The simulation with no advection or diffusion showed that phytoplankton 

 production in the cold offshore waters, although reduced when compared to inshore 

 waters , is sufficient to deplete nutrients from the entire surface layer within 

 the 72-day simulation (fig. 13a). Therefore, temperature-controlled, in situ 

 production alone was not sufficient to produce the persistent offshore gradients. 



The second simulation illustrates that vertical mixing, together with in 

 situ production, is sufficient to reproduce the observed biological and chemical 

 patterns. The major effect of vertical and horizontal advection is to smooth 

 the nutrient gradients through increased mixing caused by repeated reversals in 

 flow direction. The same is true for plankton along the relatively dilute, 

 cold-water boundary (fig. 13d). 



It appears that the distribution of chemical and biological properties in 

 the vicinity of the 4°C isotherm is controlled primarily by the interaction of 

 in situ processes and the differences in vertical mixing on either side of the 

 isotherm. Shoreward, the water mass is weakly stratified vertically. This 

 reduces the mixed depth and allows increased biomass production and subsequent 

 nutrient depletion (Sverdrup 1953, Stadelmann et al. 1974). Lakeward, deep 

 vertical mixing keeps a significant portion of the phytoplankton removed from 

 the sunlit surface layers and therefore inhibits their growth. 



The region near the 4°C isotherm has been referred to as the "thermal bar." 

 Many notions regarding this region have suggested, as the name implies, that 

 elevated biomass concentrations shoreward of the isotherm are caused by some 

 barrier to their transport offshore. The above analysis demonstrates that in 

 fact no such barrier exists. Biomass, concentrated in surface waters shoreward, 

 are simply diluted vertically throughout the water column when transported 

 offshore. In this fashion, the presence of this convergence zone near the 4°C 

 isotherm in fact enhances offshore transport. 



LIMITS OF ECOLOGICAL MODELS 



Compensating errors - Ecosystem models that are faithful to extant theories 

 relating various processes of nature tend to become complicated, nonlinear 

 collections of equations. Often, verification of these more mechanistic models 

 is not possible by usual techniques because it is difficult to obtain complete 

 and independent data sets. This is because sampling all of the properties 

 simulated in more mechanistic models is difficult and expensive (e.g., zooplankton 

 biomass). Even when such data sets are available and these models have been 

 "verified" by usual techniques, one is left with serious questions concerning 

 model reliability because these generally nonlinear models have increased degrees 

 of freedom. 



Increased degrees of freedom, in this context, means that more than one 

 set of coefficient values will satisfy the usual tests for calibration and 

 verification. The basis for increased degrees of freedom is the cyclic nature 

 of mechanistic models. Since these models generally simulate ecosystem cycles, 

 one would not expect material to accumulate excessively in one particular 

 component but rather to flow among all of the components. Then, because of 

 principles of mass conservation, one could expect that, if flow rates were 

 increased or decreased proportionately, state variable concentrations would not 



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