Selected water quality parameters for a river and a tributary are as follows $($a tributary is a stream that goes into river$)$:

$\backslash$table$[[$Parameter$,$River,Tributary$],[$Flow rate $(cfs),50,25],[$Total phosphorus $(m\frac{g}{L}),0\mathrm{.}02,0\mathrm{.}3],[$Chlorophyll $(m\frac{g}{L}),0\mathrm{.}05,0\mathrm{.}1]]$

Recognizing that algae growth in this system is limited by phosphorus concentrations less than $0\mathrm{.}015m\frac{g}{L},$ the longitudinal profile of the algae concentration in the main river downstream of the confluence point will probably:

A$)$ stay the same

C$)$ decrease continuously

B$)$ decrease first and then increase

D$)$ increase first and then decrease