Biochemical influence of amphidromous shrimps on stream ecosystems in Japan

Abstract Not only through species interactions but also through excretion of nutrients, consumers can have profound effects on the ecosystem structure. While many studies in lentic ecosystems address both effects in combination, little is known about their linkages in lotic ecosystems. By a combination of field manipulative experiment, excretion measurements of consumes, and field surveys, we evaluated biochemical effects of amphidromous shrimps on stream ecosystems. The field manipulative experiment showed that the presence of shrimp suppressed the total aquatic insect biomass by 9% but increased the total benthic macroinvertebrate biomass including the shrimps by 196%. The biomass-specific NH4+ excretion rate by shrimp was similar to aquatic insects, and the calculated mean NH4+ excretion by benthic macroinvertebrate assemblage was 144% higher in the presence of shrimps. On the other hand, shrimps excreted much less PO43− than aquatic insects, and the PO43− excretion by the benthic macroinvertebrate assemblage did not change by the presence of shrimps. The field survey showed a positive correlation between NO3− concentration in stream water and shrimp density, suggesting that the excess NH4+ was nitrified and raised NO3− concentration. In contrast, PO43− concentration in stream water was negatively associated with the shrimp density, indicating that the dominance of shrimps that excrete little PO43− decreased the PO43− concentration. While nutrient concentration of stream water is often attributed to the condition of the watershed area, the results of this study indicate downstream connectivity to the ocean can also influence the nutrient dynamics of the stream through the density of amphidromous shrimps.