Seasonal dynamics of upper sublittoral assemblages on Mediterranean rocky shores along a eutrophication gradient

•Seasonal dynamic of macroalgae assemblages along a eutrophication gradient has been studied.•Nutrients showed different seasonal patterns depending on the distance to pollution.•Development of Ulva and Corallina-dominated assemblages peaked in early spring (March).•Total coverage of Cystoseira-dominated assemblages increased in May and they maintained a high coverage until September.•Nutrients supply did not affect the seasonal cycle of macroalgae.

Changes in the seasonal dynamics of Mediterranean macroalgal-dominated assemblages from the upper sublittoral zone are described along a gradient of sewage pollution. Algal coverage and composition were measured for more than one year at approximately monthly intervals. Nutrients concentrations (nitrites, nitrates, ammonia and phosphates) showed different seasonal patterns depending on the distance to the pollution focus. Ulva-dominated assemblages appearing close to the sewage outfall showed maximal coverage during early spring and started to decrease in May. Corallina-dominated assemblages - replacing ulvacean algae at intermediate levels of pollution - followed the same pattern, peaking in March and decreasing in May. In contrast, Cystoseira-dominated assemblages, present at sites with low or no influence of sewage, increased coverage in May and continued with high coverage until the end of summer. Neither ephemeral algae (Ulva spp.), nor stress-tolerant algae (Corallina elongata), nor canopy-forming algae (Cystoseira spp.) have a seasonal growth cycle governed by eutrophication. Results demonstrate also that the period from May to July is the best time of the year for Water Framework Directive (WFD) monitoring purposes as less variability is observed within the assemblages.

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