BENTHIC FORAMINIFERA AND PORE-WATER OXYGEN PROFILES: A RE-ASSESSMENT OF SPECIES BOUNDARY CONDITIONS AT THE WESTERN IBERIAN MARGIN

Abstract

The depth distribution of living (Rose Bengal-stained) benthic foraminifera in near-surface sediments was analyzed along 4 short cores from the western Iberian Margin and Gulf of Cadiz at water depths from 800 to 1920 m. Pore-water oxygen, Chloroplastic Pigment Equivalent (CPE), and total organic carbon (C org ) concentrations were measured in the same or adjacent cores. These values were used to constrain the limiting oxygen and nutrient levels that defined the ranges in sediment depth and abundance maxima of foraminiferal species. The population density showed a good correlation with pore-water oxygenation indicating that oxygen is a limiting factor for endobenthic foraminiferal assemblages at mid-depths between 800 and 1200 m. At the deep-water site, however, the fauna appeared nutrient limited. More than half of the benthic species occupied extended sediment depth habitats and a wide range of oxygen levels among the sites studied. Thirty-nine of 103 species occurred in two or more cores and showed distinct abundance maxima, 36 in the uppermost 2.0 cm where the CPE concentrations were high. Living foraminifers were not found at depths where the CPE-values were lower than 0.1 μg/g. This level indicates trophic conditions critical for the survival of endobenthic species. The frequency distribution of the lower oxygen range limits of all species showed maxima at the high oxic/low oxic (3 ml/l), low oxic/suboxic (1.5 ml/l), and suboxic/dysoxic (0.3 ml/l) boundaries and thus depicted the ecological significance of these previously recognized ecolimits. Only 30 % of the species display consistent patterns in relation to pore water oxygen. Dysoxic indicators were Globobulimina affinis, Chilostomella ovoidea and Bathysiphon capillare. Many species previously designated as suboxic indicators were ubiquitous or preferred microhabitats at oxic levels. Boundary conditions for the dysoxic indicator G. affinis were pelagic flux rates higher than 3.5 g C m −2 yr −1 , a habitat depth below the homogeneously bioturbated layer in areas outside of high productivity zones, and CPE concentrations of 0.1 to 1.0, preferentially 0.7 to 0.8 μg/g. The distribution pattern suggested that G. affinis thrives mainly on dysoxic bacteria and actively seeks this nutrient resource.