The root/rhizome system of seagrasses: an asset and a burden

Abstract

Large-scale declines in seagrass vegetation have been frequently observed in recent decades. Many of these declines can be traced to the reduction of light levels in the water column. In this paper, it is argued that the root/rhizome system offers a competitive advantage in nutrient-poor waters, but that it makes the plant vulnerable when changes in water quality lead to reduction of incident light. Seagrasses are capable of exploiting the nutrient stocks of both the water column and the sediment pore water, by leaves and roots, respectively. A survey of the literature shows that the median concentrations of water-column ammonium and phosphate in seagrass beds worldwide are 1.7 and 0.35 μM, respectively, whereas the same compounds in the pore water of the root zone reach median concentrations of 60 and 6.5 μM. The dual possibilities for nutrient uptake may underlie the apparent lack of strongly developed nutrient conservation strategies in seagrasses. The possession of roots becomes a disadvantage when the photosynthetically active radiation available to the plants decreases. At saturating light levels, the maximum rate of net photosynthesis (measured as O 2 production) of the leaves typically exceeds leaf respiration (measured as O 2 consumption) about 5 times. In low-light environments, the respiring below-ground biomass (which can greatly exceed the above-ground biomass) can be a considerable burden to the carbon balance of the plant, limiting its survival potential. In addition, secondary and tertiary effects of light reduction involving the roots and rhizomes may undermine plant vitality as well. Leaf photosynthesis is the major source of oxygen for the roots and rhizomes. Hence, decreased photosynthetic activity following light reduction may lead to hypoxic or anoxic conditions in the below-ground organs, presumably making them vulnerable to carbon starvation. A decreased flux of oxygen to the roots and rhizomes also restricts the possibilities for oxidation of sediment sulphide, a known phytotoxin, because release of oxygen into the rhizosphere will diminish. The cascade effects of light reduction which involve the below-ground organs of seagrasses are little studied. More insight into the functioning of the root/rhizome system and into the interactions between this system and the sediment environment is needed to fully understand the vulnerability of seagrasses to light reduction.