Spatial distribution of deepwater seagrass in the inter-reef lagoon of the Great Barrier Reef World Heritage Area

Abstract

Seagrasses in waters deeper than 15 m in the Great Barrier Reef World Heritage Area (adjacent to the Queensland coast) were surveyed using a camera and dredge (towed for a period of 4 to 6 min); 1426 sites were surveyed, spanning from 10 to 25°S, and from inshore to the edge of the reef (out to 120 nautical miles from the coast). At each site seagrass presence, species, and biomass were recorded; together with depth, sediment, secchi, algae presence, epibenthos, and proximity to reefs. Seagrasses in the study area extend down to water depths of 61 m, and are difficult to map other than by generating distributions from point source data. Statistical modeling of the seagrass distribution suggests 40 000 km 2 of the sea bottom has a probability of some seagrass being present. There is strong spatial variation driven in part by the constraint of the Great Barrier Reef's long, thin shape, and by physical processes associated with the land and ocean. All seagrass species found were from the genus Halophila. Probability distributions were mapped for the 4 most common species: Halophila ovalis, H. spinulosa, H. decipiens, and H. tricostata. Distributions of H. ovalis and H. spinulosa show strong depth and sediment effects, whereas H. decipiens and H. tricostata are only weakly correlated with environmental variables, but show strong spatial patterns. Distributions of all species are correlated most closely with water depth, the proportion of medium-sized sediment, and visibility measured by Secchi depth. These 3 simple characteristics of the environment correctly predict the presence of seagrass 74% of the time. The results are discussed in terms of environmental dynamics, management of the Great Barrier Reef province, and the potential for using surrogates to predict the presence of seagrass habitats.KEY WORDS: Seagrass · Trawling · Depth · Marine park · Halophila spp. Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 392: [57][58][59][60][61][62][63][64][65][66][67][68] 2009 their connectivity to the coral reef ecosystem, the impacts of fishing, global threats from climate change, global declines of coral reef ecosystems and on the biodiversity of the system as a whole (Day et al. 2000).In the early 1990s coastal management agencies recognised the value of seagrass meadows to coastal fisheries in the northern Australian region , Watson et al. 1993, and subsequent studies have reinforced the value of seagrasses as part of coastal ecosystems worldwide (Larkum et al. 2006. Seagrasses are a functional grouping of vascular flowering plants which can grow fully submerged and rooted in soft bottom estuarine and marine environments. Seagrasses are rated the third most valuable ecosystem globally (on a per hectare basis), and the average global value for their nutrient cycling services and the raw product they provide has been estimated at US$ 19 004 ha -1 yr -1 (at 1994 prices; Costanza et al. 1997). In tropical and subtropical waters, seagrasses are also important as...

Long-term climate-associated dynamics of a tropical seagrass meadow: implications for the future

“…The data from each individual observer was only used if the linear regression had an R 2 greater than 0.95, indicating a 95% certainty in the observers estimates, leaving a residual and quantified observer error of < 5%. This technique has been successfully used to determine small changes in biomass within a range of empirical field and experimental studies (Rasheed 1999, 2004, Coles et al 2009). …”

“…Aboveground seagrass biomass was determined at each of the sampling sites by means of a technique that visually estimates biomass as described by Mellors (1991) and is commonly used as a nondestructive method for assessing seagrass biomass change (Lee 1997, Mumby et al 1997, Rasheed 1999, 2004, Rasheed et al 2008, Coles et al 2009). In Karumba, observers ranked seagrass biomass by referring to a series of 12 reference quadrat photographs that represented the range of biomass likely to be encountered (Mellors 1991, Kutser et al 2007).…”

Reef sharks and inshore habitats: patterns of occurrence and implications for vulnerability

“…The spatial data sources previously mentioned were used to identify the presence of these biological features and distance to net samples. Additional data used for this analysis included seagrass distribution models (Coles et al 2009, Grech & Coles 2010, seagrass monitoring data from Seagrass Watch (Mellors et al 2008), and reef extent data from the Great Barrier Reef Marine Park Authority. Seagrasses, reefs, and mangroves were selected a priori as key biological features because they are significant features for a number of species, including reef sharks (Stevens 1984, White & Potter 2004, Knip et al 2010, White & Sommerville 2010, and could be mapped using the data available.…”

Spatial and temporal variability of seagrass at Lizard Island, Great Barrier Reef

“…were right skewed, with higher frequency of occurrence of lower percentage cover levels. The low coverage of Halophila ovalis and H. spinulosa was potentially a consequence of the shallow depth of survey ( < 5 m), as these species are often found in deeper water (Coles et al 2009). In particular, H. spinulosa is known to occur in lower light environments, and has been found at up to 61 m water depth in the GBR lagoon (Coles et al 2009).…”