Origin of Circular Beds of Thalassia (Spermatophyta: Hydrocharitaceae) in South Biscayne Bay, Florida, and their Relationship to Mangrove Hammocks

Comparisons of aerial photographs of a mangrove shoreline and an adjacent estuarine area in southwestern Biscayne Bay, Florida, showed the presence of numerous circular to teardrop-shaped areas. The circular areas on shore are hammocks of mangroves and other tropical trees and they are over depressions in the bedrock which are filled with mangrove peat. The circular areas in the estuary are beds of Thalassia testudinum. These nearly always occur over depressions in the bedrock that are filled with autochthonous mangrove (Rhizophora) peat. The peats reach a pH of 4.9 and a depth of 5 meters. Thalassia beds are often surrounded by a white halo of worm and callianassid burrows in the broken area at the periphery of the depressions in the bedrock. The effect of sediment depth on density and length of blades is shown. Dated mangrove peat from beneath a Thalassia bed 3 meters below present sea level was 3680 years old. Interpretation is that in this area the mangrove shoreline receded as sea level rose, and that the beds of Thalassia then colonized the planed-off hammocks.