The Placencia Lagoon - Coastal Zone Management Authority and ...

The salinity maps presented in Appendix 2 display shifts in salinity distribution within
the lagoon. This could be attributed to the effects of evaporation, freshwater influx from
the Placencia catchments and the net water exchange between the entire estuarine system
and the sea. The fact that the salinity is a conservative parameter allows us to make
certain conclusions about the character of the lagoon. For instance, the monitoring
surveys in the peak of two dry seasons (2001 & 2003) indicate that the upper section of
the lagoon maintains a much lower salinity relative to the mouth of the lagoon. In effect
this is a clear indication that the flushing rate of the upper part of the lagoon during the
dry season is very low and is strongly dependent on wind and tidal forces. This is one
natural attribute of the lagoon that highlights its vulnerability to the impacts of human
settlement and aquaculture developments. Hypothetically, if large volumes of wastewater
or untreated effluents were discharged into the upper section of the Placencia Lagoon, its
water quality would rapidly decline and affect the natural ecological balance.
Another characteristic of the lagoon is that during the dry season there are minimal
differences between surface and bottom salinities. Based on this observation, the
Placencia Lagoon is classified as a moderately stratified estuary.
Dissolved Oxygen concentration is one of the best indicators of an estuary’s health. An
estuary with little or no oxygen cannot support healthy levels of animal or plant life. The
aerobic bacteriological decomposition of organic matter requires Oxygen. Quite often
this process depletes the water of oxygen and makes the environment uninhabitable for
other organisms.
Most animals and plants can grow healthy and reproduce when DO levels exceed 5 mg/L.
However, when DO levels falls to 3-5 mg/L living organisms may become stressed. The
results of the surveys indicate that the Placencia Lagoon is a healthy environment
displaying a wide range of dissolved oxygen concentration (between 5.79 and 8.02
mg/L).
Several studies in estuaries have shown that although excess nutrients from human
activities are a major cause of hypoxia and anoxia, these conditions may also occur in
estuaries relatively remote from human impact. However, the severity of low DO and the
length of time that low oxygen conditions persist are less extreme. This is another good
reason for a proactive approach to manage the impact of anthropogenic activities in the
lagoon catchments. Offsetting the balance of the healthy Placencia Lagoon towards an
anoxic and unproductive environment can be avoided.
During the monitoring surveys the turbidity levels in the lagoon were influenced
primarily by the intensity and duration of the wave action. The amount of mixing
ultimately controlled the re-suspension of silt and organic matter into the water column.
The perturbation in the extremely shallow parts of the lagoon was more vigorous as
opposed to deeper parts and this was reflected in the heterogeneous turbidity levels.
Generally, the turbidity levels were low as indicated by the averaged values ranging
between 4.32 and 10.64 NTU. During the final survey (July, 03), some parts of the
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