Effect of temperature on nitrogen-limited growth rate and chemical composition of Ulva curvata (Ulvales: Chlorophyta)

Abstract

Ulva curvata (Kutz.) de Toni growing in shallow estuaries experiences a highly variable supply of dissolved inorganic N, which can limit growth rates. The effects of N supply variability and annual temperature and light variation on growth rates and chemical composition were assessed in plants grown in outdoor tanks supplied with running seawater and either pulsed or continuous N additions for 8 d. Environmental variables were measured every 12 h, growth rates every 2 d, and plant nitrogenous constituents every 4 d. The experiment was repeated 7 times over the course of a year. The slopes of growth rate-tissue N and growth rate-N supply plots increased with temperature and varied at periods at least as short as 2 d. Temperature explained 44% of variation in growth rate, and temperature, light, and N supply together explained 53% of variation in growth rate. N-limitation occurred only above 20°C, the approximate temperature for the maximum growth rate. Growth rate and tissue N were not affected by N supply frequency. Tissue N, ribulose-bisphosphate carboxylase activity, and chlorophyll contents were correlated with each other and with environmental variables but not with growth rate. U. curvata thus does not maintain balanced growth in the strict sense, but rather stores nutrients supplied as pulses for use when they become growth-limiting. Since the relative importance of any single growth-limiting factor is highly dynamic, assessing the importance of multiple limiting factors requires longterm, high-frequency sampling of principal limiting factors and seaweed growth rates.