Apparent Redox Potential Discontinuity (RPD) Depth 
When there is oxygen in the overlying water column in 
coastal areas, near-surface sediment will have a higher reflectance 
value relative to hypoxic or anoxic sediment underlying it. This 
is because the oxidized surface sediment contains particles coated 
with ferric hydroxide (an olive color when associated with 
particles), while the sulphidic sediments below this oxygenated 
layer are grey to black. The boundary between these two sediment 
types is called the apparent redox potential discontinuity (RPD). 
The term "apparent" is used because the true RPD (Eh = 0, as 
measured with microelectrodes) is usually located at a shallower 
depth in the sediment than the bottom of the high reflectance 
layer. This phenomenon is related to organisms mixing grains with 
ferric hydroxide coatings downward below the true RPD (Eh = 0). 
Once transported below the true RPD, the oxidized coatings may be 
metastable for several weeks to months in an otherwise anoxic 
(Eh<0O) environment. This oxidized, high-reflectance area is 
digitized, measured to scale, and divided by the prism window width 
to obtain a mean depth for the apparent RPD. The RPD depth is a 
sensitive indicator of the biological mixing depth, infaunal 
successional status, and within-station patchiness. In the absence 
of bioturbating infauna, the RPD will achieve a maximum depth of 
2 mm in fine-grained sediments solely by diffusion (Rhoads, 1974). 
Sedimentary Methane 
At extreme levels of organic-loading, pore-water sulfate 
is depleted and methane bubbles are produced in the sediment 
column. These gas-filled voids are readily discernable because of 
their irregular, generally circular aspect and glassy texture (due 
to the reflection of the strobe off the gas). If present, the 
number and total areal coverage cf all methane pockets is measured. 
Infaunal Successional Stage 
The mapping of infaunal successional stages is based on 
the theory that primary succession results in "the predictable 
appearance of macrobenthic invertebrates belonging to specific 
functional types following a benthic disturbance. Because 
functional types are the biological units of interest..., our 
definition does not demand a sequential appearance of particular 
invertebrate species or genera" (Rhoads and Boyer, 1982). The term 
disturbance can refer to a natural process such as seafloor 
erosion, changes in seafloor chemistry, macrofaunal foraging 
disturbances which cause major reorganization of the resident 
benthos, or anthropogenic impacts such as dredged material or 
sewage sludge dumping, trawling, thermal effluents from power 
plants, industrial discharge, etc. 
Pioneering benthic assemblages (Stage I) usually consist 
of dense aggregations of near-surface, tube-dwelling polychaetes. 
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