Fig. 1.

Application of mechanistic resource competition theory (3, 19) to explain the mechanisms of DCM formation. Lines indicate resource combinations at which the net population growth of a phagotrophic (P) and a mixotrophic (M) organism are zero, the zero net growth isoclines. Sections i–iv show resource combinations allowing the growth of M+P(i),M(ii),P(iii), and neither M nor P (iv, only theoretically). F_0,P and F_0,M, refer to the zero net growth food concentrations in the dark. In the context of Lake 111, food is represented by phototrophs (Chlamydomonas) plus bacteria. The consumption vectors (arrow) indicate resource changes by grazing assuming no variation of PAR attenuation due to varying predator or prey densities. Starting in i, grazing by M or P reduces the food abundance until the population growth of the superior competitor equals zero (thick gray line), indicated by the lower F₀ derived from the zero net growth isocline and a given PAR. In shaded strata (DCM), F_0,P determines the food abundance. F^L_0,M represents phototrophic abundance at the surface as a function of PAR at half epilimnion depth (PAR′), assuming vertical mixing.