Saltwater is heavier than freshwater. Coastal freshwater aquifers which are often the source of drinking water in cities are increasingly under threat as saltwater invades. In South Florida, cities like Fort Lauderdale and Miami in search of freshwater are tapping the inland aquifers of the state because those wells closest to them are contaminated by saltwater. South Florida isn’t alone in facing this threat. Texas and other coastal communities from Louisiana to New England face saltwater intrusion into their freshwater sources.

Desalination of seawater is increasingly seen as the solution for those areas of the U.S. coast under threat. Current desalination technology, however, remains pricey and wherever it is constructed, poses a threat to marine and freshwater habitats in proximity to these facilities.

New Orleans has a different freshwater challenge than saltwater intrusions into local aquifers. The Mississippi River for the last few years has faced prolonged drought in those areas of the country that feed freshwater into it. The result has been a drop in water levels and a decrease in the rate of flow. New Orleans relies on the river as its freshwater source which is all well and good if the areas from where you draw the water are not compromised by these changing river conditions.

So what is happening near the mouth of the Mississippi River? It appears that with changes to the continental climate conditions of the river’s watershed, saltwater from the Gulf of Mexico is creeping upstream. This saltwater intrusion is a direct threat to the 1.2 million who live in Greater New Orleans.

In September, Joe Biden declared the situation to be a state emergency and authorized funding and federal government resources to implement protective measures to secure the city’s water supply. Described as a wedge of saltwater, New Orleans has been monitoring its progress which until a few days ago appeared to be putting most of its freshwater treatment plants under threat. Since then the saltwater has retreated almost 9 kilometres downstream. Conditions inland, however, don’t hold much promise for a return to normal Mississippi River levels and rates of flow. The result is that federal funds and resources will help to build a new pipeline to draw freshwater from further upstream.

The changes in the Mississippi are not all caused by climate change. The river provides for the movement of goods throughout much of the continental United States. Dredging by the Army Corps of Engineers to deepen the river near the Port of New Orleans to accommodate larger ships is enabling saltwater to move upstream. The Army Corps is no stranger to the city and the river. It has been re-engineering the latter since the 19th century. It has built barriers on the river floor to block saltwater intrusion. The latest this year constructed a 7.5-metre (25-foot) barrier on the river bed to inhibit the saltwater intrusion. The barrier failed with saltwater flowing over it.

The new pipeline is a temporary fix because climate conditions in the Mississippi’s extensive river valley and surrounding watershed, according to climatologists, will continue to see the drying of the continental interior.

Tyler Antrup, at the Tulane School of Architecture, who is a green infrastructure expert recently stated “A lot of folks here don’t understand the way that the river works…We’re not expecting everyone to be hydrologists…I think the fact that the primary driver of this event is a drought in the Ohio River Valley is something that is very difficult for an average resident to understand….We need to work on how do we better demystify the whole watershed for our residents because it passes by our front door.” The Ohio River is the main eastern tributary of the Mississippi.

The Netherlands has faced the challenge of saltwater intrusion for centuries. Dutch hydrogeologists point to alternative sources for freshwater supply. These include rainwater catchments, stormwater capture systems, and greywater recycling by homes and businesses to reduce the need to draw water from the Mississippi.

Uprooting the people who live in New Orleans and other coastal communities under similar threat may be, in the end, the ultimate solution as sea levels continue to rise. But it is not our nature to give up on places where humans have established roots for centuries. The Netherlands is a prime example. So I expect desalination and other technologies along with freshwater conservation will help New Orleans get through much of the remainder of this century.

Bounded by ocean currents, the Sargassum occupies an area known as the Northern Atlantic Subtropical Gyre with its western boundary the Gulf Stream, northern, the North Atlantic Current, eastern, the Canary Current, and southern, the North Atlantic Equatorial Current. As you can imagine if the currents fluctuate for different reasons then the boundaries of the Sargasso Sea get equally altered.

What would cause the currents to fluctuate? Warmer than normal surface temperatures in the North Atlantic. This is a phenomenon that scientists have been observing in the North Atlantic north of the equator, the area of the ocean most commonly associated with the birth of hurricanes. So if the ocean surface is warmer and ocean currents show greater signs of oscillation, then the boundary confinement of the Sargasso Sea becomes more tenuous allowing the algae to spread.

But there is another phenomenon at play here. According to research done at the University of South Florida based on two decades of satellite data, the Amazon is playing a significant role in destabilizing the amount of Sargassum in the ocean causing the blooms to grow. What is actually happening is runoff from the Amazon is bringing fertilizer discharge into the North Atlantic Equatorial Current which is then feeding algae blooms on a massive scale outside the traditional boundaries of the Sargasso Sea.

The authors of the South Florida study call this new phenomenon the great Atlantic Sargassum Belt, and it is spreading since 2011 to cover an area from the West African coast to the Caribbean, and the Gulf of Mexico. It is over 8,850 kilometers (almost 5,500 miles) in extent, “the world’s largest macroalgal bloom” that “may become the new normal.” The estimated mass of the bloom in 2018 was 20 million tons, bigger than the previous peak volume in 2015 (see graph below), and appearing to be growing larger each year. Large volumes of Sargassum are drifting onto the shores of the Yucatan, Caribbean islands, the Florida coast, and are being picked up by the Gulf Stream and being deposited along the Carolina coast and even further north.

Other than beach despoliation, is this Sargassum mega growth a problem?

The Sargasso Sea is a giant nursery for the ocean’s tiniest inhabitants, the basis of the food chain. It provides a hatchery for shrimp, crabs, fish, turtles, and other marine species. But in excess as it drifts to shorelines it becomes a biohazard as it rots, deoxygenating the water and turning it into toxic sludge.

Add to the this, Red Tide, algae far more toxic and responsible for killing sea life on a colossal scale annually, and you have a deadly environmental feedback mechanism. Sargassum, can harbour within it Red Tide algae which when it releases its neurotoxins, even causes the host to die further contaminating shorelines and coastal waters.

The change to ocean chemistry combined with rising ocean surface temperatures is a double threat that can only be countered by changes in human behaviour. In places like Amazonia, it requires governments to restrict the use of human-created fertilizers containing nitrogen, phosphorus, and ammonia. Only then can the algal blooms begin to reduce in volume.

That doesn’t let the planet off the hook. Because other countries, not just Brazil, are equally culpable in dumping fertilizer runoff into our oceans. Just in the last week, discharge from the Mississippi River fuelled the growth of a blue-green algae known as cyanobacteria that is harmful to fish and other marine wildlife let alone humans who contact it. The Mississippi discharge was necessary to relieve flooding and high water levels further inland. So far these floodwater abatement strategies killed 48 dolphins in May and June, Compare that to the 91 known dolphin deaths in one year from the BP Deepwater Horizon spill, and you can see just how negatively significant algal blooms can become in the marine environment. The current Mississippi bloom has created a deoxygenated zone in the Gulf of Mexico that is over 20,000 square kilometers (approximately 7,829 square miles).