Reconstructing the impact of land-use changes on coastal lagoons in the Caribbean

Thesis Report

Coastal lagoons are rich in biodiversity and offer various ecosystem services. The close relation to the terrestrial ecosystems causes coastal lagoons to be vulnerable to impacts that are caused by human activities on land. The state of the coastal lagoons and the impact of human activities, is unknown for the Wider Caribbean Region. This study analyzes nitrate (N), phosphate (P), heavy metals, sedimentation rate and diatoms to reconstruct the recent development of four coastal lagoons in the Caribbean. Diatoms are used as proxy as they quickly respond to changes in water quality. Using short sediment cores, for four lagoons human impact is assessed. The lagoons have varying degrees of impact, from high (Fresh Pond, Sint Maarten), to medium (Spanish Lagoon, Aruba; Santa Martha, Curacao) to low impact (Saliña Bartol, Bonaire). Surface sediment samples were collected from eleven additional sites to provide a spatial context. Expected was that 1) levels of N, P, and heavy metals increase with increased human impact, that 2) biodiversity increases with decreasing human impact, and that 3) diatoms are a valuable and useful proxy for reconstructing water quality and environmental conditions. Results of heavy metals and N and P impact show that indeed human impact is most strongly pronounced in the site that was selected as high impact site (Fresh Pond, Sint Maarten). The medium and low impact sites do not show clear signs of human impact. Furthermore, results show that there is no correlation between diatom diversity and P, Chromium (Cr), Copper (Cu), and Nickel (Ni) concentrations. Although the Zinc (Zn) concentrations seem negatively correlated with diatom diversity, this is mainly caused by the low diversity of diatoms in the ponds on Sint Maarten. Observed diatom diversity is influenced by dissolution of diatom valves. Poor diatom preservation can bias results. As alkalinity, salinity and morphology of diatoms can strongly influence preservation, these factors are analyzed. Data suggests that species that are morphologically robust are less susceptible to dissolution in hyper saline conditions. A comparison of diatom assemblage between modern and sediment surface samples show that differences in observed diatom assemblage are substantial.