There are sensitive colorimetric reagents for phosphate detection that use ammonium molybdate combined with Malachite Green dye. They can measure phosphate concentrations down to about 1 micromolar and can be done in a 1-cm cuvette or a microplate. The reagent can be purchased, or you can make it yourself according to the literature. Here is one supplier:

http://www.enzolifesciences.com/BML-AK111/biomol-green/

Measuring Methods and Digestion

There are two methods available for determining phosphate

or phosphorus concentrations:

• Molybdenum blue method

• Vanadate/molybdate method (yellow method)

Both techniques are based on the measurement of orthophosphate.

Digestion of both dissolved organic as well as

particulate phosphorus compounds is therefore mandatory

for determining the total P content. In addition, an unfiltered

sample must be acquired in order to include all solid

matters in the digestion process.

Digestion is usually performed by heating the sample with

peroxodisulfate and sulfuric acid.

http://www.ert.pt/24/images/stories/catalogos/WTW/Lab/Online/Phosphate.pdf

Thank you Adam B Shapiro and Moaed E. Algazally for the response, it's very helpful.