Thirty years ago, according to grain size contour maps created by 
Goodell and Gorsline (1961), roughly 32% of Hillsborough Bay surface 
sediments were fine grained sediments (mean phi >4). Because they did 
not intend to map fine grained sediments specifically, the areal coverage 
of these sediments was not as well defined as the "mud" areas delineated 
by the BSG (COT 1986a). The disparity of mapping techniques used by 
Goodell and Gorsline (1961) and the BSG prevent any conclusions as to the 
increase or decrease of fine sediments in Hillsborough Bay during the 
past 30 years. However, it is apparent that relatively large areas of 
fine grained surface sediments also existed 30 years ago. 
A representative cross section of Hillsborough Bay sediment types 
was provided by combining subtidal sediment data from the BSG sediment 
mapping effort with intertidal sediment data from another BSG project 
(COT 1988). For descriptive purposes, sediment types were partitioned 
into four groups based on percent sand composition. The sediment groups 
listed in Table 1 are shallow sand, deep sand, intermediate and soft. 
Table 1. Results of grain size and carbon analyses of major sediment 
types in Hillsborough Bay. 
Shallow 
Sand 
Deep 
Sand 
Intermed. 
Soft 
% Sand 
98-100 
82-91 
34-64 
1-16 
% Silt 
0-2 
3-7 
11-35 
24-39 
% Clay 
0-1 
6-11 
21-34 
51-75 
Mean phi 
2.4-3.0 
2.9-3.5 
4.5-5.9 
6.8-7.9 
SD Mean phi 
% Total 
0.4-0.8 
1.8-2.0 
2.2-2.6 
1.2-2.4 
Carbon 
1-7 
5-9 
18-37 
35-56 
% Organic 
Carbon 
0-3 
1-3 
6-10 
15-17 
Shallow sand samples were composed almost entirely of well sorted 
fine quartz sand with a mean grain size of 2.62 phi. Shallow sands occur 
on intertidal and shallow subtidal flats usually at depths less than six 
feet and encompass about 20% of the total bay bottom area. Important 
depositional forces include tides and waves generated by wind and ship 
traffic. Although most areas lack vegetation, some have macroalgae or 
sparse seagrass coverage. Microscopically examined, these sediments 
appear as light colored sand grains intermixed with dark brown 
invertebrate fecal pellets. 
Deep sand samples contained between 83 and 91% of well sorted very 
fine sands with a mean grain size of 3.16 phi. These sediments occur in 
subtidal zones at estimated depths of six to ten feet and cover roughly 
134 
