Is there a link between cannabis and depression?

Link does exist. Chronic cannabis use has been associated with depression and other mood disorders. (adverse effects of cannabis on health: an update of the literature since 1996. Kalant h. Prog neuropsychopharmacol biol psychiatry. 2004;28(5):849). Those who self-treat depressive symptoms with cannabis can end up making the mood worse.
No. Some research has shown that marijuana can actually worsen depression and in fact it was shown that more marijuana smokers were diagnosed with depression then people who didnt smoke. This does not mean it is a cause as there are many causes or reasons for depression but there is a link.
Cannabis. Can trigger mood changes in predisposed ones, such as ones with depression/anxiety, family history of mood problems, and ofcourse history of adhd & conduct problems as a child. Both chronic use as well as withdrawal can precipitate mood changes.