Geology topics

Michael D. DeGrandpre

Publications and source records attributed to Michael D. DeGrandpre.

3 recordsLinked to original sources

USGS2023

Increasing hypoxia on global coral reefs under ocean warming

Ocean deoxygenation is predicted to threaten marine ecosystems globally. However, current and future oxygen concentrations and the occurrence of hypoxic events on coral reefs remain underexplored. Here, using autonomous sensor data to explore oxygen variability and hypoxia exposure at 32 representative reef sites, we reveal that hypoxia is already pervasive on many reefs. Eighty-four percent of reefs experienced weak to moderate (≤153 µmol O 2 kg −1 to ≤92 µmol O 2 kg −1 ) hypoxia and 13% experienced severe (≤61 µmol O 2 kg −1 ) hypoxia. Under different climate change scenarios based on four Shared Socioeconomic Pathways (SSPs), we show that projected ocean warming and deoxygenation will increase the duration, intensity and severity of hypoxia, with more than 94% and 31% of reefs experiencing weak to moderate and severe hypoxia, respectively, by 2100 under SSP5-8.5. This projected oxygen loss could have negative consequences for coral reef taxa due to the key role of oxygen in organism functioning and fitness.

Nature Climate Change↗

USGS2017

Carbonate buffering and metabolic controls on carbon dioxide in rivers

Multiple processes support the significant efflux of carbon dioxide (CO 2 ) from rivers and streams. Attribution of CO 2 oversaturation will lead to better quantification of the freshwater carbon cycle and provide insights into the net cycling of nutrients and pollutants. CO 2 production is closely related to O 2 consumption because of the metabolic linkage of these gases. However, this relationship can be weakened due to dissolved inorganic carbon inputs from groundwater, carbonate buffering, calcification, and anaerobic metabolism. CO 2 and O 2 concentrations and other water quality parameters were analyzed in two data sets: a synoptic field study and nationwide water quality monitoring data. CO 2 and O 2 concentrations were strongly negatively correlated in both data sets ( ρ = −0.67 and ρ = −0.63, respectively), although the correlations were weaker in high-alkalinity environments. In nearly all samples, the molar oversaturation of CO 2 was a larger magnitude than molar O 2 undersaturation. We used a dynamically coupled O 2 CO 2 model to show that lags in CO 2 air-water equilibration are a likely cause of this phenomenon. Lags in CO 2 equilibration also impart landscape-scale differences in the behavior of CO 2 between high- and low-alkalinity watersheds. Although the concept of carbonate buffering and how it creates lags in CO 2 equilibration with the atmosphere is well understood, it has not been sufficiently integrated into our understanding of CO 2 dynamics in freshwaters. We argue that the consideration of carbonate equilibria and its effects on CO 2 dynamics are primary steps in understanding the sources and magnitude of CO 2 oversaturation in rivers and streams.

Global Biogeochemical Cycles↗

USGS2005

Biogeochemical controls on diel cycling of stable isotopes of dissolved 02 and dissolved inorganic carbon in the Big Hole River, Montana

Rivers with high biological productivity typically show substantial increases in pH and dissolved oxygen (DO) concentration during the day and decreases at night, in response to changes in the relative rates of aquatic photosynthesis and respiration. These changes, coupled with temperature variations, may impart diel (24-h) fluctuations in the concentration of trace metals, nutrients, and other chemical species. A better understanding of diel processes in rivers is needed and will lead to improved methods of data collection for both monitoring and research purposes. Previous studies have used stable isotopes of dissolved oxygen (DO) and dissolved inorganic carbon (DIC) as tracers of geochemical and biological processes in streams, lakes, and marine systems. Although seasonal variation in δ 18 O of DO in rivers and lakes has been documented, no study has investigated diel changes in this parameter. Here, we demonstrate large (up to 13‰) cycles in δ 18 O-DO for two late summer sampling periods in the Big Hole River of southwest Montana and illustrate that these changes are correlated to variations in the DO concentration, the C-isotopic composition of DIC, and the primary productivity of the system. The magnitude of the diel cycle in δ 18 O-DO was greater in August versus September because of the longer photoperiod and warmer water temperatures. This study provides another biogeochemical tool for investigating the O 2 and C budgets in rivers and may also be applicable to lake and groundwater systems.

Montana↗