The relative importance of water motion on nitrogen uptake by the subtidal macroalga Adamsiella chauvinii (Rhodophyta) in winter and summer

The influence of seawater velocity (1.5-12 cm . s(-1)) on inorganic nitrogen (N) uptake by the soft-sediment perennial macroalga Adamsiella chauvinii (Harv.) L. E. Phillips et W. A. Nelson (Rhodophyta) was determined seasonally by measuring uptake rate in a laboratory flume. Regardless of N tissue content, water velocity had no influence on NO3- uptake in either winter or summer, indicating that NO3--uptake rate was biologically limited. However, when thalli were N limited, increasing water velocity increased NH4+ uptake, suggesting that mass-transfer limitation of NH4+ is likely during summer for natural populations. Uptake kinetics (V-max, K-s) were similar among three populations of A. chauvinii at sites with different mean flow speeds; however, uptake rates of NO3- and NH4+ were lower in summer (when N status was generally low) than in winter. Our results highlight how N uptake can be affected by seasonal changes in the physiology of a macroalga and that further investigation of N uptake of different macroalgae (red, brown, and green) during different seasons is important in determining the relative influence of water velocity on nutrient uptake.