Estimate of the Time and Space Scales Associated with Material Exchange across the Land-sea
Interface
Paula G. Coble
University of South Florida
An unsolicited proposal submitted to NASA in support of mission planning for GEO-CAPE
October 16, 2009
This proposal was formulated to address the following GEO Oceans Task:
“Exchange across land-sea interface. A large uncertainty in carbon budgets is associated with
processes occurring in tidal marshes, estuaries and river mouths, where land-derived materials
are processed and organic matter is produced, remineralized, deposited locally or exported to the
ocean. These environments are not simple conduits of material from land to ocean but "reactors"
where rapid processes occur. The time/space scales of this exchange are poorly constrained and
need more definition for the GEO-CAPE mission. (STM questions #1 and 2)"
The key processes across the land-ocean interface are 1) subaerial and subaqueous deposition, 2)
resuspension, 3) riverine export of dissolved and particulate material, 4) river plume dynamics,
5) phytoplankton production, 6) intertidal plant pump, and 7) groundwater discharge. The
overall approach to resolving critical temporal and spatial scales required to define the GEOCAPE mission will be literature review of physical and biogeochemical processes, and available
satellite-derived models and estimates of the key processes. Additional satellite imagery will be
produced as needed to support conclusions and recommendations.
II. Subaerial and subaqueous deposition, resuspension, and riverine export of dissolved
and particulate material
A. Significance
Particulate transport processes are expected to be driven by the annual cycle in river discharge.
Suspended sediment load is known to be sequential, such that initial pulses of sediments can be
deposited and resuspended along the riverbed numerous times before final release into the
coastal ocean. While this process is thought to roughly balance over a period of one to several
years, delivery to the oceans is likely to be triggered by an extreme event such as a flood or
severe storm. Thus time scales of hours to days are thought to be critical for accurate estimates
of suspended load transport processes. Approximately 30% of a rivers sediment load is trapped
in lowland floodplains and marshes, referred to as subaerial deposits. These areas can be
critically affected by human activities and extreme events. While they are more amenable to
remote sensing monitoring than subaqueous deposits, similar temporal and spatial scales
probably apply to both processes.
Dissolved transport processes are likewise primarily driven by the annual river discharge cycle,
but extreme events such as floods and droughts can cause large fluctuations over time scales of
hours to days. The occurrence of maxima in dissolved material transport is not synchronous with
the maximum in particulate material transport in the Mississippi River plume region (Green et
al., 2008). Suspended materials were found to be best predicted by wind speed, river discharge,
and SST, indicating the importance of vertical mixing in suspended particle distributions. In
contrast, east-west wind speed, river discharge, and river discharge lagged by 1 month were the
best predictors of dissolved materials, indicating the importance of advective process controlling
dissolved material distributions.
While land-sea processes may be common across river systems, the importance of large rivers to
global carbon transport cannot be underestimated. The world’s 25 largest rivers drain 50% of the
earth’s land mass, deliver 50% of the freshwater supply to the ocean, and transport 40% of the
total sediment load (Bianchi and Allison, 2009). These large rivers and their deltas are also
heavily impacted by human activities, most notably activities associated with food production.
This had led to a worldwide decrease in discharge of both water and sediments as well as an
increase in pollutants, especially nitrogenous pollutants. Dams and reservoirs built to generate
hydropower and provide water for irrigation also retain suspended materials to the extent that
deltas are in retreat, hence exacerbating coastal ecosystem issues in a time of sea level rise.
Roughly 60% of the world’s population lives along coasts and coastal regions are also the source
for 90% of global fisheries catch.
B. Approach
The North American rivers in the top 25 worldwide are #6 Mississippi/Atchafalaya, #13 St.
Lawrence, #16 Mackenzie, #17 Columbia, and #19 Yukon. Of these, the delta system of the
Mississippi/Atchafalaya is the most well-studied and will be the focus of efforts related to
defining temporal and spatial scales for deposition, resuspension, and riverine export.
II. River plume dynamics and phytoplankton production
A. Significance
Once river discharge reaches the coastal ocean, a portion of the water remains at the surface in
the form of a plume, containing an enrichment of dissolved organic and inorganic carbon as well
as nutrients. Thus, these plumes play a key role in controlling the magnitude and direction of
air-sea carbon flux. One result of decreased suspended load in rivers, a consequence of human
activities, has been a shift in the composition of carbon delivery degradation-resistant terrestrial
carbon to highly degradable phytoplankton carbon. This autochthonous carbon is more readily
converted to CO2 and also more directly available as sustenance for coastal fisheries, thereby
altering carbon cycle dynamics. In addition, this riverine productivity is not well constrained in
terms of magnitude and areal extent. Past estimates of air sea flux in coastal regions have often
assumed that coastal areas are a source due to organic loading of river water, however decreased
pCO2 from phytoplankton growth has now been observed in several coastal areas (Cai).
Spatial and temporal variability in plume dynamics influence the areal extent (size), plume
thickness, and location of plume (i.e., water column depth, local impact on ecosystem). Factors
such as winds, storms, seasons, tides, droughts, floods, human activities have long and short term
influences. Some have persistent sign to their effect, such as river floods produce positive
export. Others such as severe storms may have large variability in both sign and magnitude of
the effects.
B. Approach
The Mississippi River region will again be the major focus of these efforts, due to the availability
of existing literature relating ocean color data to parameters of interest. MODIS data have been
successfully used to estimate CO2 flux in good agreement with field studies (Lohrenz and Cai),
and SeaWIFS data have been used to estimate river discharge and DOC flux for the Mississippi
River plume (DelCastillo and Miller, 2008). Secondarily, data for the Arctic rivers will be
reviewed, both due to the expected magnitude of climate change effects in this region and the
expectation that temporal and spatial scales of importance may differ markedly from those in
temperate regions. Predictions of increased temperature, rainfall and runoff in Arctic, combined
with melting permafrost presents huge potential for C and sediment export in this region, where
some effects are already being observed.
III. Intertidal plant pump
A. Significance
Salt marsh grasses and mangroves are highly productive coastal biomes, and may have a
profound impact on estimates of carbon exchange. Productivity of mangroves takes up more
CO2 than is released by the surrounding aquatic system, and these regions may therefore be a
sink of carbon that is transported laterally to the open ocean (see refs. in Jahnke 2008). The
productivity of intertidal plant communities is largely controlled by their areal extent and by
inherent species control over photosynthetic rate. Rapid changes in areal extent can be caused by
extreme events, such as storm erosion. Spatial considerations relate primarily to the scale
necessary for accurate determine of community composition for productivity estimates. Longer
term changes due to species migration or climatology may be important, but are not likely to
drive GEO-CAPE requirements.
One unresolved issue here is the contribution of the exposed intertidal areas as both source and
sink of carbon. These areas are known sources of DOM and CDOM and efflux is influenced by
the overlapping cycles of daylight and tides, although this is still poorly understood (Millbrandt
et al, in prep). Spatial scales for observation of this process are likely to be small, and perhaps
beyond a reasonable resolution for GEO-CAPE.
B. Approach
The magnitude of effect of the intertidal plant pump is still largely a research question, and not
likely to drive GEO-CAPE requirements. However, a survey of existing literature related to time
scales of extreme events and space scales associated with accurate productivity estimates will be
undertaken. Assessment of marsh productivity may well be approached using similar land plant
tools and models. The investigation will include available information of exposed intertidal areas
to determine feasibility of GEO-CAPE to resolve these transports.
IV. Groundwater discharge
A. Significance
Groundwater comprises 95% of unfrozen freshwater reserves, and contributes 10-30% of the
total global discharge to the ocean. It supplies nutrients as well as CDOM to coastal areas,
although it is not included in river discharge measurements. The relationship between ground
water discharge and surface water discharge is uncertain, however, in Tampa Bay groundwater
discharge becomes a major source of freshwater during the dry season (Conmy, 2008).
B. Approach
Assessment of the contribution of groundwater to coastal ocean carbon, nutrient and color is
nacent area of research. Existing literature will be reviewed to determine the feasibility of this
process within GEO-CAPE capabilities.
V. Summary and Timeline
We will investigate the time and space scale requirements for the GEO-CAPE mission resulting
from the study of the following processes: 1) subaerial and subaqueous deposition, 2)
resuspension, 3) riverine export of dissolved and particulate material, 4) river plume dynamics,
5) phytoplankton production, 6) intertidal plant pump, and 7) groundwater discharge. The
investigation will primarily involve review of existing pertinent literature, along with interviews
with investigators actively research the areas of interest. Satellite imagery will be used, as
needed, to substantiate resulting conclusions and recommendations. Dr. Chuanmin Hu has
agreed to provide the imagery needed to support this project at no cost to the project.
Work will commence Jan. 1, 2010, or as soon as funding is received. Literature review will be
completed within four months from the start, the following two months will be used to assemble
satellite imagery and finalize the report to NASA and to the GEO-CAPE mission team. Total
anticipation duration of the project is six months.
VI. References
Bianchi, T.S., and Allison, M.A. Large-river delta-front estuaries as natural ‘‘recorders’’ of global environmental
change. PNAS, May 19, 2009, vol. 106, no. 20, p. 8085–8092.
Conmy, R.N. (2008) Temporal and Spatial Patterns in Optical Properties of Colored Dissolved Organic Matter on
Florida’s Gulf Coast: Shelf to Stream to Aquifer. Ph.D. Dissertation, USF, 134 pp.
Del Castillo, C.E., and Miller, R.L. On the use of ocean color remote sensing to measure the transport of dissolved
organic carbon by the Mississippi River Plume. Remote Sensing of Environment 112 (2008) 836–844.
Green R.A., Gould, R.W., Jr., and Ko, D.S. Statistical models for sediment/detritus and dissolved absorption
coefficients in coastal waters of the northern Gulf of Mexico. Continental Shelf Research 28 (2008) 1273–
1285.
Jahnke, R. 2008. Maybe It’s Not Just About Air-Water Gas Exchange. Oceanography Vol. 21, No.4, p. 42-3.
Milbrandt*, E.C., P.G. Coble, R.N. Conmy, A.J. Martignette, and J. Siwicke. Evidence for production of CDOM in
subtropical coastal regions. (in prep).
VII. Budget
Salaries (Including insur. and fringe)
P. Coble (1 mo. Summer)
Grad. Res. Assistant (6 mos.)
TOTAL SALARY AND FRINGE
Travel (to GEO-CAPE meeting for 2)
Tuition
Indirect Costs (47% excl. tuition)
$8,875
$12,714
$21,589
$4,000
$12,000
$12,027
TOTAL COSTS
$49,616