selecting maximum values

Sorry for disturbing you with another newbie question!
I have a data frame about coastal waters quality
parameters: for some parameters (e.g. NH3) I have only
1 observation for each sampling station and each
sampling date, while in other cases (chlorophyll) I
have 1 obs for each meter-depth for each station and
date. How can I select only the max chlorophyll value
for each station/date?

example

row  station         date        depth     chlorophyll
1     Castagneto      21/06/01     -0.5         2.0
2     Castagneto      21/06/01     -1.5         2.2
3     Castagneto      21/06/01     -2.5         2.4
4     Castagneto      21/06/01     -3.5         2.1
5     Ancona          23/06/01     -0.5         2.4
6     Ancona          23/06/01     -1.5         2.5
7     Ancona          23/06/01     -2.5         2.2
8     Ancona          23/06/01     -3.5         2.1
9     Ancona          23/06/01     -4.5         1.9
...

I'd like to select only row 3 and 6, the ones with max
chlorophyll values, or have the mean for the rows 1:4
and 5:9

Thanks