Biomass, production and decomposition of a deep water seagrass, Halophila decipiens ostenf.

Abstract

Beds of Halophila decipiens Ostenfeld, at depths between 15 and 27 m, were studied using a saturation diving facility (NULS-1: Hydrolab) in the Salt River submarine canyon off St. Croix, US Virgin Islands. Distribution, biomass, production and decomposition of the seagrass were studied during two missions in June 1983 and July 1984. Biomass of H. decipiens ranged from 5 to 12 g m −2 during summer months. Due to the rapid turnover of H. decipiens, biomass distribution was closely related to concurrently measured growth rates. Production estimates using oxygen production techniques or rhizome elongation were similar ranging between 100 and 500 mg C m −2 day −1. Decomposition of H. decipiens occurred rapidly, losing over 50% of its original weight in approximately 3 days. Under most conditions, weight loss from litter bags occurred more rapidly than nitrogen loss. H. decipiens exhibits a number of adaptations to a low ambient light environment, including a high ratio of leaf tissue to non-photosynthetic tissue, low leaf area index to reduce self-shading, high turnover leaf material and the ability to rapidly colonize sandy bottoms when light conditions are suitable.