Growth of the seagrass Heterozostera tasmanica limited by nitrogen in Port Phillip Bay, Australia

Abstract

Sediments beneath H. tasmanica at five sites in the bay were enriched in spring (September) with nitrogen (1000 g m-2) and phosphorus (20 g P m-2); responses were measured 5 mo later in late summer (February). At one site, Rye, N and P were added both separately and together in a 2 × 2 factorial design; at the other sites N and P were added together. At the Rye site, N-enrichment resulted in an increased concentration of total N in the roots/rhizomes and leaves of H. tasmanica and in a significant increase in dry weight of leaves (50%), density of leaf clusters (40%) and canopy height (20%). Phosphorus enrichment resulted in an increased concentration of phosphorus in leaves, but no change in leaf dry weight, density of leaf clusters or canopy height. Neither N nor P enrichment caused an increase in dry weight of epiphytes on seagrass leaves, in dry weight of non-epiphytic macroalgae or in the concentration of chlorophyll in seagrass leaves. At three of the four other sites, N + P enrichment resulted in an increase in growth of H. tasmanica. During spring and summer nitrogen apparently limits the growth of H. tasmanica throughout most of Port Phillip Bay. -from Authors