Dear Editor,
The smell now emanating from Yallahs (small) Pond — there are two ponds — is due to gases; mainly hydrogen sulphide, apparently resulting from the growth of algae. This is a characteristic of brackish ponds in the tropics, but it is not a common occurrence.
I have visited the pond hundreds of times over the past 40 years — as a birdwatcher it is one of my favourite locations — and have taken note of the seasonal changes year after year.
The water level in the pond rises during the rainy season and falls due to evaporation during the hot dry summer months. In drought years the level gets extremely low, and the salt level very high. I once analysed this very salty liquid and found it three times as salty as the sea.
The phenomenon of algal growth and offensive smell occurs infrequently — maybe twice per decade — and I suspect is triggered by an optimum salt concentration. Since this year, it has occurred in August/September after the lowest pond level began to be diluted by rainfall. Heavy rains will further reduce the salinity and the algae will disappear; but this has not happened yet.
Although they look fragile, the ponds are ancient and appear on the earliest maps of Jamaica. I once found a Taino arrowhead there, which is not unexpected as they depended considerably on shellfish. The only time the ponds are joined to the sea has been when exceedingly heavy rains raise the water level and the excess water pressure breaks through the "safety valve" at the lowest point of the protecting ridge, where NWA is now excavating.
This has happened to my knowledge three times in the past forty years, and the event I remember best was in September 1979 when the water rose more than two feet overnight and burst through to the sea. A gap about thirty feet wide let in the sea, and the pond became, in effect, a bay. However, when I went back three months later the breach was completely closed by natural wave action on the seashore — I hope and expect that the same natural corrective action will close the gap now being created by the NWA.
Incidentally, at this same time September 1979, the famous Mother Rock Spring on the hill behind the pond came back to life after many dormant years
Although the pond is quite shallow, it contains a lot of fish, which are caught and used by local people. These fish are able to adapt to changing levels of salinity up to a point, but when it passes that point all the fish die and another source of offensive smell develops as the dead fish rot. But this is short-lived, as an army of John Crows come in to feast, leaving only bones. Strangely, as soon as the rains replenish the ponds and water levels rise, a new generation of fish appear.
John Fletcher
johnofletcher@gmail.com
So that explains the smell!
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The smell now emanating from Yallahs (small) Pond — there are two ponds — is due to gases; mainly hydrogen sulphide, apparently resulting from the growth of algae. This is a characteristic of brackish ponds in the tropics, but it is not a common occurrence.
I have visited the pond hundreds of times over the past 40 years — as a birdwatcher it is one of my favourite locations — and have taken note of the seasonal changes year after year.
The water level in the pond rises during the rainy season and falls due to evaporation during the hot dry summer months. In drought years the level gets extremely low, and the salt level very high. I once analysed this very salty liquid and found it three times as salty as the sea.
The phenomenon of algal growth and offensive smell occurs infrequently — maybe twice per decade — and I suspect is triggered by an optimum salt concentration. Since this year, it has occurred in August/September after the lowest pond level began to be diluted by rainfall. Heavy rains will further reduce the salinity and the algae will disappear; but this has not happened yet.
Although they look fragile, the ponds are ancient and appear on the earliest maps of Jamaica. I once found a Taino arrowhead there, which is not unexpected as they depended considerably on shellfish. The only time the ponds are joined to the sea has been when exceedingly heavy rains raise the water level and the excess water pressure breaks through the "safety valve" at the lowest point of the protecting ridge, where NWA is now excavating.
This has happened to my knowledge three times in the past forty years, and the event I remember best was in September 1979 when the water rose more than two feet overnight and burst through to the sea. A gap about thirty feet wide let in the sea, and the pond became, in effect, a bay. However, when I went back three months later the breach was completely closed by natural wave action on the seashore — I hope and expect that the same natural corrective action will close the gap now being created by the NWA.
Incidentally, at this same time September 1979, the famous Mother Rock Spring on the hill behind the pond came back to life after many dormant years
Although the pond is quite shallow, it contains a lot of fish, which are caught and used by local people. These fish are able to adapt to changing levels of salinity up to a point, but when it passes that point all the fish die and another source of offensive smell develops as the dead fish rot. But this is short-lived, as an army of John Crows come in to feast, leaving only bones. Strangely, as soon as the rains replenish the ponds and water levels rise, a new generation of fish appear.
John Fletcher
johnofletcher@gmail.com
So that explains the smell!
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