Continental shelf sediments on the South Atlantic Eight (SAB) consist
of relict sands that, at depths ranging from 14-45 m, fall within the
photic zone and are sites in which significant rates of benthic primar
y production are observed. Thus, SAB seafloor sediments are a source o
f organic matter to the shelf system and are possibly a sink for nutri
ents regenerated within the sediments. We have investigated the nutrie
nt dynamics in SAB shelf sediments along two transects (off the coasts
of Georgia and Florida) as part of a study that addresses the signifi
cance of benthic primary production in terms of overall shelf biogeoch
emistry. Sandy sediments were sampled with a newly designed corer that
permits retention of pore water in highly permeable sands and thus av
oids commonly encountered ''washout'' problems. Nutrient (ammonium, si
licate, nitrate + nitrite, and phosphate) distributions from sediments
along both transects show substantial variation in concentration magn
itudes and profile shapes over short horizontal spatial scales (meters
). Laboratory experiments and numerical models of diagenetic processes
in these sediments suggest that (1) pore-water advection, driven by c
urrent flows over wave ripples and bioturbational features, is likely
an important transport process in promoting nutrient exchange in these
porous sands (more so than irrigation), and (2) reaction rates in SAB
sediments are rapid and are comparable to those in nearshore muddy ha
bitats. In spite of this variation and the apparently high rates of ad
vective transport, time series measurements show a gradual increase in
depth-integrated nutrient concentrations at one station along the Geo
rgia transect from the spring through the fall. This increase is proba
bly related to elevations in temperature and metabolic rates in sedime
nts during the warmer summer months. Measured oxygen and nutrient flux
es across the sediment-water interface in paired transparent ''light''
chambers and opaque ''dark'' chambers did not conform to trends obser
ved in shallow water systems, where oxygen is typically evolved and nu
trients are consumed in the light (versus oxygen that is consumed and
nutrients that are evolved in the dark). The absence of measurable or
consistent fluxes on the SAB shelf is likely the result of low concent
rations of nutrients in both the water column and the upper sediment l
ayers. However, rapid regeneration rates in sediments and generally hi
gher nutrient concentrations in deeper sediment layers suggest that se
diments are an important source of the nutrients that fuel benthic pri
mary production. Although ammonium is rapidly produced in these oxic s
ands, nitrate concentrations are uniformly low, and measurable nitrate
generally appears in the oxic sedimentary layers. The absence of high
nitrate concentrations in the suboxic or anoxic zone suggests that de
nitrification may not be an important process in SAB shelf sediments.