My understanding is that algal blooms form from
excess nitrogen and phosphorous where there is a carbon and oxygen source that is then consumed. I'd be hesitant to use a lot of manure or other high phosphorus materials unless a bioinfiltration system was in place. While manure is much slower at leaching phosphorus and other nutrients, it can still leach a lot with a lot of rain. If it's all you have, you can always buy some cheap nutrient test strips, simulate or wait for a downpour, and
take a water sample and measure it. Inorganic fertilizer can be much much worse however.
One Aussie superphosphate farm study found that “Over 92% of P, 76% of total dissolved C, and 93% of S was lost in overland flow.”
Other farm nutrient runoff studies I've seen have shown that surrounding them with a trench of woodchips,
biochar, or creating a biodiverse green boarder some meters wide that is then left to act as a bioinfiltration/bioretention system, do work.
Studies that inoculated bags of woodchips with fungi and distributed them along waterway edges also worked.
A
swale on contour that was planted out with a diverse range of plants, or a
hugelkultur with high
carbon content and plants should also work, but I haven't seen any documented studies.
When I was learning about Terra Preta and the African Dark Earths with high charcoal/biochar content, I noticed they had high levels of phosphorus which tells me the soil carbon was likely filtering and retaining it for plants and soil organisms to consume.
My advice would be to at least have a green strip on contour, swaled or not, with high plant diversity such that water is infiltrated
into the landscape and doesn't simply runoff into the lake when it rains, especially under heavy downpours. And also to reduce the soil bulk density as much as you can so water infiltrates the rest of the fields.
A plant diversity study showed that using at least 16 species produced the best water infiltration and soil bulk density results, but that may depend on the climate.
Another 5 year tree study showed that for compacted soil compost tea improved soil bulk density as much as 2" of compost did (many composts have high levels of phosphorus), but that 6" woodchips (low phosphorus) performed twice as good, and grew twice as much, mostly because it was able to maintain soil moisture and keep the microbes active and improving soil bulk density and water infiltration.
Ramial woodchips are a good option. Mulch, living or dead, matters!
Keeping the carbon loving
microbes that build the soil organic carbon that then helps sequester phosphorus and nitrogen, is very important.
A study of Brazilian forests showed that soil organic carbon was correlated with 86% of soil nitrogen, so the more soil organic carbon may mean the less nitrogen runoff. Higher carbon content also
increases the moisture holding capacity of soils.
Soils are complex though, and what works for one soil may not another, or may cause detrimental effects, case in point was a study on a forest that researchers added calcium to replenish acidic clay soils affected by acid rain. Adding the calcium resulted in increases in dissolved organic carbon runoff into streams, not what the researchers expected!
The best way to really know how what you're doing affects nutrient leaching and runoff, is to observe and test.
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