Benthic foraminifera biomass on the continental slope and São Paulo plateau of Santos Basin (SE Brazil): Comparison of different estimation methods

Abstract

The biomass of benthic foraminifera stands out in marine environments compared to other meiofaunal organisms. Estimating the biomass of these organisms is a valuable biotic descriptor for understanding the carbon cycle and the flow of particulate organic matter. This study estimates the biomass of benthic foraminifera on the slope and plateau of the Santos Basin (SE Brazil) using the biovolume and cytoplasmic occupancy methodology. This work applies a non-destructive biovolume method of estimating biomass by the direct visualization of cytoplasmic occupancy (DV) compared with pre-established (in the literature) percentages of cytoplasmic occupation of the test volume: an average of 32 % (GE) and 35 % (AL). Although there was no statistically significant difference (p < 0.05) between DV and cytoplasmic percentages, it is recognized that the calculation of GE and AL can bias the results. We, therefore, emphasize the need to assess biomass according to species composition and ultrastructural variability. The species with the biggest individuals (>250 μm) contributed most to biomass in the Santos Basin and are considered k-strategists. In contrast, the species with smaller specimens (250–63 μm) are considered r-strategists, contributing more to the density in the study area. This study highlights the discrepancy between the abundant species biomass results by measuring all specimens and the extrapolation of the population for less sample effort, probably due to biological (e.g. intra/inter species variations) and environmental (e.g. availability of food and oxygen) factors. Based on the ecology of the species, the biomass estimates seem to reveal a relationship with the quantity and quality of organic matter, which will be investigated in the future. Future studies should consider the internal structures and address potential errors resulting from cytoplasm dehydration when staining and drying are performed.