Biomass of primary producers in bottom-up ecosystems is controlled by:
“Abiotic factors and nutrient supply” for biomass of primary producers in bottom-up ecosystems is controlled by. This relationship follows from the ecological mechanism represented by the terms in the item, not merely from an association between their names. Ecosystem processes are constrained by energy conservation and by the cycling of matter. Energy enters mainly through primary production, is lost as metabolic heat at every transfer, and therefore cannot be recycled in the way that carbon, nitrogen, phosphorus, or water can. The remaining alternatives—“Predator abundance”, “Competition”, “Secondary consumers”—refer to different states, processes, or scales and therefore do not express the same causal relationship. The relevant inference should follow the pathway from resource supply to organismal uptake and then to ecosystem-level flux. Productivity, trophic transfer, decomposition, and nutrient regeneration are connected, but each measures a different part of that pathway. Field observations could test this account by measuring the proposed driver and the demographic or ecosystem response while controlling plausible confounding factors.
Ref: Fundamentals of Ecology, Odum & Barrett, 5th Ed., Ch. 4