The June onset of cyanobacteria blooms in NH lakes has been a wake-up call for many about the fragile health of lakes. The main culprit is warming water temperature that has been trending up for many years (see ice-out history). This will continue as climate change worsens, but there are things in our control that can be done. First, reduce phosphorus to lessen the role of warmer water temperature in triggering blooms. It's not just from fertilizer but septic systems that run off into the water (must be inspected to know for sure), stormwater runoff from impervious surfaces (go out when it's raining and see where water goes, it needs to soak in where it falls or close to it), animal waste (don't feed the ducks or let animal defecate near water), and erosion (don't create boat wakes in shallow water which pulls out aquatic plants needed to take up phosphorus and add oxygen to water. Deep wakes can disrupt phosphorus-rich sediment which add phosphorus to water). In addition to reducing phosphorus, add vegetation near the shoreline to prevent run-off and take up phosphorus, maintain trees and bushes at shoreline to shade and cool the water, don't disrupt aquatic plants, have septic systems evaluated. A study done in 2014 for areas of LW identifies needed action:
https://winnipesaukeegateway.org/lak.../introduction/
Finally, become educated and educate others on what a bloom looks like. You cannot rely on DES to stay up to date as not all blooms are reported and tested and they drift. Don't go into water during or just after a bloom as toxins take time to degrade. The cyanotoxin, BMAA is linked to ALS, even breathing droplets from a distance away, so the threat is not just water contact. See this presentation for more on that research:
https://www.youtube.com/watch?v=dfDtMqnBjvk&t=1452s The folks at DES are great, but they are short-staffed and underfunded. NH LAKES is a great resource, too, on proactive measures.