Mudbank off Alleppey, India: A bane for foraminifera but not so for carbon burial

Calm conditions and extensive fishing, during monsoon season in the mudbank off Alleppey (Kerala), India creates a unique environment, associated with high suspended particulate matter. The effect of processes associated with mudbank formation, on benthic foraminifera, however, has not been documented. We have studied, seasonal foraminiferal distribution, to understand foraminiferal response to physico-chemical changes associated with the mudbank formation. Additionally, seasonal changes in total carbon, calcium carbonate (CaCO₃), organic carbon (C_org) and C_org/nitrogen (C_org/N) were also measured to understand the effect of mudbank formation on carbon burial. We report a low foraminiferal abundance in the mudbank. Benthic foraminiferal diversity is also low in the mudbank, during both pre-monsoon and monsoon season, clearly suggesting a stressed environment. Agglutinated foraminifera dominate the living benthic foraminiferal population in the mudbank, suggesting that the area is carbonate undersaturated and under fresh-water influence. Ammobaculites dilatatus and Ammobaculites exiguus are the dominant agglutinated species abundant in the mudbank and thus can be used to reconstruct past changes in the mudbank. The CaCO₃ is consistently low during all seasons, at one of the core mudbank stations. The %C_org is, however, higher in the core mudbank as well as the northern peripheral region. The C_org/N is consistently uniform at all the stations indicating a similar source of organic matter in all the seasons. The higher %C_org and constant C_org/N suggest, that food availability and its source is not a major factor affecting benthic foraminifera in the mudbank. Instead, increased turbidity and low bottom water salinity are the main cause of seasonally stressed environment in the mudbank. Additionally, C_org degradation coupled with fresh water influx induced drop in bottom water pH is responsible for low foraminiferal population in mudbank region, in all the seasons. The reduced calcareous benthic foraminiferal abundance, however, does not affect the carbon burial in the mudbank, due to higher %C_org.

The prediction of PAHs bioavailability in soils using chemical methods: state of the art and future challenges

The evaluation of the available fraction of hydrophobic organic contaminants (HOCs) is extremely important for assessing their risk to the environment and human health. This available fraction, which can be solubilized and/or easily extracted, is believed to be the most accessible for bioaccumulation, biosorption and/or transformation by organisms. Based on this, two main types of chemical methods have been developed, closely related to the concepts of bioaccessibility and freely available concentrations: non-exhaustive extractions and biomimetic methods. Since bioavailability is species and compound specific, this work focused only in one of the most widespread group of HOCs in soils: polycyclic aromatic hydrocarbons (PAHs). This study aims at producing a state of the art knowledge base on bioavailability and chemical availability of PAHs in soils, clarifying which chemical methods can provide a better prediction of an organism exposure, and which are the most promising ones. Therefore, a review of the processes involved on PAHs availability to microorganisms, earthworms and plants was performed and the outputs given by the different chemical methods were evaluated. The suitability of chemical methods to predict bioavailability of the 16 US EPA PAHs in dissimilar naturally contaminated soils was not yet demonstrated, being especially difficult for high molecular weight compounds. Even though the potential to predict microbial mineralization using non-exhaustive extractions is promising, it will be very difficult to achieve for earthworms and plants, due to the complexity of accumulation mechanisms which are not taken into account by chemical methods. Yet, the existing models could be improved by determining compound, species and site specific parameters. Moreover, chemical availability can be very useful to understand the bioavailability processes and the behavior of PAHs in soils. The inclusion of chemical methods on risk assessment has been suggested and it is promising, despite some methods overpredict risks.