what is the effect of the increase of nutrients in water on the growth of algae?

Linear effect. As you increase the water nutrient, algae growth also increases,

"Daylily" is right in what has been presented.I may add that:infresh waters the P:N ration if it's more than 1:20,then the growth is lower,so the limiting factor is the P.In marine media,the ratio is usually 1:10,if it goes beyond 15 to 20,again growth is low.
2) For algal growth more than the nutrients,it's the source of light,intensity of light,period of light & temperature are important.Algae are far lower in the evolutionary scale,& hence they like lower temperature - so higher temperature lowers its growth.

The article below (I couldn't copy and paste because it's pdf) states that as phosphates and nitrogen increase, so does the algae growth, but the rate may vary. Light and temperature may reduce or accelerate growth, regardless of nutrient levels.

www.grinnell.edu/files/downloads/5-TlrsV4.pdf

Algal productivity is often correlated to levels of nitrogen (N) and phosphorus (P) (See N:P ratio, above), but other nutrients are required including carbon, silica, and other micronutrients.

The intensity, duration, and quality of light influence the dominance of algal species and the structure of algal communities. Likewise, water temperature influences the metabolic and reproductive rates of algae. Although algal growth rates can be relatively lower during periods of cold water conditions, the standing crop or biomass of algal communities can be comparatively large because of the absence or inactivity of grazing organisms. Discharge and velocity conditions also affect algal communities through scouring and washout. However, modest increases in current velocity may enhance rates of algal accumulation because nutrient uptake and boundary layer diffusion increases with current velocity (Stevenson 1996). During stable hydrologic conditions, algal communities can develop in streams and rivers within several weeks of colonization and reproduction. However, such communities may vary considerably within river reaches in relation to current velocity, depth, light intensity, and water chemistry factors. Further, seasonal changes in the abundance and composition of algal communities may occur (Porter et al 1993).

http://www.krisweb.com/krissheepscot/krisdb/html/krisweb/stream/nutrients.htm