Aquatic life depends on the tiniest of building blocks: dissolved organic matter (DOM). These dissolved molecules sustain the microbes and phytoplankton that form the foundation of aquatic food chains. The DOM in Earth’s waters holds about as much carbon as the atmosphere, and DOM can also be a major source of nitrogen and phosphorous, key ingredients in healthy aquatic ecosystems.
To track this major player in the carbon cycle, benchmark studies must monitor DOM’s status in sensitive coastal ecosystems. As the climate changes, scientists expect precipitation in areas such as the inland United States to also change, which will alter the delivery of organic matter by rivers out to the coasts. At the same time, rising sea levels will bring marine DOM toward the coast.
Letourneau and Medeiros examined DOM along Georgia’s coastline, focusing on the mouth of the Altamaha River and the Sapelo Sound estuary. The researchers collected surface samples each month for a year, running each sample through a battery of tests, including optical and molecular (ultrahigh-resolution mass spectrometry) analyses, to determine the DOM composition.
They found that DOM composition at both sites changed with the seasons. River flow increases over the winter, peaking in early spring. During high-flow times, DOM was made up of aromatic, earthy molecules eroded from terrestrial sources upstream, and the concentration of dissolved organic carbon increased. These changes were related not to local rainfall patterns but to the seasonal changes in river flow. The same pattern appeared in the estuary site, although the correlation wasn’t as strong.
During the rest of the year, when river discharge was lower, other environmental DOM sources became more important. In the river system, marshes contributed a greater percentage of DOM composition. In the estuary, lower river discharge meant greater influence of DOM flowing in from the sea.
After Hurricane Matthew, a category 4 storm that hit the East Coast in the fall of 2016, the scientists noticed that DOM followed the high-discharge patterns even though the storm occurred at a low-discharge time of year. The hurricane brought twice as much dissolved carbon as the year’s maximum, and terrestrial material again outstripped marsh and marine materials to become the dominant source of DOM.
Although local rainfall was not related to DOM, seasonal flow from inland sources and an extreme storm both boosted dissolved organic carbon levels and changed DOM composition along Georgia’s coast. The carbon bump also led to increased bacterial consumption during times of heavy river flow.
This study demonstrates that changing seasons and extreme weather affect DOM dynamics and microbial carbon processing along Georgia’s coast. (Journal of Geophysical Research: Biogeosciences, https://doi.org/10.1029/2018JG004982, 2019)
—Elizabeth Thompson, Freelance Writer