Spatiotemporal Variations in Macrozoobenthic Communities at the Mouth of the Mezitli Stream, Mersin Bay (Northeastern Mediterranean)

This study evaluated the ecological status of macrozoobenthic communities at the mouth of Mezitli Stream, Mersin Bay, under spatiotemporal variability and pollution pressures. Sediment samples were collected from six stations during dry and wet seasons; Shannon-Weaver Diversity Index, Margalef's Species Richness Index, and AZTI's Marine Biotic Index (AMBI) were used to assess ecosystem health. In addition, the oxidation-reduction potential (ORP) of the sediment was measured as a supportive indicator of environmental conditions. During the dry season, low Shannon and Margalef values, together with low ORP levels, were observed, especially at Stations 5 and 6, due to possible oxygen deficiency and degraded benthic structure. These stream-influenced stations stood out as the most degraded areas with high AMBI scores, dominance of tolerant species (Nereis sp., Corbula gibba), and low dissolved oxygen levels. During the wet season, increased freshwater inflow and improved oxygen resulted in richer and more balanced communities (e.g. Alvania discors, Tellina sp. and Mangelia sp.). Despite these conditions, Stations 5 and 6 exhibited lower Shannon, Margalef, AMBI scores and ORP levels. The findings reveal that seasonal changes in organic load and oxygen levels are the main factors determining the structure of the benthic ecosystem in the Mezitli Stream mouth area.