Decks, Patios, Porches and Docks - new deck cracking badly

Ok. I just put in a brown pt deck last summer. I only applied water sealer to the cut ends. I live in manitoba canada. The lumber yard told me to wait unitil the following summer to stain. I used a system that allowed me to secure the deck from the bottom. This consisted of metal brackets (shadow track). There was no cracks in any of the wood and no screws coming down from the top. The top was perfect. Now this summer I see that the wood is cracking like crazy. The ends are splitting and cracks run down form one end of boards to the other. I'm worried that this deck is not going to last 10 yrs never mind 40 yrs like I planned. I put a lot of work to make it look nice and clean and now it looks like crap.
Can someone tell me what I can do to stop the cracks from getting worse?What I did wrong? I plan on staining it my next days off with a good stain after I clean it properly.
I'm thinking of putting on a protective covering over the wood so that the cracks won't bother me any more. Anyone can suggest some good products that I could buy that would cover the decking completely protecting it from water damage and uv. I'm looking for something that would be glued on and would last 40yrs. I would like it to cover the exsisting decking.
Thanks for help!