**Title: Coastal greenhouse gas fluxes: A poorly quantified climate
feedback**

**Statement of Problem:** We are in the midst of a large unintended
experiment in coastal environments, in which large amounts of
anthropogenic nutrients are being conveyed to coastal environments by
rivers, atmospheric inputs, and groundwater. As a result, biological
productivity and rates of many biogeochemical processes in coastal zones
are increasing. Well-documented consequences of this widespread nutrient
enrichment include depletion of oxygen in bottom waters and sediments.
High-profile examples include the Gulf of Mexico, Long Island Sound, and
Chesapeake Bay. Coastal embayments throughout the world are undergoing
drastic biogeochemical and ecological changes as a result of increasing
eutrophication and anoxia.

An important consequence of the anoxic diagenesis of organic matter is
production of nitrous oxide (N~2~O) and methane (CH~4~), significant
anthropogenic greenhouse gases. The climate forcing of CH~4~ and N~2~O
is enhanced by \~25 times (CH~4~) to 320 times (N~2~O) on a
molecule-for-molecule basis relative to CO~2~. Because of this, CH~4~
and N~2~O collectively contribute roughly a quarter of the current
anthropogenic radiative forcing (IPCC Summary for Policy Makers,
February, 2007). Despite the well-known high productivity and
anthropogenic changes occurring in coastal environments, greenhouse gas
fluxes from these regions are poorly known, are probably underestimated,
and are largely ignored in global budgets (Bange, 2006a, b).

We hypothesize that coastal fluxes of CH~4~ and N~2~O are increasing and
are important to the global budget. As evidence, we note that
groundwaters in many coastal regions contain a legacy of nitrate
pollution from agriculture and treatment of anthropogenic waste. Such
loadings are expected to increase with time, and this pollution will
continue to exacerbate the existing anoxia problems in coastal waters.
Furthermore, nitrate contamination of coastal groundwater may be
directly linked to increased fluxes of N~2~O to the atmosphere. Under
anoxic conditions in aquifers and in sediments, microbial reduction of
nitrate occurs, a process that produces N~2~O. Therefore, elevated
nitrate concentrations in groundwater are likely to lead to significant
fluxes of both N~2~O and CH~4~ from the coastal zone for decades to
come.

**Objectives:** We propose to address the deficit in our understanding
of N~2~O and CH~4~ fluxes by initiating a research program aimed at
quantifying greenhouse gas fluxes in coastal environments, focusing
primarily on methane (CH~4~) and nitrous oxide (N~2~O).

**Work Strategy:** The initial focus of this work will be on examining
fluxes from the intertidal zone of impacted estuaries, for three
reasons: 1) such areas are key locations of discharge of
nitrate-containing groundwater and possibly of N~2~O or CH~4~; 2) any
gas fluxes from the intertidal zone would occur directly to the
atmosphere and would not be accounted for in coastal water, meaning the
intertidal zone could be an underappreciated source; and 3); PI
Sundquist has offered use of his soil benthic chambers for examining
fluxes in the intertidal zone, and his expertise in designing and
interpreting the related experiments. Initial testing of the techniques
would take place in impacted estuaries close to Woods Hole, such as West
Falmouth Harbor, due to considerable background information available
from ongoing work (e.g. http://www.eeb.cornell.edu/biogeo/nanc/wfh/).
Additional fluxes would be examined in impacted estuaries that are the
sites of ongoing field efforts, such as Long Island Sound and Chesapeake
Bay. Given adequate resources, we envision scaling up in the future to
examine fluxes along a transect that extends from land, through the
intertidal zone, to estuaries, using a variety of approaches.

**Relevance and Impact:** Identifying ways to control or mitigate
anthropogenic greenhouse gas emissions is a topic that is garnering
increasing attention in both the political and scientific arenas. Yet
any effort to manage these emissions requires that we have some
confidence in the magnitude of important sources of the gases. This
project will help fill a void in our understanding of the important
anthropogenic sources of N~2~O and CH~4~ to the atmosphere (Bange,
2006a,b). This is a topic of global importance that can be addressed at
the local scale in any of a number of locations. Ironically, any N~2~O
released to the atmosphere represents nitrogen that is removed from the
coastal zone and is therefore no longer available as a nutrient to fuel
eutrophication. Therefore, by quantifying the N~2~O flux to the
atmosphere we will simultaneously be helping to quantify the degree of
natural remediation of nitrate by the biogeochemical process of
denitrification.

**Partnerships:** This work will take advantage of existing soil
chambers of Sundquist, as discussed above. Furthermore, both Sundquist
and the CMGP group have instrumentation in hand capable of measuring
N~2~O and CH~4~. We will benefit from synergy with ongoing efforts to
assess nitrogen loading to coastal waters via groundwater, including a
CMGP-funded effort in Chesapeake Bay, a pending proposal in western Long
Island Sound and another pending propoal in Great South Bay (south of
Long Island). This project would also be well poised to interact with a
number of other USGS and external programs, including:

-   USGS/DOE gas hydrate project (CH~4~ release)

-   USGS programs studying sea level rise (which could exacerbate CH~4~
    fluxes)

-   International Polar Year field programs (USGS, NSF; to study melting
    of permafrost)

-   US Global Change Research Program

-   Ocean Carbon and Biogeochemistry Program (researchers funded by NSF,
    NOAA, DOE, NASA, etc.)

# References 

Bange, H.W., 2006a. Nitrous oxide and methane in European coastal
waters. Estuarine Coastal And Shelf Science, 70(3): 361-374.

Bange, H.W., 2006b. New directions: the importance of oceanic nitrous
oxide emissions. Atmospheric Environment, 40: 198-199.