Do you mean in the water, or in the algae cell matter?

For the water, I normally see it reported as Total N : Total P. Suggest you check the lab standard methods if you're interested in the lab methods for TN and TP, but I think that uses the filtered sample.

Thank u sir for your valuable suggestion.. i am asking about water ,,i have some of resuls in Nitrate, Phosphate and silicate for a period of i year. in this connection how i will calculate N:P:Si ratio 

in the paper below you find an overview of various indices of N:P ratios and how they are used to predict nutrient limitation. There is however at best a weak link to phytoplankton diversity (in the way that co-limitation should favor higher diversity - this was often proposed, but is not easily seen in data)

Article Performance of the Redfield Ratio and a Family of Nutrient L...

One should use these indices with caution. Very often (and very easily, especially in the past) I see people claiming that some patterns are due to limits imposed by a certain value of N:P. But then, looking at the absolute values, nor N nor P is limiting. If the availability of a single element is not limiting, how can their ratio be limiting?

You have to make chemical analysis and calculate mass or molar ratio between TP to TN

The only acceptable method would be: 1) Identify (at species level) and count the individual of each species in many samples. 2) Calculate their diversity (Shannon's or any other acceptable diversity index, NOT species numbers). 3) Measure organic C and N in these samples and calculate the C:N quotient (by atoms). 4) Look at the relationships between diversity and C/N. So simple.

Low N/P ratio initiate favorable conditions for Nitrogen Fixers Cyanobacteria. But, Ratio is not the whole story because if N and P levels are high together with low ratio all other algal species enjoy as well. Therefore, the concept is slightly modified to: Under Nitrogen deficiency and P sufficiency and reasonable inoculum, Cyanobacteria flourish. It is worldwide known that under Nitrogen defficiency,  and  N/P mass ratio below 29    N-Fixing cyanobacteria produce blooms.

There is only a good method: 1) Identify at species level and count the individuals of each species of a number of samples. 2) Calculate the diversity of each sample (either Shannon's or any other reliable index, NOT what is at present known as biodiversity, the old species richness or number of species). 3) Analyze the organic C and N contents of each sample (or aliquots if you don't want to destroy the whole sample) and calculate the C:N quotient (by atoms or weight). 4) Calculate the regression equations relating C:N quotients with diversity. As simple as that. 

It is good that you have measured Si. I have seen lakes with very low Si where the diatoms are depauperate, presumably as a consequence. Green algae seem to handle the low Si better.

If you only have nitrate as you mention, your N:P and N:Si ratios wll be flawed, Most phytoplankton species will use not only all types of inorganic N (and ammonia is generally abundant, especially in an estuarine environment), but also at least some types of organic N.

This said, I suggest a couple of additional readings, apart from the interesting paper suggested byRobert. unfortunately for one I can only find the abstract.  (I guess that I filed the full paper so well that I cannot find it). On the other hand, perhaps it is time to look at things under different perspectives. I found not long ago  a challenging and interesting paper I want to share

Here they go and good luck