In the microbial loop, heterotrophic bacteria utilize dissolved organic matter (DOM) as an energy source. DOM becomes remineralized into inorganic material and nutrients available for primary production. As the amount of nutrients increase, the abundance of each trophic level increases, which is known as the bottom-up effect. This study investigates the effect of the increasing human population density on an intertidal community along the waterfront of Kralendijk, Bonaire. DOM, fecal indicator bacteria (enterococci, Escherichia coli, and coliform bacteria), nutrients (nitrate, phosphate, and ammonia), primary producers (percent cover of macroalgae), and herbivorous consumers (density of sea urchins) were sampled. There was no pattern between the variables and the increase of the adjacent population density. Factors such as rainfall, changes over time, tides, and herbivore grazing may have influenced the results. When graphed over time, rainfall impacted the concentrations of nutrients and fecal indicators. Nitrate, ammonia, and coliform bacteria increased, while phosphate, enterococci, and E. coli decreased. Concentrations of ammonia were found to exceed the threshold for a healthy coral reef ecosystem (6x). No correlation was found between DOM and heterotrophic bacteria, although concentrations of E. coli and nutrients were high at one site. This intertidal ecosystem does not appear to be influenced by bottom-up controls, as there was neither a correlation found between the percent cover macroalgae and nutrients or the density of sea urchins. The site with the highest percent cover of macroalage had the lowest density of urchins and vice versa.
This student research was retrieved from Physis: Journal of Marine Science XIX (Spring 2016)19: 91-99 from CIEE Bonaire.