Geology topics

Andrew M. Graham

Publications and source records attributed to Andrew M. Graham.

3 recordsLinked to original sources

USGS2021

Rebounds, regresses, and recovery: A 15-year study of the coral reef community at Pila‘a, Kaua‘i after decades of natural and anthropogenic stress events

Pila‘a reef on the north shore of Kaua‘i, Hawai‘i was subjected to a major flood event in 2001 that deposited extensive sediment on the reef flat, resulting in high coral mortality. To document potential recovery, this study replicated benthic and sediment surveys conducted immediately following the event and 15 years later. Coral cores were analyzed to determine coral growth rates and density. Our results suggest that significant reduction in terrigenous sediments has led to partial ecosystem recovery based on coral species and colony increases, more balanced size frequency distributions, improved coral condition, and enhanced coral recruitment despite lack of recovery of large dead coral colonies. However, within this 15-year period, episodic storms and a bleaching event impeded the recovery process, preventing full recovery and continuously threatening the coral reef community. As climate change progresses, the intensity and frequency of these disturbances are predicted to increase.

Hawaii↗

USGS2013

Effect of dissolved organic matter source and character on microbial Hg methylation in Hg–S–DOM solutions

Dissolved organic matter (DOM) is a key component of fate and transport models for most metals, including mercury (Hg). Utilizing a suite of diverse DOM isolates, we demonstrated that DOM character, in addition to concentration, influences inorganic Hg (Hg(II) i ) bioavailability to Hg-methylating bacteria. Using a model Hg-methylating bacterium, Desulfovibrio desulfuricans ND132, we evaluated Hg-DOM-sulfide bioavailability in washed-cell assays at environmentally relevant Hg/DOM ratios (∼1–8 ng Hg/mg C) and sulfide concentrations (1–1000 μM). All tested DOM isolates significantly enhanced Hg methylation above DOM-free controls (from ∼2 to >20-fold for 20 mg C/L DOM solutions), but high molecular weight/highly aromatic DOM isolates and/or those with high sulfur content were particularly effective at enhancing Hg methylation. Because these experiments were conducted under conditions of predicted supersaturation with respect to metacinnabar (β-HgS(s)), we attribute the DOM-dependent enhancement of Hg(II) i bioavailability to steric and specific chemical (e.g., DOM thiols) inhibition of β-HgS(s) growth and aggregation by DOM. Experiments examining the role of DOM across a wide sulfide gradient revealed that DOM only enhances Hg methylation under fairly low sulfide conditions (≲30 μM), conditions that favor HgS nanoparticle/cluster formation relative to dissolved HgS species.

Environmental Science & Technology↗

USGS2012

Dissolved organic matter enhances microbial mercury methylation under sulfidic conditions

Dissolved organic matter (DOM) is generally thought to lower metal bioavailability in aquatic systems due to the formation of metal–DOM complexes that reduce free metal ion concentrations. However, this model may not be pertinent for metal nanoparticles, which are now understood to be ubiquitous, sometimes dominant, metal species in the environment. The influence of DOM on Hg bioavailability to microorganisms was examined under conditions (0.5–5.0 nM Hg and 2–10 μM sulfide) that favor the formation of β-HgS(s) (metacinnabar) nanoparticles. We used the methylation of stable-isotope enriched 201 HgCl 2 by Desulfovibrio desulfuricans ND132 in short-term washed cell assays as a sensitive, environmentally significant proxy for Hg uptake. Suwannee River humic acid (SRHA) and Williams Lake hydrophobic acid (WLHPoA) substantially enhanced (2- to 38-fold) the bioavailability of Hg to ND132 over a wide range of Hg/DOM ratios (9.4 pmol/mg DOM to 9.4 nmol/mg DOM), including environmentally relevant ratios. Methylmercury (MeHg) production by ND132 increased linearly with either SRHA or WLHPoA concentration, but SRHA, a terrestrially derived DOM, was far more effective at enhancing Hg-methylation than WLHPoA, an aquatic DOM dominated by autochthonous sources. No DOM-dependent enhancement in Hg methylation was observed in Hg–DOM–sulfide solutions amended with sufficient l -cysteine to prevent β-HgS(s) formation. We hypothesize that small HgS particles, stabilized against aggregation by DOM, are bioavailable to Hg-methylating bacteria. Our laboratory experiments provide a mechanism for the positive correlations between DOC and MeHg production observed in many aquatic sediments and wetland soils.

Environmental Science & Technology↗