Invisible life is vanishing from the Mediterranean - and scientists know why

A long coastal record from the northwestern Mediterranean reports a steady drop in marine viruses since 2011.

The decline tracks warmer, clearer, and leaner waters, a change linked to climate pressure in a region already flagged as a hotspot.

The data come from Blanes Bay, a coastal site in Girona where scientists have sampled surface waters every month for nearly two decades.

The pattern is clear, and it matters because viruses help set the pace of ocean chemistry and food webs, from microbes to fish.

Marine viruses are fading

The study was led by Xabier López-Alforja, a marine microbial ecologist at the Institute of Marine Sciences (ICM-CSIC) in Barcelona.

López-Alforja’s work explores how viruses interact with their microbial hosts and how those relationships shift under changing climate conditions.

Western Mediterranean waters are warming faster than many other seas, according to an IPCC regional assessment.

The warming builds on a longer trend of dwindling nutrient supply, a process called oligotrophication – the gradual loss of nutrients that leaves the sea less productive.

Scientists watching Blanes Bay report a sustained fall in viral abundance that begins in the early 2010s and persists through recent years.

The team also noted rising water clarity and less phytoplankton, tiny drifting plants that form the base of marine food webs.

Losing marine viruses

Marine viruses aren’t minor players. They destroy microbes, triggering what’s known as the viral shunt – a recycling process that releases carbon and nutrients back into surface waters.

This recycling process can also redirect carbon downward when infected cells clump and sink, a pathway called the viral shuttle. If viruses become scarcer, fewer cells may rupture, and those nutrient and carbon flows can slow.

“Climate change is reshaping even the tiniest microbial communities that keep the ocean healthy,” noted study co-author Dolors Vaqué.

“As viruses decline, nutrient cycles shift, threatening coastal ecosystems and the fishing communities that depend on them.”

What the long record shows

The Blanes Bay team gathered monthly samples for nearly 18 years, capturing regular seasons and unusual years alike.

The researchers used generalized additive mixed models, flexible statistical models for nonlinear time patterns, to separate seasonal swings from long trends.

The models reveal a clear turning point around 2011. That’s when viral counts begin to decline, and the once-strong seasonal rhythm starts to fade.

This change coincides with rising sea temperatures and a steady drop in nutrients and chlorophyll levels. These trends mirror regional signs of nutrient depletion and reduced productivity across the western Mediterranean.

Earlier research even documented a yearly decrease in chlorophyll near the Balearic Islands, pointing to a broader pattern of ecological slowdown.

Factors predicting viral abundance

The team also used neural networks to identify which factors best predict viral abundance.

The passage of time – represented by the variable “year” – emerged as the strongest driver, reflecting long-term environmental pressure.

Meanwhile, nutrient levels and the abundance of host microbes accounted for the shorter, month-to-month fluctuations.

Marine viruses in a warming sea

Heat does not act alone, but it sets the stage. The Mediterranean has seen repeated marine heatwaves and a strong warming signal in models and observations, a pattern echoed by broader climate studies focused on this basin.

At the same time, fewer nutrients mean slower host growth. Experiments show nutrient stress can delay infection and shrink the burst size, the number of viruses released when a cell breaks.

Phosphorus scarcity appears especially important because viruses are rich in nucleic acids. Previous research found that phosphorus shortage can suppress lysis and tilt infections toward quieter states.

Changing virus patterns

When nutrients run low, viruses can slip into lysogeny, a quiet state where viral DNA hides inside the host and waits for better times.

Evidence from polar seas shows that higher pCO2 and certain temperatures can raise lysogeny relative to active killing, as reported in experimental results.

The season still shapes life in Blanes Bay. Winter usually brings more nutrients and a bump in chlorophyll, while summer tends to be clearer and leaner, conditions that can slow infection.

The record also tracks changes in potential hosts, like bacteria and cyanobacteria. These hosts often peak before viruses, a sign that host growth helps set the viral calendar.

The future of marine viruses

Genetic diversity is the next question. The team plans to sequence decades of archived viral material to see whether fewer particles also means fewer kinds of viruses.

Management questions follow closely behind. Fishing communities rely on stable biogeochemistry, the cycling of elements that feeds plankton, and shifts in viral activity ripple through that system.

Long-term records are rare – and essential. Without them, natural cycles of rise and fall can mask the slow, steady shifts that only emerge after many years of observation.

The Blanes Bay record shows that even the smallest players are sensitive to heat and hunger. That sensitivity tells us where the system may bend next.

The study is published in the journal ISME Communications.

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