Historical Baseline Assessment of Coastal Pollution in Bou Ismaïl Bay (Algeria) Using a Multi- Compartment Biomonitoring Approach

The health of the ocean is often measured by the life it supports, particularly the vast underwater forests of seagrass that line the Mediterranean coast. These meadows, formed by a plant called Posidonia oceanica, act as the foundation for the local ecosystem, providing food and shelter for countless creatures while stabilizing the seabed. Because these plants are sensitive to changes in their environment, their presence, density, and overall structure serve as a reliable barometer for water quality. When the water is clean and stable, the meadows thrive; when pollution enters the system, the plants struggle, and the entire community suffers. Alongside the seagrass, scientists also look at the sediment trapped within the plant roots and the animals that live among them, such as sea urchins, to understand how long-term pollution accumulates and moves through the food web. This approach allows researchers to see not just the immediate state of the water, but the history of contamination that has settled into the environment over time.

In a study focused on the central coast of Algeria, researchers set out to establish a clear picture of the environmental history in Bou Ismaïl Bay and its surrounding areas. They chose three distinct locations to compare: a relatively quiet coastal zone near Tipaza, a heavily industrialized and urbanized area around Bou Ismaïl, and a busy tourist peninsula at Sidi Fredj. By sampling the water, the seagrass itself, the muddy sediment beneath the plants, and the tissues of sea urchins, the team created a multi-layered view of the bay's health. Their goal was to determine how human activity—from agriculture and industry to tourism and sewage—had altered the natural balance of this marine environment.

The results painted a stark picture of degradation driven by human pressure. In the quietest area near Tipaza, the seagrass meadows were dense and healthy, with hundreds of plant shoots growing in every square meter. However, as the researchers moved toward the more developed zones, the health of the underwater forests collapsed. In the industrial and urban areas of Bou Ismaïl and Sidi Fredj, the number of seagrass shoots dropped by more than half, and the plants covered far less of the sea floor. The researchers calculated a conservation score for the meadows, which measures the ratio of living plants to dead, decaying matter. In the healthy zone, this score was high, indicating a thriving ecosystem. In the impacted zones, the score plummeted, signaling that the living meadows were being replaced by dead mats of organic debris, a sign of severe ecological stress.

This decline in plant life was directly linked to the quality of the water and the sediment. The water in the Bou Ismaïl area was found to be heavily contaminated with nutrients, particularly ammonium and phosphates, which are common in untreated sewage and agricultural runoff. These nutrients caused the water to become cloudy and fueled the growth of algae that choked the seagrass. The water also contained high levels of bacteria from fecal matter, far exceeding safe limits for swimming, which pointed to a massive influx of raw sewage and waste. The sediment beneath the plants acted as a long-term storage tank for these pollutants. The mud in the most affected areas was loaded with high concentrations of metals like iron, aluminum, and zinc, which had settled there over time from industrial discharges and urban runoff.

The study also revealed that these pollutants were not just sitting in the mud but were entering the food chain. The sea urchins, which graze on the seagrass and live in the sediment, showed signs of accumulating these metals in their bodies. While the levels in the animals were generally lower than in the sediment, the presence of these elements in living tissue confirmed that the pollution was bioavailable and moving through the ecosystem. One particularly striking finding was a localized spike in mercury levels found in the seagrass roots near Sidi Fredj. This specific contamination likely stemmed from historical activities at the nearby recreational marina, such as the use of antifouling paints on boats, which had left a chemical footprint that persisted in the environment long after the initial discharge.

Ultimately, the research highlights how different types of human activity leave distinct and damaging marks on the coastal environment. The industrial and agricultural zones suffered from a broad mix of chemical and nutrient pollution that degraded the water and suffocated the seagrass. The tourist areas, while perhaps cleaner in some respects, still showed signs of localized contamination from marina operations. The study concludes that without significant improvements in wastewater treatment and stricter control over industrial and agricultural runoff, the coastal ecosystems of this region will continue to deteriorate. The data collected serves as a crucial historical baseline, providing a reference point to measure future changes and the effectiveness of any efforts to restore these vital underwater forests.