Effects of light deprivation on the survival and recovery of the seagrass Halophila ovalis (R.Br.) Hook
Introduction
The abundance and extent of seagrass ecosystems around the world is declining (Walker and McComb, 1992). Seagrass loss has occurred through factors such as cyclones (Poiner et al., 1989) and diseases (Den Hartog, 1996). However, the primary cause of seagrass loss is reduced light availability, often caused by anthropogenic changes (Shepherd et al., 1989). Most light reduction is caused by increased water turbidity and an increase in the biomass of epiphytes on the seagrasses leaves (Orth and Moore, 1983, Shepherd et al., 1989, Walker and McComb, 1992). Increases in water turbidity and seagrass epiphyte loads often result from human activities such as urbanisation, dredging, use of industrial fertilisers and poor catchment management (Dennison et al., 1993, Onuf, 1994, Short et al., 1996).
Seagrasses respond to reduced light availability with a variety of morphological and physiological adaptations, many of which increase the efficiency of light harvesting (Abal et al., 1994). The morphological photoadaptive responses of seagrasses to light reduction include an increase in canopy height, a decline in shoot density, and broader thinner leaves (Backman and Barilotti, 1976, Bulthuis, 1983, Neverauskas, 1988, Abal et al., 1994, Grice et al., 1996; Abal and Dennison, in review). Physiological photoadaptive responses to light reduction include an increase in chlorophyll content and a decrease in the chlorophyll a:b ratio (Wigington and McMillan, 1979, Dennison and Alberte, 1982, Dennison and Alberte, 1985, Abal et al., 1994). Despite the photoadaptive abilities of seagrasses, their minimum light requirements are high in comparison to algae, phytoplankton and terrestrial plants (Abal et al., 1994), with the minimum light requirements for different species of seagrass ranging between 5 and 29% of surface irradiance (Duarte, 1991).
It has been suggested that the duration of time a species of seagrass can survive below their minimum light requirements is related to the ability of that species to store carbohydrates (Czerny and Dunton, 1995). Seagrasses such as Thalassia testudinum have extensive rhizomes and are capable of storing large quantity of storage carbohydrates. These species are able to survive longer than species such as Halodule wrightii, which have small rhizomes and therefore little capacity to store carbohydrates (Czerny and Dunton, 1995). Seagrass survival during light limitation may, however, depend solely upon the quantity of carbohydrates stored within the roots (Kraemer and Alberte, 1995). Alternatively, the duration of time a species of seagrass can survive below their minimum light requirements may depend on the ability of the seagrass to tolerate and/or disperse the phytotoxic end products of anaerobic root respiration (Pregnall et al., 1984, Smith et al., 1988).
Transient periods of turbidity which result from factors such as sediment resuspension and flooding rivers, have the potential to affect the distribution and abundance of seagrasses by depriving them of nearly all light (Zimmerman et al., 1991, Preen et al., 1995). Apart from the inferred evidence from the Hervey Bay (Queensland, Australia) seagrass die-off, where 1000 km2 of Halophila spp died-off after two large flood events within a 3 week period (Preen et al., 1995), there is very little information about the decline and recovery of seagrasses in response to transient light deprivation events. The devastation that occurred in Hervey Bay highlights the necessity to understand the effects of light deprivation on Halophila spp. The present study focused on Halophila ovalis, a small, leafy and fast growing seagrass. Halophila ovalis is a markedly tolerant seagrass, capable of living in a range of salinities, sediment types and water temperatures (Den Hartog, 1970, Hillman et al., 1995). Due to the low minimum light requirements of H. ovalis, it can survive in a broad range of water depths, from shallow intertidal environments to very deep water (up to 50 m) (Lee Long et al., 1996).
The aim of this study was to investigate the responses of H. ovalis (R.Br.) Hook during and after a period of total light deprivation (i.e. total darkness). Total light deprivation was used to simulate conditions during flood events such as in the Hervey Bay seagrass die-off. The primary objectives of the study were to measure changes in (i) biomass, (ii) storage carbohydrate content and (iii) pigment characteristics (chlorophyll a fluorescence and chlorophyll content), following total light deprivation. To meet these aims and objectives, three separate experiments were conducted; two outdoor aquaria studies referred to as (i) the `Dark Incubation' experiment and (ii) the `Dark Incubation-Recovery' experiment; and one field experiment refereed to as (iii) the `In situ Dark-Recovery' experiment.
Section snippets
Plant collection
Halophila ovalis plants for the aquaria experiments were collected from subtidal (1.5 m below mean low water; MLW) monospecific seagrass beds located in One Mile Harbour on the eastern shores of Moreton Bay, Queensland, Australia (27°29.62′S; 153°23.87′E). Plants with intact roots, rhizome and shoots were collected by removing cores of seagrass with a sediment corer (12 cm diam.×15 cm depth), and placing them in flower pots lined with a plastic bag. The plants were then immersed in seawater and
Biomass
The total biomass of Halophila ovalis in the first 3 days of the Dark Incubation experiment was between 48 and 60 g m−2, and this declined to 17 g m−2 after 24 days (Fig. 2a). The above-ground biomass decreased more rapidly than the below-ground biomass, with the above-ground biomass starting to decline after 3–6 days in the dark, compared with 9–12 days for the below-ground biomass (Fig. 2a). As a consequence of a slower decrease in below-ground biomass than above-ground-biomass, the ratio of
Biomass and growth responses
The die-off of Halophila ovalis after 30 days of light deprivation was considerably more rapid than has previously been reported for other species of seagrass (Table 5). Light reduction studies show that seagrass species vary widely in their tolerance to a reduction in light below their minimum requirements (Table 5). For example, the small seagrass Heterozostera tasmanica, like H. ovalis, displayed limited tolerance to severe light reduction, surviving for only 2 months under 2% of surface
Conclusions
The duration of time H. ovalis plants are light deprived, and the light history before frequency of light deprivation events, appear to be the primary factors affecting the survival of this seagrass in environments that experience transient light deprivation events. Furthermore, there are many species of Halophila sharing similar morphologies and niches to H. ovalis, which are also very likely to show limited tolerance to light deprivation. If a light deprivation event caused by factors such as
Acknowledgements
Our thanks to Simon Costanzo, Eva Abal and Andrea Costas for their assistance in the field and/or laboratory. Bob Pendry is acknowledged for assistance in setting up the outdoor aquaria at CSIRO Marine Laboratories.
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