From an article on LiveAquaria.com...
"The nitrogen cycle is the process of various bacteria breaking down toxic waste into less harmful components. It involves 4 steps:
The first step is the decay of waste products of fish, plants, and invertebrates, along with any dead organisms or uneaten food. As these materials decay, ammonia is produced, which at even low levels will burn the gills of fish and choke off their oxygen supply.
Bacteria called Nitrosomonas consume this ammonia, and in the process, create a chemical byproduct called nitrite. Although nitrite is toxic (preventing blood from carrying oxygen), fish can withstand roughly twice the amount of nitrite in their water compared to ammonia.
Then, other bacteria called Nitrobacter consume the nitrite, and in turn, release a less toxic chemical called nitrate.
Nitrate requires anaerobic conditions to turn it into harmless nitrogen gas. The parameters needed to create this condition are not commonly present in most aquariums. Hence, water changes are necessary to dilute nitrate levels."
LR in sufficent quantities and of the right type (porous) provide the surface area for all three types of bacteria to grow. You probably can not have enough LR for the last stage of the process and where a DSB, macro algea, denitrifyer, and pwc's come into play. The bacterial population will grow and contract in relation to amount of waste in the tank. Less waste than normal and bacteria will start to die, more waste and bacterial population will grow. If the population can not grow fast enough for the increased waste then the water parameters will go askew. First ammonia, then nitirite, then nitrate -- a mini-cycle until the population reaches equilibrium.
The LR is not producing nitrate. It's actually helping to some degree. Nitrate under 40 ppm is acceptable for a FO or FOWLR tank. At nitrates of 10 ppm you can even have mushrooms and other softies. SPS require 0 or near 0 trAtes.
Just take it slow and you'll be fine. You haven't even had your diatom bloom yet