A Review of Nutrient Allocation and Variation Within Seagrass Tissue.

Brief description

Data on nitrogen (N) and phosphorus (P) concentrations in above-ground and below-ground plant parts of 19 species of seagrass world-wide were compiled from published sources. Additionally, data on growth and nutrient concentrations of Halophila ovalis from the Swan River Estuary, subject to experimental manipulations of either sediment organic matter enrichment or reduced light under controlled growth conditions, were pooled to investigate relationships between growth and nutrient stoichiometry of the seagrass.

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Statement: This paper compiles data published post-1990 for concentrations of nitrogen (N) and phosphorus (P) in both leaf (n = 150) and below-ground plant parts (n = 80) for 19 species of seagrasses world-wide. Data are compared to pre-1990 data on leaf nutrient concentrations published by Duarte (1990). Additionally, a substantial data set of nutrient concentrations in the leaf (n = 103) and below-ground plant parts (n = 70) of the seagrass Halophila ovalis are compared with the multi-species data set. The data set of H. ovalis is derived from culture experiments completed over consecutive summers (2003 - 2005). In these experiments H. ovalis, collected in intact cores with site sediment from the Swan River Estuary, Western Australia, was grown under controlled conditions (18 deg C, 12 hour light / 12 hour dark, ~ 300 umoles of photons m-2 s-1 photosynthetically active radiation (PAR) unless otherwise stated). Growth rates of these plants were manipulated by either addition of organic matter (particulate in the form of wrack, or dissolved as sucrose), or light reduction. Full details of growth conditions for the H. ovalis experiments can be found in Chapter 3 of the thesis. Plants were grown in 90 mm cores (containing multiple apices) in culture for 2 - 7 weeks before plant tissue was destructively harvested. Plant material, initially frozen immediately post experiment, was separated into root, rhizome and leaf. Since initial biomass within each core could not be determined prior to experimental manipulation, leaf number was counted at the outset of experiment and used to estimate growth rate. Leaf growth, as mg apex-1 day-1, was calculated by difference between initial and final leaf number multiplied by average leaf mass, then divided by the number of apices present at the end of the experiment and the length of the experiment. Leaf growth rate was converted proportionally via relative biomass allocation of above and below-ground parts to give total growth rate expressed as mg apex-1 day-1. Epiphytes were removed from leaves by wiping with a tissue, and scraping with a razor blade when necessary. Samples were dried to a constant weight at 60 °C. All plant material within cores was homogenized prior to analysis for tissue nutrients. Phosphorus and nitrogen in leaves, roots or rhizomes was determined by methods depending on the mass of sample available. If greater than 200 mg of plant tissue was available then plant samples were digested with sulphuric acid and hydrogen peroxide, prior to analysis for nitrogen by autoanalyser and phosphorus determined colourimetrically (Murphy & Riley 1962). The majority of root and rhizome samples had less than 200 mg of plant tissue and these samples were finely ground in an eppendorf ball mill grinder. Ground plant tissue was then digested and analyzed for phosphate by a method modified from Solorzano and Sharp (1980) by Fourqurean and Zieman (1992). Total nitrogen in small samples was determined by mass spectroscopy (Tracermass Ion Ratio Mass Spectrometer with Roboprep preparation system - Europa Scientific). Detection limits of these methods were 7 umoles g-1 DW N and 2 umoles g-1 DW P.

Notes

Credit
Strategic Research Fund for the Marine Environment (SRFME)