The marine ecosystems of the Amazon Continental Shelf (ACS) are strongly influenced by the region’s rivers, especially the Amazon River and the Pará River estuary, which discharge more than 30% of all the freshwater that reaches the world’s oceans. That’s impressive, isn’t it?
But that’s not all. These rivers also carry nutrients to the sea, which act as essential “food” for microscopic organisms such as algae. These organisms perform what we call primary production: they transform sunlight into energy, which then fuels the entire marine food chain. It is estimated that the Amazonian rivers contribute around 25% of this energy production in coastal areas!
This importance stems from the large volume of nutrients (substances that help organisms grow) that the rivers carry to the sea. These nutrients are crucial for the growth of microalgae—tiny marine plants that help convert carbon dioxide (CO₂) into less harmful substances, benefiting the environment.
Image 1. Sargassum. Avaiable at: https://fadliqnoze.blogspot.com/2011/10/sargassum-sp.htmFadli Edward D’Silva Qnoze: Sargassum sp.html
Since 2011, scientists have observed an increase in large patches of seaweed known as Sargassum. It forms floating “islands” in the ocean. These algae do not grow on the seabed and, although they are natural, when they become too abundant, they can harm marine life, tourism, and fishing. The exact cause of this overgrowth is still not fully understood, but it is believed that changes in ocean temperature are influencing the phenomenon. Sargassum. Ela forma “ilhas” flutuantes sobre o oceano. Essas algas não crescem no fundo do mar e, apesar de serem naturais, quando crescem demais, podem prejudicar a vida marinha, o turismo e a pesca. A causa desse crescimento exagerado ainda não é totalmente entendida, mas acredita-se que as mudanças na temperatura dos oceanos estejam influenciando esse fenômeno.
To investigate this, researcher Rafael Aquino from the Marine Environmental Research and Monitoring Laboratory (LAPMAR) conducted a study between 2018 and 2019. He analyzed 82 points along the ACS during different times of the year: the rainy season, the dry season, and transitional periods. During the study, data such as pH, water temperature, salinity, dissolved oxygen, turbidity, and nutrients like ammonium (a type of nutrient that supports plant growth) were measured.
Image 2. Map of the collection point. Avaiable at: https://www.sciencedirect.com/science/article/pii/S0048969722055310
The results showed that during the dry season, the seawater was saltier, warmer, and had higher oxygen levels. In the rainy season, the water became more turbid, and more acidic. The amount of chlorophyll a (an indicator of the presence of algae) varied throughout the year. Ammonium was the most abundant nutrient, especially during the rainy season when the volume of water in the Amazon River and Pará River estuary increased. clorofila a (um indicador de presença de algas) variou ao longo do ano. O amônio foi o nutriente mais encontrado, especialmente na época de chuvas, quando o volume de água do rio Amazonas e estuário do rio Pará aumenta.
Here’s the key point: this increase in ammonium coincided with a greater presence of Sargassum, suggesting that the nutrients carried by the rivers may be fueling the overgrowth of these algae. However, human activities also play a role in this process. The study indicated that deforestation, excessive use of fertilizers in agriculture, and untreated sewage dumped into rivers can all increase nutrient levels, leading to a process known as eutrophication. In eutrophication, the excess of nutrients causes algae to grow uncontrollably, harming the entire ecosystem. Sargassum, o que indica que os nutrientes que vêm dos rios podem estar ajudando essas algas a crescerem demais. Mas a ação humana também tem influência nesse processo: o estudo indicou que o desmatamento, o uso exagerado de fertilizantes na agricultura e o esgoto sem tratamento jogado nos rios podem causar aumento dos nutrientes, causando a chamada eutrofização – quando há tanto nutriente que as algas crescem de forma descontrolada, prejudicando todo o ecossistema.
Therefore, understanding how the Amazonian rivers influence the ocean is essential. It helps protect biodiversity and ensures the safety of coastal communities and ocean-based activities.
