Research

My research explores biogeochemical connections between land and water at landscape to macrosystem scales.  I use a combination of field/lab methods, satellite remote sensing, and GIS analyses to examine ecosystem processes of estuaries, rivers, and lakes. My CV is available here. If you have questions about my research, please don't hesitate to contact me!

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DISSOLVED ORGANIC MATTER IN the ARCTIC 

The Arctic is rapidly changing in response to climate warming. Aquatic systems and estuaries dominate much of the Arctic, and are vital to understanding carbon cycling across the entire region. In the Arctic, my research uses satellite remote sensing and laboratory analyses to address three questions: Where does riverine organic matter (OM) come from? What is the fate of OM as it moves from headwaters to estuaries? How are OM dynamics changing in response to climate?

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REMOTE SENSING OF WATER QUALITY IN TEMPERATE LAKES

The Upper Midwest (MN, MI, WI) is dotted with tens of thousands of lakes of different colors, trophic states, sizes, and watershed features.  Remote sensing allows us to map colored dissolved organic matter (CDOM) across the region, and use that data to explore watershed controls on lake chemistry; trends in lakes DOM across Minnesota, associated with climate, atmospheric chemistry, and land use; and model lake DOM storage.

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STORM-DRIVEN NITROGEN EXPORT TO TEXAN ESTUARIES

Nitrogen from agricultural run-off and wastewater is a major driver of estuarine ecosystems. In coastal Texas, alternating wet-dry years results in large variations in nitrogen delivery from rivers.

 
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Aquatic Organic Matter in the Arctic

Remote SEnsing of DOM

Colored dissolved organic matter (CDOM) is a useful proxy for dissolved organic carbon in Arctic rivers, an important part of the carbon cycle and source of energy and nutrients to aquatic and estuarine microbes. I built remote sensing models for six large Arctic rivers to estimate CDOM (Griffin et al., 2018), then applied those models to historical imagery from 1985 - 2017 (Griffin, 2016).  Time series analyses showed increased discharge-normalized CDOM in the Ob' River, where warming likely led to DOM mobilization from extensive peatlands. The adjacent Yenisey River, in contrast, decreased in discharge-normalized CDOM. The Yenisey lacks the deep, extensive peatlands on the Ob', and as permafrost thawed, hydrological flowpaths deepened and flowed through more mineral soils.

River export of OM

I have been lucky to be involved with the Arctic Great Rivers Observatory, which provided the data used for my remote sensing methods. Methodologically-consistent, seasonally explicit sampling of large river mouths allows researchers to quantify how much terrestrial material enters the Arctic Ocean, and use biogeochemical signals as indicators of widespread change throughout a watershed. Through Arctic-GRO, I have contributed to work on fluxes of particulate organic matter (POM), addressing the variability in export driven by both seasonal and annual hydrology (McClelland et al., 2016). Organic matter in Arctic rivers generally increases in age by the end of the ice-free season. Radiocarbon dating of lignins demonstrated that such ancient pools of carbon are far more prevalent in the Kolyma River than the Mackenzie River (Feng et al., 2017).

 

 
 
 
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CDOM in Midwestern Lakes

Watershed controls on Lake chemistry

Our group on Remote Sensing of Water Resources at University of Minnesota is working to produce maps of remotely sensed CDOM, chlorophyll-a, and suspended sediment in 10,000+ lakes across MN and parts of WI and MI. With such a rich dataset, I am exploring what watershed characteristics control the wide variety of lake types within this geographic region. Wetlands and land cover, lake connectivity, and lake morphology all influence CDOM, and can limit the reliability of CDOM as proxy for DOC (Griffin et al., in review).

Time Series analysis of CDOM

Using Google Earth Engine, our remote sensing algorithms developed recently can be applied to historical Landsat imagery dating to the 1980s. Other boreal and north temperate regions, particularly the Northeast USA and Scandinavia, have shown increases in DOC and CDOM over the past thirty years, but this has not been well constrained for the Upper Midwest. Preliminary time series analyses show little unidirectional change in CDOM in our study area, however, possibly owing to differences in acid rain impacts and climatic change compared to the NE and Scandinavia.

 
 
 
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Nitrogen in Texas Rivers

Storm Driven Nitrogen Export

Water quantity in Texas rivers is dominated by two factors: reservoirs and storms. While the Texas legislature has mandated episodic releases of water from reservoirs to help maintain salinity regimes in estuaries, the difference in nutrient export from these releases and more "natural" storm events are poorly understood. We studied six rivers across a climatic and land use gradient in coastal Texas to examine these two types of hydrological events. Storm events, especially in small rivers, dominated both water and nitrogen export (Griffin and McClelland, in prep) except in the Nueces River, which has a reservoir very low in the watershed.

climate, land use, and estuarine production

The data collected along these six rivers was part of a larger project linking climate, land use change, river routing (Tavakoly et al., 2016), and nitrogen export as drivers of estuarine productivity. The holistic approach of this project aimed to integrate  climate, hydrological, and productivity models from land to sea.