Release of dissolved organic carbon from seagrass wrack and its implications for trophic connectivity

The export of old leaves and stems (wrack) from seagrass meadows provides a mechanism for trophic connectivity among coastal ecosystems. As little of this wrack is consumed by mesograzers, leached dissolved organic carbon (DOC) may determine the importance of wrack as a trophic subsidy. However, few studies have examined the effect of seagrass type or age on the release of DOC or its bioavailability. We examined the amount and composition of DOC released from different wrack: Posidonia sinuosa, Amphibolis antarctica and the alga Laurencia sp. We then examined the effect of age on DOC leaching from P. sinuosa wrack. The bioavailability of the DOC was also assessed using a bacterial bioassay. The rate of DOC leaching from P. sinuosa leaves decreased exponentially with time. According to that exponential model, ~50% of the total DOC release occurred in the first 14 d and it would require a further 2.94 yr to release the same amount again. Fresh algae Laurencia sp. leached the greatest amount of DOC in the first 16 h (6.7 g kg–1 fresh weight (FW) wrack), followed by fresh P. sinuosa leaves (1.7 g kg–1 FW), A. antarctica leaves (1.1 g kg–1) and stems (0.6 g kg–1), 4 wk old P. sinuosa (67 g kg–1) and fine detritus (74 g kg–1). In all cases, the composition of the DOC was similar and dominated by the hydrophilic component (in P. sinuosa, predominantly sugars and amino acids). Leachates from all fresh wrack supported bacterial growth over 24 h. Leachate from older wrack either failed to support bacterial growth or only supported it for a limited time. Given the exponential decay in DOC release rate, the interacting timescales of transport and leaching will affect the value of wrack as a vector for trophic subsidies.