Abstract
This study compares living (LA) and dead (DA) benthic foraminiferal assemblages and identifies different factors that possibly cause differences in the distribution of both assemblages in the Aveiro Continental Shelf (Portugal). A total of 44 sediment samples was collected during summers of 1994 and 1995 along transects (east-west direction) and between 10 and 200 m water depth. Complex statistical analyses allow us to compare the abundance and composition of the LAs and DAs in function of depth, grain-size and total organic matter in all studied stations even in those where the numbers of individuals were rare in one or both assemblages. The highest densities and diversities of the LAs are found in the middle continental shelf on gravel deposits (coarse and very coarse sands) mostly due to the substrate stability, reduced deposition of fine sedimentary particles, availability of organic matter with high quality related to oceanic primary productivity likely induced by upwelling events, and oxygenated porewaters conditions. The DAs have, in general, higher densities and diversities than the LAs. In the outer continental shelf, the dissimilarity between both assemblages is higher due to the accumulation of tests, low dilution by sedimentary particles and scarcity of living foraminifera. Based on the comparison of LAs and DAs and considering the characteristics of the study area and the species ecology, it has been possible to understand the cause of temporal deviation between the LAs and DAs of benthic foraminifera. This deviation is much more pronounced in the inner shelf where the energy of the waves and the currents induce very dynamic sedimentary processes preventing the development of large LAs and the preservation of DAs. Some deviation also occurs in the middle shelf due to the seasonal loss of empty tests. The most well-preserved time-averaged DAs were found in the outer continental shelf.
Highlights
The evaluation of environmental quality represents a priority, in coastal and transitional marine settings where most of the anthropogenic activities are placed
Densities of dead assemblages (DAs) increase, in general, towards the deeper continental shelf zones correlated with an increase in fine-grained sediment and Total Organic Matter (TOM) due to reduced removal of foraminiferal tests by hydrodynamic activity and low ‘dilution’ of tests by terrigenous inputs at these depths
The low density of living foraminifera is found in shallower stations under the influence of the continental drift, due to sediments instability, and in the stations of the outer continental shelf and shelf break probably because of the lack of food quality, conditioned by the occurrence of weak upwelling events during months preceding the sampling campaigns
Summary
The evaluation of environmental quality represents a priority, in coastal and transitional marine settings where most of the anthropogenic activities are placed. In narrow shelves [3] as new compositional states present in LAs are not yet captured by DAs. A lower level of fidelity between living-dead molluscan assemblages was, for instance, documented in areas with enhanced anthropogenic eutrophication [1,3]. The deviation of LAs-DAs might represent a useful tool by which identifying the source of biases such as the environmental degradation and identifying areas suitable for paleoenvironmental and paleoecological reconstructions in both deep and recent time This actualistic approach has been applied in transitional environments like estuaries and lagoons as well as more open shelf setting and based on mollusk, ostracods, scallop and benthic foraminifera [1,2,3,4,5,6]. The LAs are widely used to determine the small-scale variations, the DAs might represent an important source of biological information such as the inventory of rare species and the estimation of anthropogenically shifted baselines [7]
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