Meiofaunal distributions on the Peru margin: relationship to oxygen and organic matter availability

A quantitative study of metazoan meiofauna was carried out on bathyal sedim ents (305, 562, 830 and 1210 m) along a transect within and beneath the oxy gen minimum zone (OMZ) in the southeastern Pacific off Callao, Peru (12 deg reesS). Meiobenthos densities ranged from 1517 (upper slope, middle of OMZ) to 440-548 ind. 10cm(-2) (lower slope stations, beneath the OMZ). Nematode s were the numerically dominant meiofaunal taxon at every station, followed by copepods and nauplii. Increasing bottom-water oxygen concentration and decreasing organic matter availability downslope were correlated with obser ved changes in meiofaunal abundance. The 300-m site, located in the middle of the OMZ, differed significantly in meiofaunal abundance, dominance, and in vertical distribution pattern from the deeper sites. At 305 m, nematodes amounted to over 99% of total meiofauna; about 70% of nematodes were found in the 2-5 cm. interval. At the deeper sites, about 50% were restricted to the top I cm. The importance of copepods and nauplii increased consistentl y with depth, reaching similar to 12% of the total meiofauna at the deepest site. The observation of high nematode abundances at oxygen concentrations <0.02mll(-1) supports the hypothesis that densities are enhanced by an ind irect positive effect of low oxygen involving (a) reduction of predators an d competitors and (b) preservation of organic matter leading to high food a vailability and quality. Food input and quality, represented here by chloro plastic pigment equivalents (CPE) and sedimentary labile organic compounds (protein, carbohydrates and lipids), were strongly, positively correlated w ith nematode abundance. By way of contrast, oxygen exhibited a strong negat ive correlation, overriding food availability, with abundance of other meio fauna such as copepods and nauplii. These taxa were absent at the 300-m sit e. The high correlation of labile organic matter (C-LOM, sum of carbon cont ents in lipids, proteins and carbohydrates) with CPE (Pearson's r = 0.99, p <0.01) suggests that most of the sedimentary organic material sampled was of phytodetrital origin. The fraction of sediment organic carbon potentiall y available to benthic. heterotrophs, measured as C-LOM/Total organic carbo n, was on average 17% at all stations. Thus, a residual, refractory fractio n, constitutes the major portion of organic matter at the studied bathyal s ites. (C) 2001 Elsevier Science Ltd. All rights reserved.