Evaluation of indicators of nitrogen limitation in deep prairie lakes with laboratory bioassays and limnocorrals

Abstract

The deep lakes in central Alberta are characterized by relatively high total nitrogen (TN) to total phosphorus (TP) ratios (33 to 64, by weight) and high TP (10 to 100 µg 1−1) in the euphotic zone in summer. These same lakes show indications of nitrogen limitation: (1) blue-green algae are relatively important in summer (7 to 88% of total biomass) and a large proportion of the filaments contain heterocysts and (2) inorganic nitrogen levels are extremely low in the surface waters (NOinf2sup−+ NOinf3sup−+ NHinf4sup+− N < 10 µg 1−1). In 1985 and 1986 inorganic nitrogen (25 to 1200 µg 1−1 − N) and/or phosphorus (12 to 100 µg 1−1 − P) was added to water in 31 m3 limnocorrals or 21 bottles from four deep prairie lakes. In most cases, phytoplankton biomass (estimated as chlorophyll a) was stimulated only by phosphorus additions. However, for a short period (8 d) after an unusual storm, phytoplankton biomass was stimulated by nitrogen additions. After this storm, TP increased by up to 75% in the epilimnion of the study lakes whereas TN did not increase. The algae which increased in these experiments tended to be the dominant algae in the control vessels, which varied with the experiment and the lake. Thus, a large proportion of blue-green algae and low inorganic nitrogen levels are not indicative per se of nitrogen limitation in lakes.