The Lagoon Monitor
Is Removing Lagoon Causeways Good or Bad?
Algae and the Lagoon
|
|
For many years environmental experts have pointed out that the earth fill causeways have been adversely affecting water quality in the lagoon and should be replaced completely by bridges to restore the lagoon's natural circulation. This concept was brought into question by a recent computer modeling study. The study suggested that, although salinity and water circulation may be improved by removal of the causeways, the increased wave action that would result may decrease water clarity by stirring up fine sediments from the lagoon bottom. If this were the case the poor water clarity would decrease the light available for seagrasses. So the critical question is: Would removing the causeways decrease water clarity? The answer is very complex. Some relevant water clarity data is collected by the Indian River Lagoonwatch Volunteer Program, which has been monitoring water quality in the lagoon every week since 1991.
The maps to the right show water clarity data, contrasting conditions in April versus August for the past two years, which are typical of the same comparison over the past ten years. The data show water clarity as measured by Secchi disk. A Secchi disk is a black and white disk lowered into the water until the black areas can no longer be distinguished from the white areas. This depth is the Secchi Depth. The lower the Secchi depth the cloudier the water and the harder for seagrasses to receive the light they need to survive.
If wave action was the controlling cause of poor water clarity in the lagoon one would expect water clarity to be poorest in the winter and spring when strong north, and north-east winds drive straight down the length of the lagoon stirring up sediments. One might also
expect that hot calm summer days in August might exhibit better water clarity. In the maps above, the April maps show extensive areas of Secchi depths of 1 meter or greater (greater than three feet) whereas the August maps show extensive areas with very poor Secchi depths of less than a meter. This is the opposite of what one might expect. Our volunteers report on water conditions such as whether the water is calm, rippled, has waves or has white capped waves. Also contrary to what one might expect there is no statistical relationship between wave conditions and water clarity as
measured by Secchi Disk. Clearly more wave activity should result in more sediment being suspended but the overall water clarity is still consistently bad in the summer months and for the most part good in the winter months with the effects of wind being almost unnoticeable. So, if it's not suspended sediments that is reducing water clarity for the most part, what is it? One factor is the effect of natural organic tannins that dye the tributaries of the lagoon a tea color. The greater the rainfall, the greater the volume of tannins and new sediment added to the lagoon. Rainfall does have the direct effect of lowering the water clarity of the lagoon but even during drought periods water clarity is very poor during the summer months. One of the major reasons for this is algae growth.
|
Next Article: Algae and the Lagoon
|
