Tidal flows link terrestrial, estuarine, and marine systems. The residence time of a water mass in a seagrass habitat (determined by the tidal fluxes) may have a profound effect on seagrass distribution. Short residence times (days) allow pollutants and excess nutrients to be flushed out of a system before harming seagrasses (Kitheka et al., 1996). In contrast, long residence times promote the accumulation of nutrients and the growth of phytoplankton and nuisance algae while suppressing the growth of seagrasses via low light availability (Rysgaard et al., 1996; Herbert, 1999).
Coral reefs, seagrass beds, and mangrove forests often co-occur in tropical coastal systems suggesting an interaction of sorts, determined by tidal flows. For example, coastal wetlands such as marshes and mangroves assimilate nutrients leaching from land and thereby reduce the nutrient level reaching adjacent seagrass systems (Valiela and Cole, 2002) during ebb flows. These authors suggested that land-derived N loads from 20 to 1,000 kg N ha-1 year-1 seem to be a critical range for seagrass survival in shallow waters. When N loads are higher, wetlands are no longer able to remove sufficient N through denitrification, and N burial, and tidal currents will carry the excess nutrients into the sea-grass beds. Excess nutrients can then lead to the loss of the seagrasses. Therefore, adjoining plant systems (wetlands and seagrasses) are not isolated units but are likely to be linked (Valiela and Cole, 2002).
Seagrass beds are among the most productive systems on the planet (Dring, 1994) and experience relatively low grazing losses with most leaf production being shed (Cebrian and Duarte, 2001; Mateo et al., Chapter 7). This amounts to a considerable quantity of detrital material which can remain within the seagrass meadow or can be exported. The fate of seagrass detritus depends, to a large extent, on the magnitude of currents, waves, and tides (Ochieng and Erftemeijer, 1999) and the nature of the leaves: some leaves float on becoming detached while others sink (Zieman et al., 1979). Floating leaves are more likely to be exported by tidal currents, but leaves that sink and form detritus locally may also be exported onto adjacent beaches or the deep sea during storm events and/or spring tides (Hemminga and Nieuwenhuize, 1990; Kirkman and Kendrick, 1997; Ochieng and Erftemeijer, 1999). It appears that in many instances detritus remains within the originating ecosystem, being recycled more or less in situ (Hemminga and Nieuwenhuize, 1991; Paling, 1991). In other cases, large amounts of sea-grass detritus are transported into adjacent estuaries contributing to the estuarine carbon cycle (Bach et al., 1986; Cebrian and Duarte, 2001; Mateo et al., Chapter 7). Seagrass fragments have even been found at great depths in ocean basins (>1,000 m) where, it is postulated, they comprise an important food source for a number of invertebrate detriti-vores (Menzies et al., 1967; Menzies and Rowe, 1969; Wolff, 1976, 1979; Suchanek et al., 1985), as well as pelagic fishes and crustaceans (Williams et al., 1987). Litter washed up onto beaches also supports a wide range of invertebrates (Kirkman and Kendrick, 1997; Ochieng and Erftemeijer, 1999), and the location where the litter is deposited (high tide line or storm line) determines where invertebrates will find the highest availability of food. Litter that remains in shallow waters provides protection from erosion (Ochieng and Erftemeijer, 1999) and a habitat for juvenile fish (Lenanton et al., 1982; Robertson and Lennaton, 1984), but the reliability of this habitat depends on the local hydrodynamic conditions.
Tidal flows do not only link adjacent communities but can also isolate them. For example, in an estuary in Kenya, organic particles efflux and reflux between mangroves and seagrasses during each tidal cycle (Hemminga et al., 1994). During the ebb, POM effluxes from the mangroves reaching the sea-grasses; during the flood, particles resuspended in the seagrass meadow (in part, particles generated in the mangroves and deposited in the seagrasses) reach the mangroves (Hemminga et al., 1994). These particles never make it to the adjacent coral reef due to trapping of the high turbidity plume by the tide and onshore winds (Fig. 13; Kitheka, 1996; Kitheka et al., 1996; Miyajima et al., 1998). Turbid waters could be detrimental to the reef-forming coral polyps (Johannes, 1975). As a result of the wave attenuation by the coral reef and the tidal isolation of the corals, mangroves, seagrasses and the coral reef can co-exist just a few kilometers apart along the Kenyan coast. This process is likely to also apply to other reef-seagrass-mangrove associations throughout the world.
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