Methodologies for monitoring and assessment of marine litter are not readily available. They need to be developed, harmonized and implemented.
Marine litter - Options and Recommendations
Following the adoption of the MSFD ComDec 2010/477/EU, the TG ML has prepared a report on the options for assessing marine litter in the marine environment. This report on options and recommendations was a direct support to the MS implementing the MSFD and the basis for further work of the TG ML.
Guidance for monitoring
The MSFD Technical Group on Marine Litter has reviewed existing approaches and provides guidance for the monitoring of marine litter. While some methodologies are based on existing approaches, adjusted to MSFD needs, new ones have also been developed:
Beach Litter
Monitoring of Litter on beaches 

Link to Guidance p.38-46
Floating Litter
Monitoring of litter floating in the sea surface layer 

Link to Guidance p47-55
Seafloor Litter
Monitoring of litter on the sea floor 

Link to guidance p 56-65
Litter Category Master List
The monitoring of marine macro litter is based on its identification according to a list of items. 

(link to TG ML Guidance for litter) p.117-123

The technical group on marine litter has started a joint effort, to review the list, ensuring its compatibility across Europe and beyond.
Microplastic
Monitoring of litter items/particles below 5 mm size
Ingested Litter
Litter ingested by biota, such Fulmars or turtles and quantified by the analysis of stranded animals.
Entanglement
Monitoring of marine species being entangled by marine litter
Riverine litter
Monitoring of litter entering the marine environment via rivers