This is an interesting manuscript, and thanks for sharing as a preprint! I have yet to read the paper in more detail but for now have the following comments.
1. I think the ms would benefit from being put also in the context of biodiversity-ecosystem functioning research. That is, to more explicitly link the results and the discussion to previous work that manipulated the number of biotic esources and looked at rates (or total) consumption. Examples include:
DeMott, W. R. 1998. Utilization of a cyanobacterium and a phosphorus-deficient green alga as complementary resources by daphnids. - Ecology 79: 2463-2481.
Fox, J. W. 2004. Effects of algal and herbivore diversity on the partitioning of biomass within and among trophic levels. - Ecology 85: 549-559.
Cardinale, B. J., Matulich, K., Hooper, D. U., Byrnes, J. E., Duffy, J. E., Gamfeldt, L., Balvanera, P., O'Connor, M. I. and Gonzales, A. 2011. The functional role of producer diversity in ecosystems. - American Journal of Botany 98: 572-592.
2. Were the urchins give food ad libitum, or why were the levels of 34 g per species in monoculture and 10 g per algal species in mixture chosen?
With best wishes,