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

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 21 per plant) than summer (33 mm d 21 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 21 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.Seagrass meadows are declining globally at a disturbing rate driven by anthropogenic effects along increasingly populated coastlines (Waycott et al. 2009). Recovery is often very slow and may take decades for large meadowforming genera such as Posidonia (Kendrick et al. 2000(Kendrick et al. , 2002Cambridge et al. 2002). Transplanting plant units from healthy meadows into denuded areas may be the only means of re-establishing seagrasses for habitat restoration (Fonseca et al. 1998;Bastyan and Cambridge 2008) but nutrient limitation (Kenworthy and Fonseca 1992) and poor root growth (Lepoint et al. 2004) have been reported to contribute to low transplant success rates. Fertilizers have been added to overcome nutrient limitation and enhance transplant success (Sheridan et al. 1998;Worm et al. 2000; Cambridge and Kendrick 2009). Seagrasses take up nutrients mainly through the roots when concentrations of nutrients in the sediment are much higher than in the water column, and nutrient imbalances or limitation may also link to nutrient availability in the sediments. Rapid development of a strong root system for anchorage and nutrient uptake seems likely to enhance transplant success but little is known about root growth in any seagrasses, particularly their response to nutrient availability (Kiswara et al. 2009). For species with extensive root systems that grow in subtidal environments, accurate sampling presents several difficulties, such as buoyant roots that a...

Season and sediment nutrient additions affect root architecture in the temperate seagrasses Posidonia australis and P. sinuosa

“…2), suggesting that other factors affecting aboveground biomass may be confounding clear responses to nutrient additions. This is supported by Hovey et al (2011), who demonstrated decreased root productivity with N additions in summer, which is a typical response when nutrients in the sediment are at adequate to saturating levels (Short 1987).…”

“…Porewater concentrations taken from inside the pots showed that nutrients were released from Osmocote fertiliser with concentrations up to 5 times higher than controls at ambient nutrient levels (Table 1). Nutrient concentrations in leaves and rhizomes were also higher when N or P was added, evidence that plants were taking up the nutrients (see Hovey et al 2011 for plant nutrient concentrations). No significant change in shoot to root ratio was detected, but it was highly variable (Fig.…”

“…In March (austral autumn) and October (austral spring) 2006, plastic pots (350 × 120 × 120 mm, 3 vertical slits per side) were assigned randomly to treatments and positioned by SCUBA divers in a 25 m 2 subtidal area. Pot preparation and placement followed methods described by Hovey et al (2011). Treatments had either (1) 10 g of pot sediment (control = no added nutrients), (2) nitrogen (N), 9 g (equivalent to 88 g N m −2 ) Osmocote ® 5 mo release, 33% N as urea, (3) phosphorus (P), 6 g (equivalent to 23 g P m −2 ) Osmocote ® 5 mo release, 18% P as mono-calcium phosphate or (4) combined N+P, 9 g of N and 6 g of P. All treatments were exposed to ambient nutrient conditions in the water column and sediment porewater, and concentrations of NH 4 and PO 4 within pots were sampled for each treatment (Table 1; Hovey et al 2011).…”

“…Transplant units consisted of 2 shoots and a length of horizontal rhizome (100 mm for P. australis and 70 mm for P. sinuosa), and roots were removed from rhizomes using a razor blade (Hovey et al 2011). One transplant was planted per pot and anchored with a 150 mm long plasticcoated wire peg.…”

Variability in patterns of macro-epiphytic leaf community of Posidonia oceanica in the Islands of Kuriate: Western coast of Tunisia

“…In the Mediterranean Sea, Posidonia oceanica (P. oceanica) represents the most important seagrass in terms of productivity, distribution and habitat structuring. It provides substratum to a species-rich epiphytic community [12,13], which achieves maximum biomass between the end of spring and the end of summer [1,14]. Monitoring of epiphyte of P. oceanica beds is becoming a useful tool to test the health of coastal environment and several countries have developed programs to study the distribution and the characteristics of seagrass beds and their epiphyte [1,[15][16][17][18].…”