For dry weight, for most taxa around 40% is carbon. For wet weight it ist much more variable and taxon-specific (in particular gelatinous vs. crustacean and others)... I think this reference will be useful: Kiørboe, T. (2013). Zooplankton body composition. Limnology and Oceanography, 58(5), 1843-1850.
1. Peter H. Wiebe et al., 1975. Relationships between zooplankton displacement volume, wet weight, dry weight, and carbon. Fishery Bulletin 73(4): 777-786.
2. Peter H. Wiebe, 1988. Functional regression equations for zooplankton displacement volume, wet weight, dry weight, and carbon: a correction. Fishery Bulletin 86(4): 833-835.
Note that the second article corrected the formulas published in the first article.