These are three different questions. First question is about phytoplankton bloom in general; second question is specifically about "harmful" blooms, and third question is about harmful blooms under climate change.

Phytoplankton bloom is defined by two characteristics: the magnitude of biomass change, and the time period over which that change occurs. Loosely put, a bloom is a large increase in phytoplankton biomass over a short period of time. Various conditions are needed for a bloom to happen; e.g. sufficient nutrients and light, low grazing pressure and mortality, low advective losses, etc. Text book case of phytoplankton bloom is the spring blooms you see in temperate and polar waters.

To address the question whether a phytoplankton bloom is harmful or not, first you have to define "harmful". The common definitions are that a phytoplankton bloom produces toxic substances (e.g. shellfish poisoning in the case of toxic dinoflagellates) or undesirable water quality change (e.g. discoloration, oxygen depletion during decay of the blooms). Experiments and field observations can be done to study the harmful effects; for example, bioassay to test for toxic effects, monitoring water quality, etc.

Your last question is more complicated. In essence the question is: will climate change favor bloom formation of known harmful species, or turn "harmless" blooms into harmful ones (e.g. uncouple grazing from phytoplankton bloom such that decay of excess phytoplankton will lead to hypoxia)? The first scenario is relatively easy to test in the laboratory by investigating the growth and toxin production of known harmful species under different environmental conditions. The second scenario is difficult to test because it essentially involves complex biological and physical interactions, which is not something that can easily be replicated in laboratory.

Thank you @Kam W Tang.

But Importantly, I want to how do you ascertain certain phytoplanktan species as bloom forming species? Is it based on data acquired over long periods or any criteria to call certain sp. as bloom forming ones?

Phytoplankton multiply by simple cell division and hence they have the *potential* to increase their biomass exponentially and quickly--- i.e. the potential to form bloom. Whether they will in reality depends on external conditions I mentioned, such as nutrients, light, mortality. In other words, you can put a species in the right conditions and it'll bloom; put it in the wrong conditions and it won't. So, the question is not whether there are "bloom forming" species (which would imply there are "non-bloom forming" species). Rather, the question is whether the conditions favor bloom formation.

@Kam W Tang : thank you very much. This answer allows me to have better clarity on topic.

Good discussion. But for riverine water how to assess bloom formation. more over cell count vary based on the river mixing, river bed morphology, land drainage, flow discharge etc.

@Nitish Venkateswarlu @Kam W Tang: Some of the phytoplankton species bloom in low nutrient profile too, designated as oligotrophic species. So its the individual requirement of a species to bloom. But yes majority of species bloom in eutrophic conditions. Additionally, the temperature plays a crucial role. whether a species is temperate or tropical. Its a combination of abiotic and biotic factors which built a baseline for a species to bloom.