Environmental gradients and macroalgae in Mediterranean marshes: the case of Pego-Oliva marsh (East Iberian Peninsula)

•Mediterranean marshes are highly diverse in macroalgae despite high nitrogen levels•Macroalgal assemblages can be used to evaluate ecological quality of Mediterranean marshes•These systems are severely threatened by human activities and climate change•Mediterranean marshes deserve to be protected and better studied as sentinels of climate change

Although Mediterranean marshes have historically suffered high anthropogenic pressure, they have maintained their remarkable biodiversity. They are severely threatened but remain comparatively unexplored systems from the algological point of view. For example, most of the indexes proposed for monitoring ecological quality are based on diatoms and very few have explored the use of macroalgae. The Pego-Oliva marsh is located in the east of the Iberian Peninsula close to the Mediterranean coast with warm annual temperature and fairly high precipitation. The aims of this study were to ascertain the ecological variables that explained macroalgal distribution in the Pego-Oliva marsh and to assess their indicator value. Macroalgal biodiversity was seen to be high (50 taxa) despite the high nitrogen concentration of the marsh. All the environmental variables studied had a broad range of variation throughout the marsh, especially conductivity (500-12290 μS/cm), temperature (14.3-31.7 °C), nitrate (9.493-64.113 mg/L) and ammonium (0.004-0.814 mg/L). A clear gradient of conductivity and dissolved oxygen was observed from fresh to saltwater. Batrachospermum arcuatum, Calothrix parietina, Chaetophora tuberculosa, Draparnaldia mutabilis, Hildenbrandia angolensis and Leptolyngbya angustissima were seen to act as indicators of low conductivity and dissolved inorganic nitrogen, and high dissolved oxygen, while Calothrix pulvinata, Ulva intestinalis, Homoeothrix violacea, Phormidium tergestinum and Thorea violacea were indicators of high conductivity and low dissolved nitrogen habitats. Cladophora glomerata, Compsopogon coeruleus, Polysiphonia subtilissima and Ulva flexuosa are the most widespread species and have a broad ecological range. Irrigation ditches have high ammonium and low dissolved oxygen concentrations and host infrequently reported species like Kumanoa mahlacensis. The data presented confirm the usefulness of macroalgae for the ecological monitoring of marshes, while increasing our knowledge of the distribution and ecological range of some species. However, more experimental work is needed to know the tolerance range of species living in dynamic systems like Mediterranean marshes.