The capacity for refugiums to produce a large amount of food can be proven by a similar situation in the wild. On the Big Island of Hawaii, there is a popular dive spot there where people can see manta rays feeding at night. Divers head out during the late afternoon to “get acquainted” with the mantas, and then they rest and wait until sunset while others secure lighting on the bottom. As the sun goes down, the divers get situated around the bottom lighting, and often a sea swirl of planktonic life surrounds them (principally polychaete worms and crustaceans).
This happens because much of plankton is “positively phototactic,” meaning that they are attracted to light. In due course, usually a dozen or more resident manta rays, and at times a transoceanic specimen or two of much greater size, swoop in repeatedly to scoop up this bounty of food. Collectively, these manta rays weigh tons, and they go to the same area night after night because this patch of sand and rock produces at least tens of pounds of food daily. Although your refugium substrate may seem puny, there is similarly a considerable amount (proportionately, of course) of live foods being continuously produced.