Seasonality, biodiversity and microhabitats in benthic foraminiferal communities

Abstract

Benrhic foraminifera (Protista: Sarcodina) are ol1l'-ceUed organisms that are widely spread over the worlds' oceans. Most of the species can be distinguished by the morphology of their sheJl, and the high preservation potential of these shells makes them a lIseful tool to date sediment layers and reconstruct ancient environments. The latter, however, reqUires sufficient insight in the t'cology. After decades of research on the ecology of benthic foraminifera, stiLi more questions than answers exist. Nevertheless, the last fn-v years we seem to have come closer to some answers through experimental and monitorIng studies. Whereas experiments are useful because of the possibility to standardise environmental parameters, monitoring studies are valuable because of repetitive sampling during one or several years. These time seriL's provide us with knowledge on how foraminiteral communities change during the seasons, and develop over the years. With tile repetitive sampling of several s[;l.tions situated within one region we also gain insight in diffcrencn in foraminiferal community structures OVLT distance. In a monitoring study of the- northern Adriatic Sea. BannawicUap et a1. (1992) found large seasonal changes in foraminiferal abundance<; and in-sediment distribution. The combination of food flux and especially oxygenation of the sediment, was found to playa major role in the foraminiferal community structures. In 1995, Jorissen et aJ. prese-nted the TROX-model. which describes the foramjniferal assemblages in the light of these two major controlling f.1ctors. food and oJo,:ygen. To further unravel the importance of food and oxygen in benthic foraminiferal communities, a rather large-scale monitoring project was starred in 1996. This project regarded two completely different regions within the Mediterranean Sea, the eutrophic Adriatic Sea and the oligotrophic Levantine Basin. [n the Levantine basin, where nutrient levels are known to be low. tht' stability of the environment was expected to be relatively high. This in contrast with the Adriatic Sea, from which \vc knew that environmenl,d circuITI<tances change drastically throughout the year, which is re{Ject by the foraminiferal abundances (Barrnawidjaja et aJ. (992). In order to srudy <;easonal changes in the foraminiferal communities both study areas were monitored bi-monthly between June '<,16 and June '98. In the Levantine basin, Wt' >tudied a down-slope transect perpendicular to the Israeli coast trom 40 to '7oom water depth. Several oceanographic properties (chlurophyll-a, temperature, salinity, and dissolved oxygen) ,,-c-rc mc-~sllrC'd <illllliunrolldy wirh rhf' samphng of sedirnents. The sediment oxygenation was measured in detail and hVlllg (Rose Bengal stained) benthic foraminifera were collected.