Water quality affects the structure of copepod assemblages along the Sfax southern coast (Tunisia, southern Mediterranean Sea)

Abstract

The Sfax southern coast (Gulf of Gabes, Mediterranean Sea) has been under increased anthropogenic pressure for many years. In the present study we investigated the effects of this anthropisation on the spatial distribution of copepod assemblages in relation to the physicochemical features of seawater at 20 stations sampled on 19 March 2013. Copepods represented 73% of total zooplankton abundance. Small planktonic copepods (<1.45mm), including pollution-tolerant species (e.g. Oithona nana, Paracalanus parvus, Harpacticus littoralis and Tisbe battagliai), proliferated exclusively in stations of ~0.5-m depth characterised by high coastal anthropogenic inputs. The largest copepod species were dominated by Calanus helgolandicus (1.45–2.5mm) in the offshore zone in depths of ~3m. Substantial numbers of Oithona plumifera (7.5%) were found at depths between 0.5 and 3m. Copepod diversity was significantly higher in the southern zone, which is less affected by sewage, than in the northern zone, which was subjected to higher pressure (Shannon–Wiener index H′=1.5–2.5 and ≤1.5 bits individual–1). A shift in the planktonic copepod community between the two zones was linked to deterioration of water quality, with higher phosphorus levels, turbidity and chemical oxygen demand (COD) in the northern zone.