Oversimplifications and Confusion Page 6

hypoxic zone. Of course, this pertains to the average conditions for the hypoxic zone;
nearer the rivers a greater percentage could be accounted for by terrigenous carbon, while
regions remote from the rivers receive insubstantial labile terrigenous carbon. Curiously,
the UA report pays no attention to the losses of terrigenous organic carbon to
sedimentation, dispersion and metabolism as it is transported over distances of 30-380
km during the several weeks”, yet devotes an appendix to calculations that seek to
demonstrate that less than 25% of the marine organic carbon produced in waters
overlying hypoxic zones is deposited to the bottom—over a vertical distance of 10 m or
less. Actual published observations from sediment traps show that there is sufficient
seston deposition from surface production, especially during the spring, to supply the
organic carbon required (Topic 1 report).

The UA report also suggests that large injections of terrigenous organic carbon during
floods may have consequences for oxygen depletion in subsequent years. Considered in
the context of the previous discussion, this would have to assume that the small labile
portion of this organic matter that is deposited on the shelf would remain largely
undegraded over six months in relatively warm Gulf of Mexico bottom waters until
stratification sets up the next year and then be actively mineralized. Also, while the
multi-year consequences of flood events deserve more attention, it is important to keep in
mind that these floods also inject elevated masses of nutrients, which by stimulating plant
production greatly magnify the pool of labile organics on the shelf (see below). Organic
matter produced in situ as a result of such pulses could also remain stored in the shelf
system until hypoxia sets up in the following spring. At least in the 1993 flood, a much
larger than normal portion of the flow through the Mississippi delta proper was lost from
the shelf environment as a result of the
strong discharge jet that propelled the
plume onto the slope and wind forcing of
currents toward the east. So, a smaller
than normal proportion of the organic
carbon spike could be “credited” to the
carbon metabolism of the Louisiana shelf.
Again, this underscores the need to
consider the importance of interannual
inputs of both carbon and nitrogen from a
dynamic and geographically realistic
perspective rather than as a simplistic
black box.

Concomitant with the greatly diminished suspended particulate concentrations
experienced in the lower Mississippi River during the last half of the 20" century (50 to
70 % reductions’), the concentrations and loadings of organic carbon have also declined.

° Mean currents available to transport DOC or POC are quite slow, <25 cm sec-I in the coastal boundary
jets and slower in most of the region experiencing hypoxia (Wiseman, W.J. and F. J. Kelly. Seasonal
variability within the Louisiana Coastal Current during the 1982 flood season. Estuaries 17:732-739. )

' Kesel, R.H. 1988. The decline in the suspended load of the Lower Mississippi River and its influence on
adjacent wetlands. Environmental Geology and Water Sciences 11:271-281.