Sentence:  The "s"-score relates to the <WSD>saprobic</WSD> zone of which the species are characteristic; a score of between 1 and 4 is assigned to each species, depending upon whether it is typically found in oligosaprobic, α-mesosaprobic, β-mesosaprobic or polysaprobic zones, respectively. 
Meaning of the word marked with <WSD> tags:  That contains dead or decaying organic material (and therefore also saprobes).