Responses of seagrass to nutrients in the Great Barrier Reef, Australia

Abstract

Declines in seagrass biomass and growth have been widely reported in response to anthropogenic irnpacts. In contrast, the distnbution and biornass of seagrass in the carbonate sedirnent around Green Island reef, part of Australia's Great Barrier Reef (GBR), has measurably increased during the past 50 yr, possibly due to increases in the availability of nutrients from local and regional anthropogenic sources Using historical aerial photography, increases in seagrass distribution at Green lsland have been mapped. The growth, morphological and physiological responses of 2 seagrass species (Halodule uninervis and Synngodium isoetifolium) to elevated sedirnent nitrogen (N; lOOx control) and/or phosphorus (P; 1Ox control) were measured to investigate whether increased nutrients could account for the observed increase in distribution. Increases in the growth rate, amino acid composition and tissue nutrient content of both species occurred in response to elevated sediment N, but not P. Concentrations of the N-rich amino acids asparagine and glutainine ~ncreased 3-to 100-fold in seagrass leaves from N treatments. The 6 " ~ values of leaves decreased in response to additions of nitrogen, probably due to increased discrimination against the I5N isotope. because N availability was surplus to demand. The low 6 " ~ value of seagrasses in the Green Island back reef suggests that their prirnary source of N is either from N2 fixation or fertilisers and that the N from sewage is not a large component of their N budget. This study is the first to dernonstrate N, rather than P, as the primary limiting nutrient for growth of seagrass in carbonate sedirnents and Supports the hypothesis that the increase in the seagrass distnbution and biomass at Green Island was caused by an increase in nutnent availability. We also hypothesise that seagrass distribution and biomass in many regions of the GBR may be limited by nutnents and that the lack of substantial seagrass meadows in the southern GBR could be due to these reefs receiving less nutrients from the mainland.