Abiotic Sponge Ecology Conditions, Limski Kanal and Northern Adriatic Sea, Croatia

Abstract

The Limski kanal, a semi-closed inlet (channel-like bay) located on the western coast of Istria (Croatia), is an extraordinary sponge habitat. Research on the marine ecosystem has been conducted there for more than 100 years. Today, 42 valid Porifera species are described. 139 species are listed for the area around Rovinj and 159 species for the northern Adriatic Sea. While several scientists described the sponge fauna, information on the abiotic situation or an explanation for the diversity differences is missing. This study interprets physicochemical and ecological parameters including depth profiles (temperature, salinity, pH-value, oxygen, water current velocity, photosynthetically active radiation [PAR], water transparency and nutrients). Their variances are linked with the distribution patterns of the prevailing sponge fauna. The channel is characterised as predominantly marine habitat. Although a shallow sediment barrier and a headland reduce the impact of the northern Adriatic Sea, differences between the channel and the open sea seem to be limited. Compared to the more homogenous water body of the Adriatic sampling locations, the channel shows variations and gradients of ecological parameters between different locations (e.g., due to freshwater influx: nutrients, temperature, oxygen content, salinity and water current velocity) – this offering habitat diversity. The sponge fauna changes along those ecological gradients. It is dominated by the photophilic species Aplysina aerophoba and Chondrilla nucula, but sciaphilic species such as Dysidea avara, Axinella polypoides and Aplysina cavernicola can be found, too. Some specialised species (e.g., Geodia cydonium, Tethya aurantium) even populate the muddy bottom of the channel.