A Problem of Size and Scale 



The increasing use of relatively small experimental ecosystems or 

 microcosms in marine research has raised a number of interesting questions 

 related to the importance of size or scale in natural, as well as experimental 

 systems. In pelagic marine environments, one of these questions for which we 

 have very little relevant information is the importance of turbulence in the 

 water. The relationship between turbulence and scale was first formalized by 

 Richardson (1926), who distinguished between classical Fickian diffusion (in 

 which scale is not a factor) and the mixing that is characteristic of turbulent 

 fluids such as the sea. While the nature of turbulence is extremely complex, 

 Richardson's concept of turbulent energy passing through a series of 

 progressively smaller eddies from the wavelength at which it is put into the 

 fluid until it is ultimately dissipated in viscosity has continued to prove 

 valuable in studying the mixing of marine waters (Okubo 1971). In natural 

 systems, turbulent energy is added at a rather large scale by winds, tides, and 

 major currents. Since none of these is usually effective in microcosm tanks or 

 bags, some artificial means of introducing turbulent energy at smaller scale may 

 be required to develop pelagic ecosystems that are credible experimental 

 analogs of the "real world" (Perez et al 1977). 



There are at least two aspects to the turbulence problem, one involving the 

 actual distribution of organisms, particles, or dissolved constituents in the 

 water — the problem of patchiness (see Steele 1974), and the other involving 

 the metabolic or behavioral responses of organisms to water turbulence. The 

 study of plankton patches usually concerns water masses on a scale larger than 

 the largest microcosms yet developed (1300 m , 10 m dia.; see Menzel and 

 Case 1977), and it is generally conceded that this aspect of the ecology of 

 marine waters is not well represented in microcosm experiments. The 

 importance of this omission is not yet known. While there have been numerous 

 studies which have documented the response of sessile plants and animals to 

 the special case of turbulence in flowing water (Fox et al 1935; KerswiU 1949; 

 Whitford 1960; Jaag and Ambiihl 1963; Whitford and Suchumacker 1964; 

 Mclntire 1966; Westland 1967; Nixon and Oviatt 1971), the responses of 

 pelagic organisms to small scale turbulent energy have received much less 

 attention. 



With the exception of an older qualitative study of the morphological 

 response of Daphnia to water motion by Brooks (1947), the recent work by 

 Pasciak and Gavis (1975) on the relationship between turbulence and nutrient 

 uptake by phytoplankton, and our own studies on marine plankton in 

 laboratory microcosms (Perez et al 1977), the effect of water turbulence on 

 the grov^h and metaboUsm of planktonic organisms is almost totally unknown. 

 While this situation is largely a result of the difficulties involved in measuring 



385 



