Filling in of Lac Bay Bonaire, causing loss of vital environments Halimeda sediment production investigated

Student Report

The mangrove forest on Bonaire, located in the south east of the island, is decreasing in area, just like many other mangroves around the world. One of the threats to Lac Bay is filling in of the open bay located in front of the mangroves. This would result in the loss of vital habitats like the seagrass fields in the open bay and the mangroves in the backwaters. Sediment production by Halimeda, a calcareous algae in Lac bay Bonaire, was determined to quantify the infilling of the bay. When this specific type of algae dies, it does not break down completely, but turns into lime sand.

To determine the sediment production of Halimeda, first biomass growth was measured by marking Halimeda plants with cable ties. Thereafter sediment production was calculated through the determination of the calcium carbonate content of Halimeda. Furthermore, the occurrence of Halimeda was mapped and the change in depth of the bay was estimated.

The results show that Halimeda grows mostly around the edges and locally in the open bay. A biomass production of approximately 14 grams/year with an average calcium carbonate content of 75% allowed to calculate the sediment production of about 640 grams/year/m2. This results in a decrease in depth of 0,5 mm per year (5 cm/century). If this sediment is transported to the edges of the mangroves, an infilling of this area of 35 cm per year can be expected.

On average for the whole bay, a decrease of 40 cm in depth was found over a period of 50 years. As an average halimeda is only contributing to infilling with 2,5 cm over 50 years and thus does not seem to influence the infilling of area of the bay much. The question remains how the bay is filled in and whether Halimeda might be contributing more location specific to the infilling of the bay.