The Collaborative International Dictionary of English v.0.48:

Benthic \Ben"thic\ (b[e^]n"th[i^]k), a. [fr. Gr. be`nqos depth
   of the sea.]
   of, pertaining to, or occurring at the bottom of a body of
   water, especially referring to the ocean depths.
   [PJC]

WordNet (r) 3.0 (2006):

benthic
    adj 1: of or relating to or happening on the bottom under a body
           of water [syn: benthic, benthal, benthonic]