The San Juan Bay Estuary in Puerto Rico is an interconnected series of lagoons and canals that weave through San Juan, the capital city and home to nearly 350,000 people. As the city has boomed, the canals and waterways connecting the ocean with inland lagoons have grown clogged with sediment, trash, and debris. As a result, conditions look drastically different than they did even in the 1970s when residents first raised concerns about water quality in the estuary.
The challenges facing the San Juan Bay Estuary are typical of coastal, tropical urban areas around the world. Although coastal areas less than 10 meters above sea level represent only 2% of the world’s land area, they are home to 13% of the world’s urban population. These urban areas also tend to have low socioeconomic status and large populations vulnerable to storm surges and tropical storms associated with climate change.
In a new study of the San Juan Bay Estuary, Oczkowski et al. point out that surprisingly little is known about urban estuaries in tropical regions, especially given their prevalence and vulnerability. The authors evaluated nutrient cycling in the estuary, as well as how debris and sediment buildup in canals influence water quality in connected parts of the bay.
Through sediment analysis, the authors found that nitrogen fixation could be a significant source of nitrogen in the most urbanized parts of the estuary, where, for example, raw sewage enters the water. Much of the nitrogen fixation could stem from sulfate-reducing microbes, which are common in mangroves but have not been previously documented in urban systems. Furthermore, the nitrogen contributions from the bacteria appear to equal or exceed those from urban runoff and sewage.
The findings help to explain anoxic conditions, fish kills, and algal blooms that have occurred in parts of the estuary. The research also highlights how San Juan’s growth and lagging infrastructure have contributed to hydrological changes and an increase in residence time for nitrogen in the water.
The study is one of the first to link the biogeochemical and hydrologic conditions of the San Juan Bay Estuary. Although San Juan was the focus of this research, the study authors lay out a plan for conducting similar research in other urban estuaries around the world. (Journal of Geophysical Research: Biogeosciences, https://doi.org/10.1029/2019JG005502, 2020)
—Aaron Sidder, Science Writer