Time period is renaissance, with the most powerful culture being in a area geographically similar to the middle east. So a cleric was trying to combat a drought in his kingdom using the power of a solar eclipse, he invoked great divine energies to accomplish this. However he went a bit overboard with his wish, he said that "It shall rain from the heavens, such is it does during the best of times, and mana able to sustain all plants and animals, shall rain from the heavens with it.", suffice to say this didn't turn out the way he was probably hoping.
The interpretation his wish wasn't quite so bad as it theoretically could have been (it could have ended up raining mana and water 24/7 for one), however it still turned out that following his instructions, the wish didn't really go so well.
In addition the spell somehow ended up effecting 6 areas instead of just his own kingdom: The areas affected were a 260 mile area around his capital which is a city comparable to Baghdad in the ancient world but more powerful, and a 130 mile area of 5 other places. One area is on the the continent directly opposite the earth in an area similar to Kansas which is moderately populated and akin to medieval era Europe. Two other areas to 90 degrees of those 2, one just being open ocean akin to the middle of the pacific, and the other being part of an island chain akin to the Philippines with Aztec level civilization. The last two are to 90 degrees same as before but are arctic, one is in a area of arctic ocean, and the other is on a continent to the north similar to Antarctica. Together the 6 locations align perfectly as each is on its own face of a cube of mystic ley-line energy. Magic is assumed to be rare enough it shouldn't affect your answers, no the effects aren't likely to be reversible for at least a millennia.
The actual effects: the spell sets precipitation equal to the average precipitation during the rainiest quarter of the year, this goes based on averages over the past few millennia. More devastating however is that whenever it rains it also rains "mana".
Mana is a substance designed to contain the necessary nutrients to nourish any plant or animal in the areas affected, this is based on life in all affected areas at the start of the spell. The resulting mixture is very sugary (to sustain hummingbirds and some insects), rich in many different nutrients and proteins, and is also rich in nitrates for plants (though with the exception of algae most would actually not respond well to such high amounts), by trying to provide the optimal nutrients for every plant and animal it ended up providing too many for the vast majority of them.
The mana falls as syrupy rain (may freeze in arctic but is nearly all sugar, plus fats, proteins, etc so it will freeze at much lower temp). The amount that falls is about equal to the amount of water rain caused by the spells effect, and as mentioned before, precipitates along with it. In case anyone's thinking about how this would increase the earth's mass over large timescales it won't, the extra rain is pulled from the oceans and the nutrients are assembled from material pulled out of the crust.
Composition of mana and amount of rainfall and manafall, is set once the spell starts and will never stop or change in any way.
The question is, this would probably be pretty bad, but just how bad? How would this affect the environment in the areas directly altered by the spell, and how far will the effects reach beyond that? How will it alter the societies in and around the areas with manafall? Not looking for answers over timescales of millennia, how life would evolve with such massive nutrient infusion will be addressed in a later question.
Some really obvious implications; I imagine that this will cause massive algae blooms in any water source, and will likely kill many plants due to too many nitrates. The sticky syrup will be a great food source for insects, but it will also get many of them trapped in its sticky syrup so I can't say what the net effect will be. I can't really predict what the effects will be beyond that(thus why I ask the question), though I highly suspect that the affected areas will reek from all the bacteria that it will cause to explode in number.
I suspect the areas affected will be bad enough that it will cause mass emigration. The mana may also cause the surrounding areas to flourish however, since it can be added to other food to sustain people (by itself it may cause hypervitaminosis, and is unappetizing as a gross syrup), thus allowing a food surplus.
This question's answers, will likely be applicable to many potential scenarios where an area has a large overabundance of nutrients suddenly introduced.