Bioaccumulation and Trophic Transfer of Mercury and Selenium in African Sub-Tropical Fluvial Reservoirs Food Webs (Burkina Faso)

<div><p>The bioaccumulation and biomagnification of mercury (Hg) and selenium (Se) were investigated in sub-tropical freshwater food webs from Burkina Faso, West Africa, a region where very few ecosystem studies on contaminants have been performed. During the 2010 rainy season, samples of water, sediment, fish, zooplankton, and mollusks were collected from three water reservoirs and analysed for total Hg (THg), methylmercury (MeHg), and total Se (TSe). Ratios of δ¹³C and δ¹⁵N were measured to determine food web structures and patterns of contaminant accumulation and transfer to fish. Food chain lengths (FCLs) were calculated using mean δ¹⁵N of all primary consumer taxa collected as the site-specific baseline. We report relatively low concentrations of THg and TSe in most fish. We also found in all studied reservoirs short food chain lengths, ranging from 3.3 to 3.7, with most fish relying on a mixture of pelagic and littoral sources for their diet. Mercury was biomagnified in fish food webs with an enrichment factor ranging from 2.9 to 6.5 for THg and from 2.9 to 6.6 for MeHg. However, there was no evidence of selenium biomagnification in these food webs. An inverse relationship was observed between adjusted δ¹⁵N and log-transformed Se:Hg ratios, indicating that Se has a lesser protective effect in top predators, which are also the most contaminated animals with respect to MeHg. Trophic position, carbon source, and fish total length were the factors best explaining Hg concentration in fish. In a broader comparison of our study sites with literature data for other African lakes, the THg biomagnification rate was positively correlated with FCL. We conclude that these reservoir systems from tropical Western Africa have low Hg biomagnification associated with short food chains. This finding may partly explain low concentrations of Hg commonly reported in fish from this area.</p></div

Relationship between the TMF of THg in fish food webs of African water bodies and FCL.

<p>Relationship between the TMF of THg in fish food webs of African water bodies and FCL.</p

Mean (±sd) total length, THg, MeHg, total selenium and molar ratio TSe/THg, %MeHg, δ¹⁵N (‰), δ¹³C (‰) and trophic position (TP) of fish from three freshwater reservoirs (Burkina Faso).

<p>Abbreviations: n1 is the sample size for THg, TSe analyses, n2 is the selected sample for MeHg analysis. TL refers to total length of fish.</p><p>Mean (±sd) total length, THg, MeHg, total selenium and molar ratio TSe/THg, %MeHg, δ¹⁵N (‰), δ¹³C (‰) and trophic position (TP) of fish from three freshwater reservoirs (Burkina Faso).</p

A comparison of THg biomagnification rates in different types of African water bodies.

<p>A comparison of THg biomagnification rates in different types of African water bodies.</p

Map of study areas showing locations of the three reservoirs in Burkina Faso.

<p>Source: Base Nationale de Données Topographiques (BNDT), 2000 and Base de Données d’Occupation des Terres (BDOT), 2002 of Burkina Faso.</p

Frequency distribution of FCL measured in THg biomagnification studies for African water bodies.

<p>The global mean (± 1 standard deviation) of food chain length in lakes is provided as a reference (Vander Zanden and Fetzer 2007).</p

Food web structure of three freshwater reservoirs from Burkina Faso.

<p>The ratio of δ¹⁵N, indicating trophic position, and δ¹³C indicating dietary carbon source of biota in the freshwater reservoirs. Error bars = ± 1 standard deviation.</p

Microbial Community Structure in Lake and Wetland Sediments from a High Arctic Polar Desert Revealed by Targeted Transcriptomics

<div><p>While microbial communities play a key role in the geochemical cycling of nutrients and contaminants in anaerobic freshwater sediments, their structure and activity in polar desert ecosystems are still poorly understood, both across heterogeneous freshwater environments such as lakes and wetlands, and across sediment depths. To address this question, we performed targeted environmental transcriptomics analyses and characterized microbial diversity across three depths from sediment cores collected in a lake and a wetland, located on Cornwallis Island, NU, Canada. Microbial communities were characterized based on 16S rRNA and two functional gene transcripts: <i>mcrA</i>, involved in archaeal methane cycling and <i>glnA</i>, a bacterial housekeeping gene implicated in nitrogen metabolism. We show that methane cycling and overall bacterial metabolic activity are the highest at the surface of lake sediments but deeper within wetland sediments. Bacterial communities are highly diverse and structured as a function of both environment and depth, being more diverse in the wetland and near the surface. Archaea are mostly methanogens, structured by environment and more diverse in the wetland. <i>McrA</i> transcript analyses show that active methane cycling in the lake and wetland corresponds to distinct communities with a higher potential for methane cycling in the wetland. <i>Methanosarcina</i> spp., <i>Methanosaeta</i> spp. and a group of uncultured Archaea are the dominant methanogens in the wetland while <i>Methanoregula</i> spp. predominate in the lake.</p></div

Rarefaction curves for bacterial and archaeal sequences.

<p>Results for two environments are shown: (A) lake; (B) wetland. Dotted lines represent the 95% confidence intervals, as computed by resampling in MOTHUR.</p

Maximum likelihood tree of bacterial 16S rRNA sequences.

<p>Major phyla, represented by triangles whose area is proportional to the number of sequences, were tested for lineage-specific differences using UniFrac. The lineage-specific analysis tests for each lineage whether the sequences have a different distribution among environments than does the tree overall and therefore highlight which lineage contributed to the differences observed (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0089531#pone-0089531-g003" target="_blank">Figure 3</a>). Nodes A-D are significantly unevenly distributed between environments; <i>p</i>-values are: <i>p</i>_A = 1.1×10⁻⁵, <i>p</i>_B = 2.8×10⁻³, <i>p</i>_C = 5.8×10⁻⁴, <i>p</i>_D = 1.3×10⁻³, <i>p</i>_E = 1.0×10⁻², <i>p</i>_F = 1.1×10⁻¹⁹. Pie charts connected to these nodes represent the distribution of environments within each lineage. Numbers adjacent to each node represent aLRT statistics (SH-like supports); only support values >0.50 are shown. Scale bar for branch lengths (expected number of substitutions per site) is shown.</p