Physical Characteristics of the Anchialine Ponds of Bermuda

There are five anchialine ponds in Bermuda of 0.5 ha or over, the largest being Mangrove Lake with an area of 12 ha. Volumes vary from 4,550 to 164,418 m3 and tidal flushing from 0.56% to 62.8%. The deepest, Walsingham Pond, has maximum and mean depths of 6.2 and 3.9 m respectively. Connections between the ponds and the sea vary from small fissures to small caves, which in all but one, Lovers Lake, connect at the surface of the pond. Mean salinities are all close to open sea water values, but in ponds with low flushing and in Lovers Lake, surface values are somewhat reduced, the lowest being 27.4‰ in Trotts Pond. Mean pond water temperatures were all very similar at about 24°C, but ranges were highly variable. Dissolved oxygen values in ponds in which they were studied in detail were quite variable, especially in summer, ranging from supersaturation to close to zero and it is possible that anoxic conditions occur occasionally. Light levels were studied in Walsingham Pond; there bottom irradiance levels exceeded 20% of surface values. All other ponds are much shallower and well illuminated. The sediments showed high organic content inversely related to tidal flushing, ranging from a mean of 42% in Mangrove Lake to 8% in Lovers Lake. The inorganic fraction of sediments was silt-clay mixed with, or overlying, a shell gravel. The percent siltclay, varying between 67 and 32%, was also inversely related to tidal flushing. The shell gravels varied in composition from pond to pond but all had the gastropod Cerithium lutosum in abundance. Although many of the shell components reflected the abundance of living molluscs in the ponds, there were also significant quantities of relict species which are not presently living there. It is concluded that some species go through large population cycles at relatively long intervals.