General Description And Role In Ecosystems

Mangrove and seagrass communities are characterised by a small number of widely distributed angiosperm taxa, having evolved mostly post-Cretaceous (within the last 60-100 million years). The relatively recent evolution of these communities may explain their comparatively low species diversity today, although this is also arguably related to the harsh environmental factors that characterise these communities' habitats. The relatively rich mangrove and seagrass floras of today are testament to their adaptive and evolutionary success for living in and adjacent to the intertidal zone. These highly specialised plants flourish in soft sediments, tapping rich estuarine nutrients with their distinctively vascular root systems. Mangroves further provide significant habitat and structure where their biomass accumulation, while readily seen as contiguous with adjacent rainforests, is also analogous to that created by coral reefs.

While important details of relevant phyletic origins remain lacking, ancestral mangrove and seagrass plant taxa are known to have reinvaded marine environments in multiple episodes from the diverse selection of an-giosperm lineages. Their evolution appears constrained by key functional attributes essential for survival in saline and aqueous settings where isotonic extremes, desiccation and hydrologic exposure combine as uniquely harsh constraints on organisms living in tidal zones and estuaries. To achieve this, mangroves and seagrasses share a number of ecophysiological traits and have evolved mechanisms to cope with life at the land-sea interface, for example, salt tolerance, translocation of gases to aerate their roots, and reproductive strategies. Both plant communities perform important ecosystem services such as sediment stabilisation, nutrient processing, shoreline protection, and provide habitat and

Figure 16.1 Distribution of mangrove species along the coastline of GBR catchments in Queensland.

nursery grounds. While the species diversity of mangroves and seagrasses is relatively low compared with adjacent communities like tropical rainforests and coral reefs, the diversity of organisms that reside in, or utilise, mangroves and seagrasses is high. Many fish and shellfish spend all or part of their life cycle in mangroves and/or seagrasses, including important commercial, recreational and artisanal fisheries.

The land-sea interface is a dynamic environment, where subtle natural changes in climate, sea level, sediment and nutrient inputs have dramatic consequences in the distribution and health of mangroves and sea-grasses. Local human disturbances include eutrophica-tion, dredging/filling, overfishing and sedimentation. The combined pressures of human disturbances and global climate change have led to mangroves and sea-grasses becoming 'endangered communities'. Small scale restoration projects have demonstrated the extreme difficulty of scaling up to effective, large scale restoration. Urgent protective measures need to be implemented to avoid further loss of mangroves and sea-grasses and the resulting environmental degradation of coastal ecosystems of the GBR coast.

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