A multi-institution team headed by VIMS researchers has received a 5-year, $1.7 million grant from the National Science Foundation to study the role that plankton play in the consumption and production of dissolved organic matter in the ocean.
Collaborators in the DOMINO project (for Dissolved Organic Matter IN the Ocean) include VIMS scientists Deborah Bronk, Walker Smith, and Deborah Steinberg. They are joined by colleagues at the University of California Santa Barbara, the University of Delaware, the University of Maryland, and Woods Hole Oceanographic Institution.
Dissolved organic matter, or DOM for short, refers to the vast pool of biologically produced carbon, nitrogen, and phosphorus compounds dissolved in seawater. Researchers have long known that plankton are the major consumers and producers of DOM in the sea. What's unique about the DOMINO study, says Bronk, is that it represents "the first time that anyone has studied the three major modes of DOM production--direct leakage from phytoplankton cells, zooplankton grazing of plankton cells, and viral infection and rupturing of plankton cells--at the same time." Simultaneous study of the three processes will allow the researchers to quantify the relative role that each plays in DOM cycling.
Another novel aspect of the study is its use of an analytic technique developed by Bronk for isolating organic nitrogen dissolved in seawater. Because phytoplankton are mostly made of carbon, isolating nitrogen and other less-abundant bodily constituents had previously proven difficult.
A better understanding of dissolved organic nitrogen compounds is vitally important, especially in heavily developed estuaries like Chesapeake Bay. In the Bay, excess inputs of nitrogen from fertilizers and car exhaust can nurture algal blooms so dense they cloud the water, shading ecologically important Bay grasses. When the algae die, their decomposition can deplete oxygen supplies.
The DOMINO team will investigate the three DOM production processes using both laboratory experiments and research cruises to Chesapeake Bay and the California coast. Bronk notes that the Chesapeake provides an ideal system for examining the processes that control DOM release in marine environments. "Much of the original work on the cycling of dissolved organic nitrogen took place in Chesapeake Bay, and there have been a number of classic studies concerning the release of dissolved organic carbon there as well," says Bronk. "The Bay also typically has a well-defined spring algal bloom, so it's an excellent place to study the fate of phytoplankton biomass once it forms."
The fate of phytoplankton biomass is particularly important in the Bay because Bay managers currently only look at nitrogen concentrations and uptake rates. "Knowing that phytoplankton biomass forms is only part of the story. What happens to that biomass is equally important," says Bronk. "For example, if most of the biomass is released in a dissolved form, it's not going to sink to the bottom and contribute to low oxygen levels. Instead, it may be carried out of the Bay into the coastal ocean."
A key goal of the project's laboratory experiments is to better understand how seawater nitrogen levels affect the amount of DOM released by phytoplankton. Nitrogen is typically a limiting nutrient for phytoplankton growth. By adding more nitrogen to their tanks, Bronk and Smith can simulate the phytoplankton blooms that occur when storms or run-off from fertilized land inject this nutrient into sunlit surface waters. By later adding zooplankton, Steinberg can measure how DOM production rates change as these creatures begin to eat the marine plants. Dr. Earl Wommack of the University of Delaware will add viruses to the mix, while Drs. Craig Carlson of UC Santa Barbara and Dan Repeta of Woods Hole will help measure the chemical forms taken by the various dissolved carbon and nitrogen compounds, and how available they are to scavenging by bacteria.
Taken together, results from the lab and field studies will help quantify how nitrogen-availability and the life-styles and stages of various plankton species affect how much dissolved organic carbon and nitrogen they produce, and what chemical forms these elements take.
Figuring out the sources and chemical forms of dissolved organic carbon compounds is particularly important to the issue of global climate change. Because the ocean's reservoir of dissolved organic carbon is so large, even small changes in its size can significantly affect other components of the global carbon cycle--including the atmospheric pool of the greenhouse gas carbon dioxide.
Results from the DOMINO lab and field studies will help support another goal of the project, which is to create a computer model that can accurately simulate the relative roles of carbon and nitrogen in the marine DOM pool. This modeling component, led by Dr. Raleigh Hood, is essential to improving the realism of the large-scale models used to predict global carbon-cycle dynamics and how they might respond to human activities.
The DOMINO project includes a significant educational component as well. The researchers will work with the VIMS public relations staff to develop a marine science mini-school to be offered annually to the general public, and create an interactive computer model of the carbon cycle that can be used by mini-school lecturers and in public displays at a number of venues around the Commonwealth.
The DOMINO project is funded under NSF's Biocomplexity in the Environment program, which supports studies that help clarify how the biological, physical, chemical, and human components of the global ecosystem interact.