Direct measurements of root growth and productivity in the seagrasses Posidonia australis and P. sinuosa

Hovey, Renae K., Marion L. Cambridge, and Gary A. Kendrick

Limnol. Oceanogr., 56(1), 2011, 394-402 | DOI: 10.4319/lo.2011.56.1.0394

ABSTRACT: The effects of nutrients and planting season on root growth were investigated in transplants of the seagrasses Posidonia australis and P. sinuosa. Difficulties with sampling and estimating root growth of these submerged plants were overcome by growing transplants, with all roots removed initially, in pots containing a standardized sand medium. Nitrogen (N), phosphorus (P), and nitrogen and phosphorus combined (N + P) were added to pots. Root elongation and productivity were measured after harvesting at 1 month, 4 months, and 6 months in consecutive experiments beginning in autumn and spring. Roots formed within a month, growing into extensive root systems. Posidonia australis root growth was three-fold less over winter (10 mm d-1 per plant) than summer (33 mm d21 per plant), with some plants producing up to 9 m of roots in 6 months. Posidonia sinuosa root-growth rates were significantly lower than those of P. australis and did not vary between winter and summer (6 mm d-1 per plant). Root productivity was lower with N and N + P addition for P. sinuosa and with N addition for P. australis in summer. In contrast, P addition had little effect on root growth, reducing only primary root growth in P. australis. Addition of nutrients did not result in the expected increases in fine roots in the zone around the nutrient source. This technique for measuring root growth demonstrated the potential of these Posidonia transplants to produce extensive root systems within months of planting, in contrast to the much slower growth of roots in mature seagrass meadows.

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