How can we prevent a depressed employee from affecting the internal environment?

Ask the employee if the depression is caused by problems at work and try to find a resolution to their concerns.
Watch out for other things that can cause depression or stress in the workplace like bullying and harrassment, unsocial hours, too much responsibility or not enough responsibility, lack of job satisfaction. 
Encourage your employees to look after their mental health.
Ensure people take lunch breaks, have a good work-life balance and exercise regularly (perhaps look into discounted gym memberships for your employees In a bad case, an employee’s stress or depression may make him or her 'disabled' for the purposes of the Equality Act.
You will then be under a duty to make reasonable adjustments to any provision, criterion or practice you operate that places him at a substantial disadvantage compared with non-disabled people.
Warning: a failure here will constitute disability discrimination. 
the approach that should be taken to identify the case ,there anything we can reasonably do to help? Advice from  professionals will help .
Immediately after the employee returns to work, you should meet with him/her to clarify your understanding of their current state of health to talk about any temporary or permanent adjustments which are to be put in place to facilitate his or her return, and to discuss any other points which the employee wants to raise.
Make decent notes so there is no confusion later as to what was agreed.

The cause for a depressed employee can be work related ( such as overwork, job insecurity, lack of training, over promotion, change, bad working environment etc.) or personal.
You have to talk to the employee in private and try to find out what the reason is.
He might know the answer himself.
If it's work related, that can be solved by you ex.
the cause is overwork then you might reduce the workload.
Don't be judgemental, try to understand and help. 
Try to motivate your employee be optimistic, lead by example.
You must keep a balance between offering support and make sure they fulfil their obligations Even they go through hard times, they have to understand that they still have to perform.
Based on the situation, you are the one who can decide, the issue can be solved or you have to move forward without certain employee.
Attitude is important.
       

First of all, work itself, environment or colleagues can be the source of depression.
Sometimes a unpleasent work environment, or colleagues can determine someone to suffer from depression; some people also have to deal with this situation as maybe they do not have other alternative  (financially speaking).
Other people simply don't fit, their way of work doesnt match the company...
  However the situation, depressed people need help.
They need moral support, and also medical care in some cases.
Depression in work it is also quite common nowadays and we should pay attention more to our colleagues and their feelings as the quality of the work it is also related with the mood of a person.....

Consider any or all of following; AS SUPERVISOR / SENIOR POSITION: 1.
 Evaluate the reason of depression and propose solutions for your sub-ordinate 2.
 As supervisor, you should ensure healthy and pleasant working envoironment within your authority.
3.
 Provide ethical and emotional support to sub-ordinate 4.
 Encourage the sub-ordinates to face critical and tough situations AS COLLEAGUE / CO-WORKER; 1.
 Evaluate the reason of depression and extend your support to ease the situation for your colleague 2.
 Provide moral support 3.
 Discuss with him the problems he is facing and propose him suggestions 4.
 Encourage positive thinkings and approach

By talking to them.

most mistakes done these days , we treat the symptomps and not the real cause of the issue , in these times in the world , who is not depressed, a real HR know his company , the individual , and every work load and personal issue, this should not be difficult if he want to fix it , or  not the choice is yours.

it always better to understand other parties feelings and situations prior concluding that he is Depressed .
He needs a a gud Consoling and avail him with self confidence  

Since90% of the people in the planet cannot leave  his / her personal problem at home so most of us are effected with our problems but we can help  each other to stop contaminate the work environment with negative impulse, all companies should consider giving all the employees NLP course