The energy consumption of various grinders, chippers, and balers in terms of energy per unit weight of material processed is well known. One can get that number merely by calling up any number of equipment suppliers.

 If one is going to gasify the biomass or process it into ethanol, the biomass is going to have to go through at least one size-reduction step. So my point is simply:  why not do that size reduction right at the point of harvest so as to also increase the bulk density of the biomass and save on transportation cost.

Harvesting pond muck is an interesting idea. As long as the concentration of organic matter is high enough and you can dewater and dry it without consuming a great deal of energy, it might have a positive energy return. It would work much better in warm rather than cool climates, particularly if you want to go with natural solar drying. Again, the economics of material handling is what can make or break such a scheme.  

Building large open ponds or lagoons is relatively cheap and easy. Buidling large ponds with a transparent cover is neither cheap nor easy. Going with long narrow trenches would help ease some of theses construction difficulties.

What about harvesting water hyacinthes?  These floating plants grow wild in Florida , grow incredibly fast, and tend to blanket whole ponds.  They soak up nutrients like crazy, which is why there have been some demonstration projects using a water hyacinthe pond to treat domestic sewage.  I happen to have a tiny koi pond by the side of my porch that is only about 5ft wide and 6 ft long. Each spring I buy two water hyacinthes for it, and by mid summer the pond is completely blanketed. In fact, I have to remove at least 2 - 3 plants each morning from mid-July through mid-September (this is in Delaware).  The plants have a pretty high water content though, so dewatering and drying would be a major energy input. Just an idea.