"Detritus" by definition is a collective noun, like "trash". It has no singular because it is collective inherently.

arts of dead organisms and cast-off fragments and wastes of living organisms. The forest floor is covered in detritus.
www.hubbardbrook.org/education/Glossary/Glossary.htm


the particulate decomposition or disintegration products of plankton, including dead cells, cell fragments, fecal pellets, shells, and skeletons, and sometimes mineral particles in coastal waters
www.coris.noaa.gov/glossary/glossary_a_k.html


Accumulated organic debris from dead organisms, often an important source of nutrients in a food web.
www.ucmp.berkeley.edu/glossary/gloss5ecol.html


Loose material such as rock fragments and organic particles, that result directly from disintegration of the parent material. Synonymous with debris.
www.wisconline.com/wisconsin/geoprovinces/glossary.html

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