The effects of nutrient additions on aquatic systems have been frequently studied. Typically, these studies report an increase in algal biomass and a decrease in species diversity in response to nutrient increases. However, it is not clear why comparable aquatic communities respond differently to nutrient additions of similar magnitudes. We tested the effects of the rate and amount of nutrient load on community structure in 760 l mesocosms; treatments manipulated the total amount of nutrients that entered an aquatic system (small versus large load) and the temporal pattern in which these nutrients entered the system (annually, monthly, or weekly). We found that the effects of the loading rate of nutrients were at least as important as the total amount of the nutrients for several response variables. Although these effects were manifested in several ways, the response to the different rates was most prominent within groups of the primary producers, which showed large shifts in composition and abundance.

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