Organic carbon transport as a key process impacting aquatic ecosystem services

Many of the ESDL’s process-based studies focus on understanding organic carbon transport, for several reasons. First, organic carbon transport in particulate form is poorly understood, requiring new development of empirical relations or theory in order to model it. Organic material often aggregates, and its transport may be dependent on the porosity of aggregates and the strength of the microbial exudates that bind them, which in turn are influenced by water quality, flow characteristics, and temperature. Second, organic carbon is a critical component of many ecosystem services. It is often a limiting factor for denitrification in streams, and its deposition as sediment in wetlands is a critical component of land building. Indeed, organic sediment accretion may be necessary for some coastal marshes to keep pace with sea-level rise. Third, organic carbon cycling is strongly linked to dissolved oxygen dynamics, which constitute a critical factor for aquatic habitat. Production of carbon through photosynthesis adds dissolved oxygen to the water column, whereas respiration depletes it. Last, organic carbon characteristics have great utility as an indicator of system dynamics and processes. Optical qualities of dissolved organic carbon, which are rapidly and cheaply assessed with scanning fluorometers and spectrophotometers, are sensitive to the presence of many different types of compounds and can indicate the provenance of the organic material (e.g., whether it originates from microbes or higher plants), its freshness/biological availability, and even some of the abiotic conditions of the environment from which it was withdrawn (e.g., redox potential).

Fluorescence excitation-emission matrices for two different water samples. These spectra are essentially a fingerprint of the fluorescent dissolved organic carbon pool.

Fluorescence excitation-emission matrices for two different water samples. These spectra are essentially a fingerprint of the fluorescent dissolved organic carbon pool.

Unrestored (left) and restored (right) stream study sites in the Chesapeake Bay watershed. This work is done in collaboration with the USGS.

Unrestored (left) and restored (right) stream study sites in the Chesapeake Bay watershed. This work is done in collaboration with the USGS.

In one ongoing study, we are using multivariate statistical techniques on 3D fluorescence spectra of particulate organic matter leachates to trace the origin of particulate organic matter transported in urban streams. Earlier work suggested that low carbon availability may limit the water quality-related ecosystem services performed by restored streams in the Chesapeake Bay watershed relative to unrestored streams, raising the question of the origin of the difference in the supply of organic carbon between the two streams. This project establishes a novel technique for quantitatively source-tracking organic sediment. Similarly, in another study we will be using fluorescence spectra and statistical analyses to determine the origin of organic carbon input to pools in intermittent coastal streams and its reactivity. Intermittent streams in California support coho salmon and steelhead trout in the southernmost part of their range. These salmonids persist in some of the pools within these streams but not others during the dry summer months. Because of the vulnerability of these threatened/endangered populations to climate change and water withdrawals, there is much interest in understanding the factors contributing to “good” salmonid habitat and understanding how they would be affected by land-use change and alternate water conservation strategies. We are testing the hypothesis that the dissolved carbon input to pools during the summer dry period determines their oxygen content and the quality of habitat for salmonids, and that the carbon input is strongly influenced by watershed land use.

Cleo Woelfle-Erskine installs a piezometer at Salmon Creek, CA.

Cleo Woelfle-Erskine installs a piezometer at Salmon Creek, CA.

We are also developing a way to label natural organic sediment aggregates using fluorescent dyes that bind to specific organic functional groups. The aim is to develop an active particulate-matter tracer that retains the surficial properties of particulate organic sediment in the natural environment. Experiments involving tracer introduction can be planned to address critical questions such as the role of direct interception of aggregates by plant stems in deltaic land-building processes or in the development of microtopography in the Everglades. A better understanding of both of these processes is critical to the success of wetland restoration projects planned in the Everglades and the Mississippi River delta.

Another poorly understood component of particulate carbon transport in the Everglades that will impact the success of restoration efforts is the tradeoff between its influences on land building and on nutrient mobilization. One of the objectives of Everglades restoration is to preserve and restore the multi-channel ridge and slough landscape structure, which my previous modeling efforts indicate requires the redistribution of flocculent organic sediment under pristine water-quality conditions. However, the Everglades has experienced decades of phosphorus enrichment—known to cause shifts in food web structure and a reduction in biodiversity—and its sediments are enriched in legacy phosphorus. Enhanced flows associated with restoration efforts will therefore also result in enhanced mobilization of particulate organic phosphorus. The ESDL is part of a multi-institutional science team established to evaluate the effects of a large-scale flow release in the Everglades, one of the showcase components of the $10 billion restoration experiment. Pre-release monitoring over the past three years has established a baseline understanding of the biophysical factors influencing landscape evolution. Flow releases scheduled for 2013 and 2014 will provide us with the opportunity to quantify tradeoffs in mobilization of excess phosphorus versus the favorable redistribution of organic sediment in ways that contribute to ridge and slough landscape topography.

Depiction of the planned flow release experiment in the Everglades. For decades, the region separated by the two levees depicted has been isolated from flow. An experimental flow release will evaluate the extent to which flow mobilizes sediment and its impacts on landscape and ecosystem processes. Photo credit: DPM Science Team.

Depiction of the planned flow release experiment in the Everglades. For decades, the region separated by the two levees depicted has been isolated from flow. An experimental flow release will evaluate the extent to which flow mobilizes sediment and its impacts on landscape and ecosystem processes. Photo credit: DPM Science Team.