When phosphorus is scarce, adding more phosphorus fuels growth. When nitrogen runs out, the system tends to favor organisms that can pull nitrogen from the air — including filamentous cyanobacteria, a group of photosynthetic bacteria that sometimes form visible surface blooms.

What are the ecological consequences?

Huseby said the trend is already reshaping the northern Baltic in visible and unwelcome ways.

"I was surprised by the clear decline in the DIN:DIP ratio, especially in the Bothnian Bay," she said. "If this trend continues, the area could become nitrogen-limited within a few decades. Nitrogen limitation favors filamentous cyanobacteria over other phytoplankton, which can disrupt ecosystems and increase the risk of harmful algal blooms."

She added: "We are already seeing more of these cyanobacteria in the Bothnian Sea, and they may become common in the Bothnian Bay as well."

Cyanobacterial blooms are a hallmark of eutrophication — the over-enrichment of waters with nutrients. The consequences include murkier water, oxygen-starved "dead zones" on the seabed, and food-web disruptions that can ripple from plankton up to commercial fish.

Where is the extra phosphorus coming from?

The researchers point to a likely culprit: phosphorus-rich water flowing northward from the Baltic Proper, the central Baltic, into the Gulf of Bothnia. That finding turns the Gulf's ecological future into a Baltic-wide problem rather than a local one.

What does this mean for management?

For decades, environmental policy in the region has assumed phosphorus was the limiting nutrient in northern Baltic waters. That assumption now looks outdated.

"To reduce eutrophication in the Gulf of Bothnia, efforts across the entire Baltic Sea must continue," Huseby said. "Much of the phosphorus increase originates from the Baltic Proper, making regional cooperation through HELCOM, the Baltic Marine Environment Protection Commission, essential."

She added that reducing dead zones, cutting nutrient inputs and addressing climate change are all needed to limit harmful cyanobacterial blooms. HELCOM coordinates environmental protection among the nine Baltic Sea coastal states.

What are the study's limitations?

The conclusions rest on long-term monitoring data, which is robust but coarse. The study documents a clear trend rather than identifying a single cause.

Climate change, agricultural runoff, wastewater treatment upgrades, and shifts in the Baltic Proper's circulation could all contribute to the phosphorus build-up, and the paper does not separate their effects.

The Bothnian Bay is not yet nitrogen-limited. Whether it crosses that threshold within a few decades depends on how phosphorus inputs and climate variables evolve together.