in 

 the eastern U.S., around 1908 in New England, and in 1916 in Poponesset 

 Bay, Cape Cod (Cottam, 1934). These events, perhaps due to disease. 



