730 VERNBERG 



3. It must be effective as to sequence, as ordered by the 

 ontogeny (development) of the organism. 



Just as a decision must be made as to which external factors 

 should be studied, a decision also must be reached as to which 

 internal functions should be investigated. Obviously this decision is 

 based on the nature of the question to be answered. As with external 

 factors, numerous studies have been published dealing with various 

 internal factors, including resistance and capacity adaptations, which 

 have ranged from overt responses of the intact organism to 

 subcellular changes. In addition, the responses of populations and 

 communities to environmental fluctuation have been analyzed. 



The purpose of this paper is to review recent research efforts 

 dealing with the influence of multiple factors on organisms in aquatic 

 systems. Since Alderdice (1972), Salisbury (1975), and Vernberg 

 (1975) have reviewed and discussed many papers dealing with this 

 topic, this review will not be comprehensive; rather, it will emphasize 

 recent literature that illustrates some distinct research areas. 



Inherent in most scientific disciplines is the problem of definition 

 of terms, and physiological ecology is no exception. Synergism has 

 been variously defined. Only two examples will be cited here. 

 Gould's Medical Dictionary defines it as, "the cooperative action of 

 two or more agents or organs." The definition in Webster's Collegiate 

 Dictionary is more comprehensive, stating, "synergism is the coop- 

 erative action of discrete agencies, such that the total effect is greater 

 than the sum of the two effects taken independently." This latter 

 definition appears to be more widely used. Synergism is not always 

 easy to demonstrate, however, and there are cases where the degree 

 of synergism quantitatively is near zero and the effects of two 

 agencies (environmental factors) appear to be additive and not 

 significantly greater thcin the sum of the two agencies acting 

 independently. A multiple factor effect is simply the expression of 

 the influence of two or more factors on some response of an 

 organism. The data from this type of study can be statistically 

 analyzed to determine the nature and significance of interactions (see 

 review of Alderdice, 1972). This paper does not deal with the 

 theoretical basis for the mechanism of interaction, but papers dealing 

 v^th this topic are cited (Lockhart, 1965; Mohr, 1972). 



LETHAL LIMITS 



One of the classic papers demonstrating the effects of three 

 factors (salinity, temperature, and dissolved oxygen concentration) 



