Estuaries are some of the most productive ecosystems on Earth. These dynamic environments, where fresh water from land meets salty ocean water, provide habitats for countless species and are nurseries for important fisheries. They also act as natural filtration systems, improving water quality by pulling excess nutrients and pollution from the water before it reaches oceanic ecosystems.
But nitrogen inputs from human activities have more than doubled since the Industrial Revolution, fueling such calamities as harmful algal blooms and oxygen-poor dead zones near the coasts and making nutrient pollution one of the biggest threats to estuaries today. But in a new review, Vieillard et al. say that managers working to restore and protect estuaries around the world from nutrient loading are limited by incomplete scientific understanding of nutrient cycling in estuaries.
The authors note that previous work on nitrogen dynamics in estuaries has been biased toward nutrient-polluted, temperate estuaries in the North Atlantic, where nutrification from agricultural and urban runoff is a chronic issue. Less attention has been paid to tropical and low-nutrient estuaries. Research that is available from these systems is limited and often decades old, but recent work suggests that tropical estuaries process nitrogen quite differently than temperate ones. Temperate estuaries tend to remove nitrogen via denitrification, for example, but even highly polluted, tropical ones appear to fix and retain nitrogen, which means that models of estuaries as nutrient filters may not always hold.
The authors also suggest that low-nutrient estuaries could provide researchers with insights into the behavior of preindustrial, temperate estuaries before agricultural and urban runoff began loading waterways with excess nutrients. Previous research suggests that before the Industrial Revolution, North American and European estuaries were net importers of nitrogen, at a time when coastal fisheries were highly productive.
Indeed, although much research has suggested that increased nutrient inputs lead to higher productivity, many low-nutrient estuaries can be as or even more productive than those with excess nutrients. This is because excess nutrients can increase phytoplankton growth at the expense of other forms of primary production, such as the growth of kelp, microalgae, and seagrasses.
To gain a more complete picture of nutrient cycling in the world’s estuaries and to predict how management decisions will affect these ecosystems, the scientists call for more research into the biogeochemistry of tropical and low-nutrient estuaries, which, they note, could serve as benchmarks for estuarine recovery amid climate change. (Journal of Geophysical Research: Biogeosciences, https://doi.org/10.1029/2020JG005766, 2020)
—Kate Wheeling, Science Writer