Unexpected Nitrogen Sources in a Tropical Urban Estuary

Abstract

Tropical urban estuaries are severely understudied. Little is known about the basic biogeochemical cycles and dominant ecosystem processes in these waterbodies, which are often low lying and heavily modified. The San Juan Bay Estuary (SJBE) in San Juan, Puerto Rico, is an example of such a system. Over the past 80 years, a portion of the estuary has filled in, changing the hydrodynamics and negatively affecting water quality. Here we sought to document these changes using ecological and biogeochemical measurements of surface sediments and bivalves. Measurements of sediment physical characteristics, organic matter content, and stable isotope ratios (δ13C, δ15N, δ34S) illustrated the effects of the closure of the Caño Martín Peña (CMP) on the hydrology and water quality of the enclosed and semi‐enclosed parts of the estuary. The nitrogen stable isotope (δ15N) values were lowest in the CMP, the stretch of the SJBE that is characterized by waters with low dissolved oxygen and high fecal coliform concentrations. Despite this, the results of this study indicate that nitrogen (N) contributions from N‐fixing, sulfate‐reducing microbes may meet or even exceed contributions from urban runoff and sewage. While the importance of sulfate reducers in contributing N to mangrove ecosystems is well documented, this is the first indication that such processes could be dominant in an intensely urban system. It also underscores just how little we know about tropical coastal ecosystems in densely populated areas throughout the globe.

Recovering From Bias: A Call for Further Study of Underrepresented Tropical and Low‐Nutrient Estuaries

“…and Yangzte River in China (Jiang & Li, 2014;Xu et al, 2005;Yin et al, 2017) have reported that DNRA dominates when the sulfide transition zone is near the sediment surface. A few studies in developing countries, such as Thailand and Indonesia (Dong et al, 2011), support the more extreme findings of Oczkowski et al (2020) of high DNRA and low-to-undetected levels of DNF, suggesting that low-latitude estuaries, like low-nutrient estuaries, likely respond differently to environmental controls than their temperate and even their subtropical counterparts.…”

“…Additionally, 83% were located in the Northern Hemisphere, Atlantic systems, and a massive 95% of the study sites were located in eutrophic or hypereutrophic systems (annual N load > 20 g N m −2 year −1 , Figure 1). By comparison, the data available from tropical and low‐nutrient or oligotrophic estuaries (annual N load < 20 g N m −2 year −1 ) are incredibly limited (Cook et al, 2004; Eyre et al, 2011; O'Meara et al, 2017; Oczkowski et al, 2020; Thomson et al, 2012). Additionally, a large proportion of studies in low‐nutrient systems are now nearly 30 to 40 years old and employed out‐of‐date methods such as the acetylene block technique (e.g., Kaspar, 1983; Kieskamp et al, 1991; Koch et al, 1992), making direct comparisons to modern studies difficult (Groffman et al, 2006).…”

“…Eyre & Balls, 1999;Oczkowski et al, 2020). Indeed, the results presented by Oczkowski et al (2020) show that in an urban, tropical estuary with restricted flow, the traditional view of estuaries as nutrient filters no longer applies. The high organic matter and sulfide levels result in an ecosystem where nitrogen-fixing, sulfate-reducing microbes produce as much N as the load from urban runoff and sewage.…”

“…Thus, maintaining and recovering functionality of estuaries is crucial from both an economic and ecological perspective. Further, the article by Oczkowski et al (2020) recently published in this journal highlights the need to turn our collective attention to understudied, tropical, urban estuaries. We seek to broaden that call to action to all tropical estuaries as well as noneutrophic, low-nutrient systems around the globe, in order to achieve a truly representative understanding of estuarine ecosystem functions.…”

“…Approximately one third of the global population lives in low‐elevation coastal zones (LECZ), over 75% of which reside in developing nations (Neumann et al, 2015). The combination of densely populated LECZs, tropical locales, low socioeconomic standing, and lower‐quality infrastructure make tropical, and particularly tropical urban estuaries, especially vulnerable (McGranahan et al, 2007; Oczkowski et al, 2020). However, without good benchmarks for comparison, estuarine restoration and recovery is likely to be compromised.…”