Promotive performance of shrimp Neocaridina denticulata on Typha angustifolia leaf litter decomposition

Abstract

Litter decomposition plays a crucial role in aquatic food webs. In the present study, we used a specially designed bucket, at the middle height of which a 1-mm nylon mesh was set, to investigate the direct (grazing) and indirect (feces) effects of shrimp Neocaridina denticulata on the decomposition of Typha angustifolia litters. During the 140-day decomposition period, the net overall effects of shrimp enhanced decomposition by 63.5%, which amounted to 32.8% of mass loss. Shrimp grazing contributed 81.7% of the enhanced decomposition, whereas feces contributed 18.5%. The early stage displayed the net effect of feces, whereas in the final days, it was mainly attributed to intensive grazing, which maintained litter breakdown at high speed. The presence of shrimp greatly improved the hydrolytic extracellular enzyme activities, especially peroxidase and phenol oxidase. Meanwhile, the presence of shrimp greatly enriched the water nutrients, total phosphorus in particular. However, selective absorption of NH₄⁺-N by microorganisms caused its shortage for decomposers, suggesting their great demands for NH₄⁺. Redundancy analysis showed that water physical and chemical properties could explain 73.5% of the variation of enzymatic activities. Besides directly grazing of shrimp, the main factors affecting decomposition were enzymatic activities in the absence of shrimp, while in the presence of shrimp, the factors changed to water physical and chemical properties (mainly pH, total nitrogen, and phosphorus, and NO₃⁻-N). Our results demonstrated that some aquatic animals, like shrimp, not only breakdown leaf litters by grazing, but also stimulate the microbial activities by their byproducts such as feces.