A wet season with increased plant growth is indicated by the fact that the population of Elks from beginning point C attained maximum population size (K100) and remained stable until point D. That's why the difference at point C could be due to the a wet year still higher plant growth.
In the process of photosynthesis, plants that contain chlorophyll utilize carbon dioxide and water using energy from sunlight that is absorbed by chlorophyll to make glucose + oxygen.
Soluble phosphorus is a measure of orthophosphate (PO4), the
soluble and inorganic filterable fraction of phosphorus, which is the most
stable type of phosphate and it is the form directly used up by the plant
cells.