Benthic zone

The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning "the depths." Organisms living in this zone are called benthos and include microorganisms (e.g., bacteria and fungi) as well as larger invertebrates, such as crustaceans and polychaetes. Organisms here generally live in close relationship with the substrate and many are permanently attached to the bottom. The benthic boundary layer, which includes the bottom layer of water and the uppermost layer of sediment directly influenced by the overlying water, is an integral part of the benthic zone, as it greatly influences the biological activity that takes place there. Examples of contact soil layers include sand bottoms, rocky outcrops, coral, and bay mud. Description Oceans The benthic region of the ocean begins at the shore line (in

Identifying contaminated sediment problems associated with urban stormwater discharges in lakes

Tom Hugo Antoine Benejam

Urban stormwater discharges during wet weather has always been a sensitive issue to deal with. By inducing combined sewer overflows, substantial impacts on receiving water are commonly feared. Aware of this problematic situation, the VSA already created the STORM guideline to cope with this issue in small rivers. However, such a guideline is still lacking for large rivers and lakes. This project is a first step towards this direction and tries to prospect the necessity of such instructions for lakes. For this purpose, the present master thesis studies the impact of urban stormwater discharged via the Flon river into the Vidy Bay of Lake Geneva, next to the city of Lausanne. A Sediment Quality Triad, meaning a combination of three assessment methods (chemistry, ecotoxicology and in situ benthic communities), was implemented to assess the impact of stormwater discharges on the sediments in this part of the lake. On a large sampling grid composed of 15 sites, heavy metals were measured and ostracod bioassays were performed with the sediment samples collected at these sites. Subsequently, on the five sites located in the central transect of this sampling grid, further experiments were led to understand the incidence of stormwater discharges on sediment quality while moving away from the Flon river’s effluent. In addition to heavy metals, PAHs and PCBs were measured at these sites. A battery of three extra ecotoxicological bioassays was also carried out. These tests were implemented with the following organisms: chironomids, macrophytes and nematodes. Finally, two kind of in situ communities were studied in these five sites: microbial communities in order to perform a pollution-induced community tolerance (PICT) and oligochaete communities in order to implement the oligochaete index of lake bioindication (IOBL). The implementation of these experiments showed that the contamination induced by urban stormwater discharges is limited to the proximity of the combined sewer overflow. A set of pollutants mainly composed of copper, zinc, PCBs and PAHs was identified within this contaminated area. Although no toxicity was observed on the sites affected by this contamination, in situ community studies revealed substantial shifts towards resistant species in both oligochaete and microbial community structures. Another contamination area, observed in remote sites and mainly composed of cobalt, chromium, iron, manganese and nickel, remains not well understood. The laboratory toxicity tests detected some toxicity in benthic invertebrates but the benthic communities studied in situ showed no apparent alteration. Modeling tools are required to get a better understanding of the source of this contamination.

2017

Responses of reef bioindicators to recent temperature anomalies in distinct areas of the North Ari and Rasdhoo atolls (Maldives)

Arthur Adams, Aurélie Angéloz, Anaël Lehmann

Assessments of reef sediments in the North Ari Atoll (Maldives) were conducted in 2015 and 2018 on reefs of three islands with different management strategies: community, resort, and uninhabited. Indices applied were the Foraminifera in Reef Assessment and Monitoring Index (FI) and the Sediment Constituents Index (SI). Both indices are based on shells or fragments of functional groups, which for the FI are foraminiferal shells and for the SI are sediment components. The FI is considered to be an indicator of water quality and the SI an indicator of water quality, community structure, and processes such as grazing and bioerosion. Both indices indicated that environmental deterioration occurred between 2015 and 2018, likely related to the intense temperature anomaly in March–June 2016 that caused widespread coral bleaching and mortality. Median FI declined from 5.1 to 4.0 overall, indicating that water quality still supports reef accretion, though the replacement of coral cover by algae and sponges likely provides more food sources for smaller, faster-growing foraminiferal species. The median SI values similarly declined from 3.8 to 3.0, reflecting a decrease in identifiable coral fragments and an increase in unidentifiable clasts, likely indicative of increased bioerosion. Although a minor component, molluscan fragments also increased by 25%, likely in response to more algal cover for grazers. In 2015, the FI and SI data indicated that the island management regime contributed to the reef health status. Uninhabited islands were associated with higher indices compared to resort and community islands. A clear distinction between management regimes was not observed in 2018, because a major decrease in FI (median: 4.9 in 2015, 2.9 in 2018) was recorded offshore from an agricultural settlement on the previously “uninhabited” island surveyed. These observations support the usefulness of these indices in reef assessment, and provide additional understanding that the FI can respond to a coral-mortality event that alters food sources in the benthic community.

2020

Responses of reef bioindicators to recent temperature anomalies in distinct areas of the North Ari and Rasdhoo atolls (Maldives)

Arthur Adams, Aurélie Angéloz, Anaël Lehmann

2020

Identifying contaminated sediment problems associated with urban stormwater discharges in lakes

Tom Hugo Antoine Benejam

2017

Identifying contaminated sediment problems associated with urban stormwater discharges in lakes

Responses of reef bioindicators to recent temperature anomalies in distinct areas of the North Ari and Rasdhoo atolls (Maldives)

Benthic communities can inform about the contamination of large lakes surface sediments: A case-study in Lake Geneva

Responses of reef bioindicators to recent temperature anomalies in distinct areas of the North Ari and Rasdhoo atolls (Maldives)

Identifying contaminated sediment problems associated with urban stormwater discharges in lakes

Benthic communities can inform about the contamination of large lakes surface sediments: A case-study in Lake Geneva