Abstract
Allochthonous detritus of terrestrial origin is one of the main energy sources in forested headwater streams, but its poor nutritional quality makes it difficult to use by heterotrophs. It has been suggested that algae growing on this detritus can enhance its nutritional quality and promote decomposition. So far, most evidence of this "priming effect” is derived from laboratory or mesocosm experiments, and its importance under natural conditions is unclear. We measured accrual of algae, phosphorus uptake capacity, and decomposition of poplar leaves in autumn in open- and closed-canopy reaches in 3 forest and 3 agricultural streams. Chlorophyll a abundance did not change significantly with stream type or with canopy cover, although in some agricultural streams it was higher in open than in closed canopy reaches. Canopy cover did not affect either phosphate uptake capacity or microbial decomposition. On the other hand, although there was no effect of canopy cover on invertebrate fragmentation rate, a significant interaction between canopy cover and stream suggests priming occurs at least in some streams. Overall, the results point to a weak or no priming effect of algae on litter decomposition in natural streams during autumn.
Highlights
Organic detritus from terrestrial origin, and leaf litter in particular, is one of the main energy sources for food webs in forested headwater streams [1,2], where riparian shading limits primary production [3]
Riparian cover at the beginning of the experiment ranged from 0 to 45% at the open reaches, whereas it was higher than 76% at the closed reaches (Table 2)
Average Dissolved inorganic nitrogen (DIN) concentrations were below 0.30 mg N L-1 in forest streams, whereas they ranged from 0.47 to 1.81 in agricultural streams, differences being, again, statistically significant (F1,4 = 16.1; p = 0.016)
Summary
Organic detritus from terrestrial origin, and leaf litter in particular, is one of the main energy sources for food webs in forested headwater streams [1,2], where riparian shading limits primary production [3]. This detritus tends to be dominated by recalcitrant compounds such as lignin and cellulose, is stoichiometrically imbalanced for the needs of consumers [4,5,6,7], and has low nutritional value [8].
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