This paper examines a system of reaction–diffusion equations
arising from a flowing water habitat. In this habitat, one or
two microorganisms grow while consuming two growth-limiting,
complementary (essential) resources. For the single population
model, the existence and uniqueness of a positive steady-state
solution is proved. Furthermore, the unique positive solution
is globally attracting for the system with regard to nontrivial
nonnegative initial values. Mathematical analysis for the two
competing populations is carried out. More precisely, the longtime
behavior is determined by using the monotone dynamical
system theory when the semi-trivial solutions are both unstable.
It is also shown that coexistence solutions exist by using the
fixed point index theory when the semi-trivial solutions are both
(asymptotically) stable.