Deciphering the Interconnections between Nutrient Supply, Demand, and Limitation

Abstract:

Studies of stream nutrient dynamics have often focused on the influence of a single potentially limiting nutrient; however, it has become increasingly evident that ecosystems are constrained by the supply of numerous limiting resources. These resources may vary seasonally as a function of temperature, sunlight, and input of carbon and nutrients from upstream or terrestrial sources. We used multiple approaches, including plateau and TASCC nutrient additions, as well as analysis of diel nitrate dynamics as three different and complementary measures of nutrient demand/limitation in New Hope Creek, a third order stream in the Duke Forest of North Carolina over the course of one year. Nitrate-N concentrations were relatively high during the winter, spring, and summer months ranging from 105 – 518 and averaging 383 µg/L between January – August. During this time frame there was low demand for and limited uptake of added nitrate during plateau or TASCC additions (i.e., addition of N did not stimulate increased N uptake) although there was processing of background N evident from diel N and oxygen analyses. During autumn litterfall ecosystem respiration increased strongly and nitrate-N concentrations dropped precipitously, ranging from 9 – 34 and averaging 23 µg/L between September – December. During this period of low in-stream nitrate-N, demand for and uptake of added nitrate was high with maximum uptake rates of 560 µg/m²/min. This highlights the importance of understanding the relationship between nutrient supply and demand and the need to determine multiple resource controls over system processing. We also suggest that nutrient addition experiments need to be interpreted within the context of the supply and demand of multiple potentially limiting resources and the that the temporal dynamics of these relationships should be considered.