Journal of Experimental Marine Biology and Ecology 250 (2000) 133–167 www.elsevier.nl / locate / jembe Review of nitrogen and phosphorus metabolism in seagrasses * Brant W. Touchette , JoAnn M. Burkholder Department of Botany, Box 7510, North Carolina State University, Raleigh, NC 27695-7510, USA Abstract Within the past few decades, major losses of seagrass habitats in coastal waters impacted by cultural eutrophication have been documented worldwide. In confronting a pressing need to improve the management and protection of seagrass meadows, surprisingly little is known about the basic nutritional physiology of these critical habitat species, or the physiological mechanisms that control their responses to N and P gradients. The limited available evidence to date already has revealed, for some seagrass species such as the north temperate dominant Zostera marina, unusual responses to nutrient enrichment in comparison to other vascular plants. Seagrasses derive N and P from sediment pore water (especially ammonium) and the water column (most nitrate). The importance of leaves versus roots in nutrient acquisition depends, in part, on the enrichment conditions. For example, a shift from reliance on sediment pore water to increased reliance on the overlying water for N and P supplies has been observed under progressive water-column nutrient enrichment. Seagrasses may be N-limited in nutrient-poor waters with sandy or (less so) organic sediments, and P-limited in carbonate sediments. On the basis of data from few species, seagrasses 2 23 1 appear to have active uptake systems for NO and PO , but NH uptake may involve both low- 3 4 4 and high-affinity systems. P uptake affinities reported thus far are much lower than values for i active ammonium uptake, but comparable to values for nitrate uptake by leaf tissues. Beyond such basic information, seagrass species have shown considerable variation in nutritional response. Dominance of acropetal versus basipetal nutrient translocation appears to vary among species as an innate trait. While some species follow classic Michaelis–Menten kinetics for N uptake, others i have exhibited sustained linear uptake with limited or negligible product feedback inhibition, perhaps in adaptation to oligotrophic environments. Zostera marina also is able to maintain nitrate reductase (NR) activity during dark periods if adequate carbohydrate reserves and substrate are available. Thus, this species can respond to nitrate pulses throughout a diel cycle, rather than being limited as most plants to nitrate uptake during the light period. Further adaptations may have occurred for seagrasses in extremely nitrate-depauperate conditions. For example, Halophila decipiens and H. stipulacea lack inducible NR and apparently have lost the ability to reduce nitrate; and a biphasic rather than hyperbolic P uptake curve, with ‘surge’ uptake, has been i described for Zostera noltii. Many seagrasses respond favorably to low or moderate N and / or P enrichment. However, excessive N loading to the water column can inhibit seagrass growth and i survival, not only as an indirect effect by stimulating algal overgrowth and associated light *Corresponding author. 0022-0981 / 00 / $ – see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S0022-0981(00)00195-7