Short-term effects of in situ nutrient enrichment of the sediment and the macroalgae Chaeotomorpha linum loads on Cymodocea nodosa: A trait-based approach

Coastal eutrophication driven by increasing nutrient inputs is a major threat to seagrass ecosystems, often acting in synergy with opportunistic macroalgae proliferation. This study investigated the short-term combined effects of sediment nutrient enrichment and varying loads of the green macroalgae Chaetomorpha linum on the structural, morphological, and physiological traits of the seagrass Cymodocea nodosa. A 90-day in situ factorial experiment was conducted in Bekalta, Tunisia, combining three nutrient enrichment levels and four macroalgal levels treatments. Seagrass structural, morphometric and physiological traits were monitored.Nutrient enrichment significantly increased porewater nutrient concentrations and initially stimulated leaf growth, shoot surface area, and chlorophyll content, indicating an initially nutrient-limited system. However, macroalgae load had a stronger negative impact on plant performance, causing marked reductions in shoot density, aboveground biomass, and growth rates, primarily through light limitation. Physiological analyses revealed that nutrient enrichment and algal shading induced metabolic adjustments, including decreased soluble proteins, sugars, and phenolic compounds, alongside enhanced antioxidant enzyme activity. These responses indicate a shift in carbon allocation toward nitrogen assimilation and oxidative stress regulation. Despite increased nutrient availability, epiphyte biomass also rose significantly, potentially exacerbating light stress.Overall, the results demonstrate that C. nodosa exhibits shortterm physiological plasticity under nutrient enrichment, but macroalgal overgrowth remains the dominant stressor affecting meadow structure. While nutrient inputs did not immediately impair plant integrity, they induced metabolic costs that may compromise long-term resilience. These findings highlight the importance of managing both nutrient loading and macroalgal proliferation to preserve Mediterranean seagrass ecosystems and their associated ecological functions.