There is credibility of source, it just depends on how much credibility. This is why critical thinking is so important.
Do you actually know people are being silenced? In the case of this article there is no evidence that anyone is being silenced, this is just what this 'Dr James' says on his blog.
If you think critically about this article, you need to ask, a few questions. Who is this so called Dr?, what is he a doctor of?, what is his history? (as far as you can determine). Why would scientists be coming to him to put out information about this oil disaster, as opposed to giving this information to someone without a reputation for being a nutbag? What type of scientists is this doctor talking about, who do they work for, etc? He wouldn't need to name names, but a little more information would be very useful to his credibility. Lastly, talk to other scientists, or do some research on geology, oil drilling etc (there is plenty of info about these topics), and form an opinion based on critical thinking, rather than just believing every bit of info thrown up onto the internet.
Critical thinking and research aren't going to save you from every scam, dodgy news reports, or dodgy people with an agenda, but it will provide you with a good set of tools to discern what most of the rubbish is.