Benthic foraminifera from the Adriatic Sea : principles of phenotypic variation

Abstract

The distribution and morphology of the benthic foraminifera in the Adriatic Sea appear to be strongly dependent on two primary controlling environmental parameters, oxygen concentration and food availability. These factors are both governed by the runoff from the Po and other Italian rivers, and by the subsequent distribution of the fluvial discharge products by the system of surface currents. The area most strongly influenced by runoff products, which is characterized by ample food availability and low oxygen concentrations at the bottom, is a strip parallel to the Italian coast at a water depth of between 20 and 60 m. In the present study we describe in detail the changes in the benthic foraminiferal faunas along transects perpendicular to this zone. The faunas along these transects react essentially in two different ways to the changes in environmental conditions. On the one hand we see qualitative and quantitative changes in the composition of the faunas, and on the other hand a number of species show a high degree of ecophenotypic variation. Statistical analysis of the faunal patterns resulted in the recognition of eight faunal clusters. One of these clusters, occurring in the near-shore zone, is consisting of obviously reworked specimens. The other seven clusters are separated on the basis of differences in the tolerances for stressed conditions and in the preferences for particular feeding niches of the various taxa. In the centre of the zone influenced by run-off products the faunas are dominated by a cluster with Bulimina marginata forma denudata and Nonionella turgida. Obviously the taxa of this cluster have the greatest tolerance for the sometimes stressed conditions found in this area. In the taxa Ammonia parkinsoniana, Elphidium granosum and Elphidium poeyanum, comparable successions of three fundamental morphologies are demonstrated. Compact morphotypes are typical for the near-shore, relatively nutrient-poor and oxygen-rich environment, in which there is a minimum influence of runoff products. In the somewhat deeper, food-enriched environment with lower oxygen concentrations, inflated morphologies occur. Still deeper, in the central part of the strip influenced by run-off products, where oxygen-poor conditions occur, these types are found with supplementary characteristics. The study of the variation in Bulimina marginata suggests that in this taxon the morphology reflects the life position of the animal. On the basis of the distribution of different morphologies in this and in similar species, it is concluded that the degree of niche separation in benthic foraminifera is dependent on the thickness of the oxygen-containing layer. If this thickness is minimal, the inbenthic microhabitat is ruled out, and only taxa adapted to an epibenthic microhabitat are found. In the final chapter some possibilities to apply the results of our study are discussed. It is argued that the bathymetrical distribution of benthic foraminifera in the Adriatic Sea is largely influenced by variations in productivity. Although the Recent situation can be described in great detail, one should be careful about using these data for estimating paleo-depths in other basins and/or other time-slices. Far more promising is the potential use of the present results for the interpretation of benthic foraminiferal faunas from deltaic environments in terms of productivity. It is demonstrated that not only variations in the productivity itself can be distilled, but also the source of such variations, such as variations in run-off, changes in circulation patterns and sea level fluctuations. It is suggested that benthic foraminifera should be used more often in pollution studies, because they can provide an accurate description of the distribution patterns of pollution phenomena in the bottom environments.