Nutrients are a good thing. They promote growth of cells in plants and animals. Food manufacturers love to advertise that their products, like breakfast cereals, are chock-full of “key” or “vital” or “healthy” nutrients.

In waterways, however, you can have too much of a good thing. Load a lake or an estuary with too much of the nutrient that limits productivity and you might choke the system with algae – the process that scientists call eutrophication because we stink at naming things. All that algae eventually decomposes, or is consumed by other organisms that eventually decompose,  which uses up oxygen crucial to other organisms. That’s a rough explanation of what has happened in the Gulf of Mexico at the mouth of the Mississippi River. Nutrients, particularly nitrogen, leach off agricultural fields and empty into the continental shelf, driving the development of a large “hypoxic” or “dead” zone each summer.

The key issue for the algae is the stoichiometry (again, we stink at naming things) or the ratio of the nutrients: the relative availability of key chemical players like nitrogen (N), phosphorus (P) and silica (Si). Say the ratio of nitrogen to phosphorus in the water is greater than what most algae need; they typically 16 nitrogen”s” for every phosphorus. Then phosphorus becomes the “limiting nutrient”. Add more phosphorus and you should get more growth because there’s already enough nitrogen available. Add more nitrogen, no dice, as there’s not enough phosphorus to match the existing nitrogen levels.

Now, past research has shown that thanks to things like fertilizer loading agricultural soils and groundwater full of nutrients, the key determinant of how much actually ends up flowing down a agricultural river is the weather. For example, a study by myself and Don Scavia showed that a wet fall (which “recharges” the groundwater) in the Midwestern U.S. followed by a wet spring means a large flux of nitrogen down the Mississippi, and all else being equal, a large “dead” zone. But most of that type of runoff and nutrient analysis has focused on individual nutrients, when what really matters to the algae, is the ratio between the nutrients.

Mississippi Delta and northern Gulf of Mexico

A recent paper led by my former student Doris Leong looked sensitivity of the nutrient ratios to variability in runoff, and hence rainfall. The idea was that, since N, P and Si come from different sources and have different solubilities and adsorptive properties, the flow of each nutrient in a watershed may respond differently to a change in rainfall and runoff*. We hypothesized that in an agricultural river basin, where there is a lot of nitrogen in a soluble form in the soils and groundwater, the N:P and N:Si ratio in the river should increase during wetter periods. An analysis using data from across the Mississippi River Basin confirmed the hypothesis, particularly for N:P:

A doubling of the discharge by the Mississippi and Atchafalaya Rivers to the Gulf of Mexico is found to increase the N:P by 10% and the N:Si by 4%. Analysis of data from upstream stations indicates that the N:P increases with discharge in subbasins with intensive row crop agriculture and high fertilizer application rates.

The effect suggests that the weather could influence nutrient limitation productivity downstream. It may also subtly influence the relative abundance of different primary producers (e.g. algae) in river or in the coastal zone. For example, research shows that if N:Si reaches well above 1:1, there can be a shift away from the production of diatoms, which require Si to build their shells.

The results of the paper suggest that, if all else is equal, a wetter climate in the central U.S. would mean higher N:P and N:Si in Mississippi water. It is important to note, however, that all else is unlikely to equal. Changes in agricultural practices, river management, erosion control, etc. will also influence the availability and movement of nutrients. From the conclusion:

The flux of nutrient ratios depends on multiple environmental characteristics of a watershed, including land use, land management, climate and geology, and the river network, and it is difficult to determine the relative importance of these different effects. The dominant nutrient species, sediment transport, presence of dams, as well as in-stream uptake of nutrients, all play a role in controlling the movement patterns of nutrients over the landscape and within rivers. High-resolution data and models that incorporate sediment and dissolved nutrient cycling are needed to understand the multiple factors that influence how nutrient ratios respond to changes in discharge. As future climate change drives an increase in hydrologic variability, the predictability of the response of nutrient ratios to discharge may be important to understanding ecosystem responses to climatic change.

* Technically speaking, we modeled the relationship between river discharge and nutrient load as a power law. Thus the hypothesis was that for a given watershed, the exponent in the power law would be different for N, P and Si.

Road repair along Nippon Causeway, 2014

My recent article in Scientific American discusses how low-lying coral atolls, like the islands of Kiribati, are a lot more resilient to sea-level rise than dire stories in the media may have you think. But don’t let the nuances of reef island geology described in the article create any doubts about the reality of sea-level rise.

The oceans are rising, and the long-term picture for places like Kiribati is not pretty. This was brought home in the past few days by storm surges again damaging Tarawa atoll’s key causeway that links the most populated islet and international port to the rest of Tarawa.

Here’s a short sidebar of the science of sea-level rise which had to be cut from the article due to space considerations:

The oceans are rising, and the rate of that rise is increasing. Global average sea-level has risen 20 cm since the beginning of the last century, and may rise up to a metre or more by the end of this century.

We can blame greenhouse gas emissions and the basic physics of water.
The planet is absorbing extra heat thanks to the human enhancement of the natural greenhouse effect. The atmosphere gets all the attention, but actually receives only 1-2% of that extra heat. The majority – roughly 93% – goes into warming those deep pools of salt water covering two-thirds of the planet. When water warms up, it expands. That thermal expansion of the ocean is responsible for over half of sea-level rise since 1900.

The key issue for future sea-level is the 3% of the extra heat in the climate system going towards melting ice, especially the Greenland and West Antarctic ice sheets. The rate of melt from these great ice sheets, which hold enough water to raise sea-level by about 12 metres, will define the coasts of the future.

The most recent Intergovernmental Panel on Climate Change report concluded that sea-level would likely rise by 52 – 98 cm by the end of this century, but allowed that far greater changes were possible. It all depends on the complicated process of ice sheet melt.

We know that during periods of Earth’s history when temperatures and greenhouse gases were at levels expected for mid-century, ice sheet melt may have raised the oceans more than 5 metres above current sea-level. We don’t know exactly how long it takes for all that ice to break off or melt. Estimates run in the centuries or longer.

The long-term change is the existential concern for a place like Kiribati. From the article:

The fact that reef islands can grow in some cases and that adaptation measures can help will not save Kiribati forever, especially if the world fails to reduce greenhouse gas emissions. Climate models project that if we stay on the current emissions path, sea-level could be rising at the end of the century at more than five times today’s rate. Even in the unlikely case that islands are able to continue, on net, to accumulate material at their current rate, they may become narrower, steeper and possess less freshwater, making them prohibitively expensive to inhabit.

After a discussion of adaptation needs and ongoing initiatives in Kiribati, the article concludes with a thought I’ve had every time my local colleagues and I return to land from collecting data:

Tarawa’s lagoon, visible to a trained eye in the distant clouds

As you travel out to sea in Kiribati, the flat islands quickly disappear below the horizon. In the old times, fishers navigated home by looking for the reflection of the shallow, greenish lagoon waters in the clouds. One day in the distant future, many of the islands of Kiribati could succumb to the sea. The people may leave, the trees may die and the land may become a submerged reef. The lagoons, still shallow in contrast to the deep open ocean, would remain green as before.

To outsiders, Kiribati would be gone. To the Kiribati people, the ghost of their former homeland would live on in the clouds.