Vertical distribution of benthic foraminifera in the northern Adriatic Sea: The relation with the organic flux

Abstract

In October 1989 the distribution of benthic foraminifera stained by Bengal Rose in the uppermost seven centimeters of the sediment has been determined in 14 sample stations in the northern Adriatic Sea. The downward organic flux, which controls the complex relation between food and oxygen availability in the benthic environment, appears to be the main factor determining the distribution of benthic foraminifera. In most bottom environments especially the oxygen concentration is limiting benthic life; low values are responsible for the low faunal densities within the sediment of some of the most organic-rich areas. As soon as the oxygen level surpasses a critical treshold value, food availability becomes the limiting factor, regulating abundance and species composition of the benthic faunas. The areas with the highest downward organic flux are typified by a number of very opportunistic taxa, which can be epifaunal as well as potentially (mobile) infaunal. These taxa are most able to profit from the combination of a high food availability and fair oxygen levels after the reoxygenation of the bottom environment in autumn. The areas with lower organic fluxes are characterized by a more stable fauna, consisting of less stress-tolerant epifaunal taxa in combination with less mobile infaunal species, which lack the possibility to track critical oxygen levels. The present data confirm that microhabitat differentiation is minimal in months of low oxygen values in the sediment. However, fossil records of such seasonal environments will mainly be determined by production in the well-oxygenated season, and therefore knowledge about microhabitat behaviour of individual taxa can certainly be useful for the reconstruction of ancient organic fluxes.