Sentence:  The physicochemical processes that control the estuarine filter must therefore be considered to operate in terms of a framework involving particulate-dissolved recycling associated with three estuarine <WSD>end-members</WSD>, i.e. river water, sea water (which may consist of more than one component <WSD>end-member</WSD>), and sediments. 
Meaning of the word marked with <WSD> tags:  A member at one end of a range or series made up of similar members.