The following article shows the utilization of waste from a marine recirculating fish as a biomass source. Hope it will be quite useful for your interest.
Biomass dry weight of marine trash can be calculated once you evaluate its density on a dry test bed of 1 sqm. But, segregation of marine trash is utmost essential before attempting to calculate weight. What you could really do is segregate the trash from your survey point into major groups and take density of each group. Then, you could proceed for plotting a weight distribution curve rather than a specific value of weight.