Selenium Distribution and Cycling in the Eastern Equatorial Pacific Ocean

Oxygen minimum zones in oceanic waters have become increasingly important to the marine environment and society. Low oxygen waters affect not only the distribution and abundance of marine organisms, but also impact the solubility and transport of trace elements that are of biological importance, with the chemical speciation and solubility depending on the actual redox poise of the waters. One redox sensitive trace element of interest is selenium, which can be both toxic and essential for organisms, depending on its chemical speciation. In 2013, the US GEOTRACES program completed the GP16 transect from Peru to Tahiti, going through the oxygen minimum zone off Peru. Dissolved selenate, selenite, and organic selenide, as well as particulate elemental selenium, were determined in water column samples. Nitrate and nitrite data were used to determine where denitrification was occurring and thus approximately where dissimilatory reduction of selenium should be occurring. Deficits in dissolved selenite+selenate showed that dissimilatory reduction of selenium (bacterial utilization of an oxidized metal as the terminal electron acceptor during respiration) occurred within the oxygen minimum zone at slightly deeper depths than where denitrification was found. The observed selenium deficits can be the result of dissimilatory reduction occurring in situ or can be laterally advected from anoxic coastal sediments. However, using a combination of advection/diffusion modeling, particulate Se(0) data, and 234Th-derived flux rates, the dissimilatory reduction of selenium was shown to mainly occur in situ, rather than via advection/diffusion from coastal sediments

Interactions between Zooplankton and Crude Oil: Toxic Effects and Bioaccumulation of Polycyclic Aromatic Hydrocarbons

We conducted ship-, shore- and laboratory-based crude oil exposure experiments to investigate (1) the effects of crude oil (Louisiana light sweet oil) on survival and bioaccumulation of polycyclic aromatic hydrocarbons (PAHs) in mesozooplankton communities, (2) the lethal effects of dispersant (Corexit 9500A) and dispersant-treated oil on mesozooplankton, (3) the influence of UVB radiation/sunlight exposure on the toxicity of dispersed crude oil to mesozooplankton, and (4) the role of marine protozoans on the sublethal effects of crude oil and in the bioaccumulation of PAHs in the copepod Acartia tonsa. Mortality of mesozooplankton increased with increasing oil concentration following a sigmoid model with a median lethal concentration of 32.4 ml L21 in 16 h. At the ratio of dispersant to oil commonly used in the treatment of oil spills (i.e. 1:20), dispersant (0.25 ml L21 ) and dispersant- treated oil were 2.3 and 3.4 times more toxic, respectively, than crude oil alone (5 ml L21 ) to mesozooplankton. UVB radiation increased the lethal effects of dispersed crude oil in mesozooplankton communities by 35%. We observed selective bioaccumulation of five PAHs, fluoranthene, phenanthrene, pyrene, chrysene and benzo[b]fluoranthene in both mesozooplankton communities and in the copepod A. tonsa. The presence of the protozoan Oxyrrhis marina reduced sublethal effects of oil on A. tonsa and was related to lower accumulations of PAHs in tissues and fecal pellets, suggesting that protozoa may be important in mitigating the harmful effects of crude oil exposure in copepods and the transfer of PAHs to higher trophic levels. Overall, our results indicate that the negative impact of oil spills on mesozooplankton may be increased by the use of chemical dispersant and UV radiation, but attenuated by crude oil-microbial food webs interactions, and that both mesozooplankton and protozoans may play an important role in fate of PAHs in marine environments.Zoe Wambaugh was supported by the National Science Foundation (NSF) Research Experiences for Undergraduates (REU) program (grant OCE- 1062745). This research was made possible by a grant from BP/The Gulf of Mexico Research Initiative through the University of Texas Marine Science Institute (DROPPS consortium: ‘Dispersion Research on Oil: Physics and Plankton Studies’). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.Marine Scienc

Effects of crude oil exposure on bioaccumulation of polycyclic aromatic hydrocarbons and survival of adult and larval stages of gelatinous zooplankton.

Gelatinous zooplankton play an important role in marine food webs both as major consumers of metazooplankton and as prey of apex predators (e.g., tuna, sunfish, sea turtles). However, little is known about the effects of crude oil spills on these important components of planktonic communities. We determined the effects of Louisiana light sweet crude oil exposure on survival and bioaccumulation of polycyclic aromatic hydrocarbons (PAHs) in adult stages of the scyphozoans Pelagia noctiluca and Aurelia aurita and the ctenophore Mnemiopsis leidyi, and on survival of ephyra larvae of A. aurita and cydippid larvae of M. leidyi, in the laboratory. Adult P. noctiluca showed 100% mortality at oil concentration ≥20 µL L(-1) after 16 h. In contrast, low or non-lethal effects were observed on adult stages of A. aurita and M. leidyi exposed at oil concentration ≤25 µL L(-1) after 6 days. Survival of ephyra and cydippid larva decreased with increasing crude oil concentration and exposition time. The median lethal concentration (LC50) for ephyra larvae ranged from 14.41 to 0.15 µL L(-1) after 1 and 3 days, respectively. LC50 for cydippid larvae ranged from 14.52 to 8.94 µL L(-1) after 3 and 6 days, respectively. We observed selective bioaccumulation of chrysene, phenanthrene and pyrene in A. aurita and chrysene, pyrene, benzo[a]pyrene, benzo[b]fluoranthene, benzo[k]fluoranthene, and benzo[a]anthracene in M. leidyi. Overall, our results indicate that (1) A. aurita and M. leidyi adults had a high tolerance to crude oil exposure compared to other zooplankton, whereas P. noctiluca was highly sensitive to crude oil, (2) larval stages of gelatinous zooplankton were more sensitive to crude oil than adult stages, and (3) some of the most toxic PAHs of crude oil can be bioaccumulated in gelatinous zooplankton and potentially be transferred up the food web and contaminate apex predators

Concentration of PAHs in body tissues (A, B), fecal pellets (C, D) and eggs (E, F) of <i>Acartia tonsa</i> feeding on <i>Rhodomonas</i> sp.(left column) or <i>Rhodomonas</i> sp. plus <i>Oxyrrhis</i> marina (right column).

<p>Experimental: copepods exposed to oil (5 µl L⁻¹). Control: non-exposed copepods. The asterisks indicate the PAH was not detected.</p

Bioaccumulation factors of PAHs in natural mesozooplankton communities from the northern Gulf of Mexico (Stations A, B and MRM) exposed to different concentrations of crude oil.

<p>Naphthalene (Nap), phenanthrene (Phe), fluoranthene (Flua), pyrene (Pyr), chrysene (Chr), benzo[b]fluoranthene (BbF). The hash symbol indicates that BAF were similar or lower than respective control treatments (non-exposed copepods). <i>n.d.</i> = no detected.</p

Relationships between mortality (%) of ephyra larvae of the scyphozoan <i>Aurelia aurita</i> and crude oil concentration after 24 (A), 48 (B) and 72 (C) hours of exposure.

<p>Regression lines based on <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0074476#pone.0074476.e001" target="_blank">Equation (1)</a>.</p

Effect of crude oil exposure (5 µl L⁻¹, 48 h, dim light) on egg production rates, egg hatching and fecal pellet production rates of <i>Acartia tonsa</i> feeding on <i>Rhodomonas</i> sp. (left column, A, C, E) or <i>Rhodomonas</i> sp. plus <i>Oxyrrhis</i> marina (right column, B, D, F).

<p>Experimental: oil exposed copepods. Control: non-exposed copepods. Error bars represent the standard deviations.</p

Concentration of the main polycyclic aromatic hydrocarbons (PAHs) detected in adult stages of ctenophore <i>Mnemiopsis leidyi</i> after 6 days of exposure to different crude oil concentrations.

<p>(Control: no oil, E1: 1 µL L⁻¹, E2: 5 µL L⁻¹, E3: 25 µL L⁻¹.)</p

Relationships between median lethal concentration (<i>LC₅₀</i>, µL L⁻¹) and incubation time (<i>t</i>, hours) of ephyra larvae of the scyphozoan <i>Aurelia aurita</i> (A) and cydippid larvae of the ctenophore <i>Mnemiopsis leidyi</i> (B) exposed to crude oil.

<p>Relationships between median lethal concentration (<i>LC₅₀</i>, µL L⁻¹) and incubation time (<i>t</i>, hours) of ephyra larvae of the scyphozoan <i>Aurelia aurita</i> (A) and cydippid larvae of the ctenophore <i>Mnemiopsis leidyi</i> (B) exposed to crude oil.</p

Total concentration of the polycyclic aromatic hydrocarbons (total PAHs) detected in adult stages of scyphozoan <i>Aurelia aurita</i> (A) and the ctenophore <i>Mnemiopsis leidyi</i> (B) after 6 days of exposure to different concentrations of crude oil.

<p>(Control: no oil, E1: 1 µL L⁻¹, E2: 5 µL L⁻¹, E3: 25 µL L⁻¹.) Error bars represent the standard errors.</p