There are plenty of estimates on salt marshes, mangroves and seagrass meadows. I am more interested in the contribution of algal forests, such as kelp or fucoides.
I was able to find estimates by:
S. V. Smith, 1981 - Marine macrophytes as a global carbon sink.
Gattuso et al., 1998 - Carbon and Carbonate Metabolism in Coastal Aquatic Ecosystems
Is there anything more current?
I also found a book chapter by Middelburg et al, 2005 that has respiration rates only..