Ammonium requirements of fast-growing ephemeral macroalgae in a nutrient-enriched marine embayment (Port Phillip Bay, Australia)

The observed high biomass of fast-growing macroalgae in Port Phillip Bay (PPB), Australia is of concern and may represent a shift from perennial macrophytes (e.g, kelps and seagrasses) to a dominance by fast-growing macroalgae. This study examined the limiting and optimum external ammonium-nitrogen concentrations for the growth of 3 fast-growing macroalgae that dominate reefs in northern Port Phillip Bay, Australia. The relationships between growth and tissue nutrients and the capacity of these algae to assimilate, store and survive on nutrients was examined. Winter and summer experiments aimed to determine the effects of tissue nutrient status on growth responses. An absence of any seasonal variation in critical N thresholds and N subsistence quotas in any of the species examined suggests that all species are able to tolerate low N availability. The utilisation of N reserves to support non-limited growth of Hincksia sordida (Clayton) and Polysiphonia decipiens (Montagne) was about 2- to 5-fold shorter in summer than winter, indicating potential N-limitation during summer. A high requirement for N by both H. sordida and Ulva sp, means that internal N reserves could support reduced growth for shorter periods compared to P. decipiens. Such high demands for N by H, sordida and Ulva sp. makes these taxa susceptible to N-limitation in summer, when inputs of N to coastal waters are low. Elevated nutrient inputs into PPB may allow these taxa to become nutrient-sufficient and colonise larger areas of nearshore reefs.