Sediment in seagrasses near Link Port,Indian River, Florida.

Grain size analyses have been made for 34 samples of the upper 5 cm of sediment
cores from seagrass beds. Sieves were used for gravel and sand; the SEDIGRAPH was used for silt
and clay. THALASSIA (13 samples), HALODULE (3), SYRINGODIUM (3) and bare sand areas (15)
within seagrass beds were sampled in the summers of 1979 and 1980. Generally, these sediments
are sands, with small admixtures of gravel, silt and clay. The average particle size is about 0.25
mm with grain size modes at 0.3 and 0.1 mm. These sediments are poorly sorted (large standard
deviation), positively skewed (excess fine particles) and are leptokurtic (excessive peakedness).
Sand content is greater than 90% by weight, and is mostly quartz. Gravel consists of carbonate
shells and shell fragments. Mineralogy of the silt fraction is not known, and the clay is judged to
be mostly kaolinite from crystal shapes seen in transmission electron microscopy. Comparison of
sediment from different species of seagrasses to bare sand areas reveals little difference in terms of
grain size characteristics. There are no data available to document seasonal differences. There is
a significantly higher content of silt (X = 1.33 %) in THALASSIA sediments than in nearby sandy
areas (X = 1.01 %). Comparison of silt grain size data (62 .5 to 3.9 µm) shows that the most
abundant particle sizes present in THALASSIA sediment are between 15-30 µm. Particle size
distribution of silt in nearby sandy areas shows less concentration in this range. Preliminary
results from late summer sediment trap measurements give a particle flux of 10-20 g m⁻² day⁻¹
during ordinary weather, which doubled during passage of storm DENNIS. It is not known how
much of this flux has come from local resuspension of sediment particles.