Macroalgae are considered nutrient sinks in coastal waters, but when they become detritus, they turn to being a nutrient source upon decomposition. Owing to reduced water flows in stands of macrophytes (including seagrass meadows and intertidal salt marshes), remarkable amounts of detritus may accumulate between blades, fronds, or stems, where it is utilized as food by various invertebrates, such as sea urchins, mollusks, crustaceans, and fish that feed on large detrital particles, but also by filter feeders that ingested suspended remnants of detritus fragmentation. Primary production in near-shore macro-phyte beds can also be exported to, and utilized by, other marine and nonmarine systems. Energy transfer from coastal to inland systems is brought about by terrestrial consumers that enter the intertidal area to feed upon, and fuel terrestrial food chains with, marine food sources that have been deposited ashore by tidal currents. The very same tidal currents may remove detritus of marine and intertidal origin from the intertidal and coastal subtidal zones into estuaries or the open sea (cf. Figure 1) where it is subject to decomposition under submerged conditions. For instance, 15% of the CO2 globally fixed in plant biomass, enters the open sea as seagrass detritus, while seagrasses make up less than 1% of the marine plant biomass.
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