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.