Final answer:
Estuaries are biomes where freshwater rivers meet the ocean, resulting in a mixture of fresh and saltwater. The productivity in estuaries is generally low compared to coastal seas. The basis of most estuarine food chains is the detritus.
Step-by-step explanation:
Estuaries are biomes where freshwater rivers meet the ocean, resulting in a mixture of fresh and saltwater. Due to the frequent tidal fluctuations, the salinity in estuaries varies considerably. As a result, the productivity in estuaries is generally low compared to coastal seas. The basis of most estuarine food chains is the detritus, which includes decaying organic matter and dead organisms. Detritus is an important source of nutrients for bacteria and other decomposers, which in turn support the estuarine food web.