As the author of your work, you hold the copyright to how your work is used by others.
When you upload your thesis onto DalSpace, you still hold copyright of your work, you have just given Dalhousie Libraries the right to distribute your thesis on DalSpace [more info].
When you sign a publishing agreement with a publisher to publish your work in a journal, the agreement may have you sign over rights to your work over to the publisher. How much of your copyright you sign over will vary depending on the journal. You may also try to negotiate changes to the default agreement set out by the publisher with an addendum [more info].
It is important for your thesis and for the future works that you will create throughout your professional career to be mindful of what copyrights you hold to those works and what impacts signing them away can have.