40 to 45% of the bay bottom. Tidal currents and waves are the major 
depositional forces. These sediments often contain benthic assemblages of 
tunicates and tube dwelling amphipods and polychaetes. 
Intermediate sediment samples contained between 34 and 64% of fine 
sands plus a relatively large fraction of clay and silt. They had a mean 
grain size of 5.17 phi, may occur at depths ranging from 8 to 12 feet, 
and cover 15 to 25% of the bay bottom. These sediments contain faunal 
assemblages similar to those found in deep sand sediments. 
Soft sediment samples contained primarily clay and silt (92%), 
with a mean grain size of 7.20 phi. Soft sediments generally occur at 
depths greater than 12 feet and occupy an estimated 15 to 20% of the bay 
bottom. Tidal currents and waves exert relatively weak forces at depths 
greater than 12 ft and so, allows considerable deposition of clay and 
silt-sized material. Dense mats of amphipod feeding tubes are commonly 
observed in some areas. These dense mats can inhibit sediment 
resuspension and may increase the settling rate of suspended particles by 
acting as baffles (Rhoads and Germano 1986). In other areas lacking 
macro-benthic organisms, soft sediments coated with a thin light colored 
sediment layer have been observed (SAIC 1987). The layer contains high 
concentrations of invertebrate fecal pellets and viable phytoplankton 
cells. This surface sediment layer can easily be resuspended and 
represents a highly nutritive energy source for benthic organisms 
including bacteria. 
The organic carbon fraction of total carbon was relatively 
constant, comprising from one-fourth to one-third of the total carbon 
measured in each sediment type. The percents of total and organic carbon 
increased proportionally with increasing percents of mud (% silt + % 
clay) in Hillsborough Bay sediment samples (Table 1). Shallow sand 
sediments were low in both total and organic carbon. Invertebrate fecal 
pellets may be the principal organic carbon source in shallow sands. In 
contrast, soft sediments were high in total and organic carbon. 
Surface Sediment and Water Column Interactions 
Phase II of the BSG’s sediment project involved the quantification 
of the oxygen demand and nutrient contributions that Hillsborough Bay 
sediments have on the overlying water column. Nixon (1981) has shown that 
a large fraction of the organic matter consumed by the benthos is 
associated with a significant flux of inorganic nutrients into the water 
column. 
The BSG made in situ measurements of SODs and NERs in Hillsborough 
Bay during 1986. The SOD chambers and the field procedures employed were 
the same as described by Murphy and Hicks (1985)Nutrients were 
1 Nutrients were analyzed by the Hillsborough County Environmental 
Protection Commission. 
135 
