Obligate groundwater crustaceans mediate biofilm interactions in a subsurface food web

Food webs in groundwater ecosystems are dominated by only a few top-level consumers, mainly crustaceans. These obligate groundwater dwellers—or stygobites—clearly interact with groundwater biofilm, but it is uncertain whether they affect the abundance and structure of biofilm assemblages. We hypothesized that crustacean stygobites would reduce bacteria and protozoan abundance and alter biofilm assemblage structure. We also hypothesized that high densities of stygobites would remove more bacteria and protozoa than would low densities, and that this difference would become more pronounced over time. First, we established that the amphipods Niphargus fontanus and Niphargus kochianus both ingest biofilm by examining their gut contents. We then conducted 2 microcosm experiments. The 1st experiment showed that both N. fontanus and the isopod Proasellus cavaticus increased protozoan abundance but that bacterial abundance was only slightly reduced in the presence of P. cavaticus. In the 2nd experiment, we determined how zero, low, and high densities of N. kochianus affected the biofilm. The high-density treatment of N. kochianus had significantly higher protozoan abundance than the control and the low-density treatment, and high densities of N. kochianus significantly increased the relative proportions of small- and medium-sized bacteria over time compared with controls. Our controlled microcosm experiments demonstrate that macroinvertebrate stygobites can influence groundwater biofilm assemblages, although the exact mechanisms are not clear. These results support the hypothesis that stygobites influence essential ecosystem services supplied by groundwater ecosystems.