Effects of invasive macrophytes on littoral-zone productivity and foodweb dynamics in a New Zealand high-country lake

Invasion of littoral zones by adventive macrophyte species can facilitate major changes in the ecology of lakes. In Lake Wanaka, a large alpine New Zealand lake, the macrophytes Lagarosiphon major and Elodea canadensis (Hydrocharitaceae) have invaded parts of the lake where they form tall dense plant beds throughout mid-depths (2––7 m) of the littoral zone. We investigated differences in plants, benthic invertebrates, fish, and food webs characterizing native and exotic plant beds in mid-depths of the littoral zone. The 3×× higher plant biomass and 2×× higher plant surface area in exotic than in native plant beds (quillworts, milfoils, and charophytes) contributed to greater standing stocks and productivity of epiphyton in the exotic plant beds. Invertebrate communities were less dense (1890/m² vs 4030/m²) and less diverse (richness == 9 vs 12) in native than in exotic plant beds because of differences in biomass, productivity, and physical structure of native and exotic plant communities. Invertebrate communities in native beds were dominated by snails, oligochaetes, and nematodes, whereas chironomids, snails, and caddisflies were dominant in exotic beds. Stable isotope signatures (¹³C and ¹⁵N) and dietary analyses indicated that Potamopyrgus antipodarum, the dominant invertebrate taxon in both bed types, consumed mostly epiphyton. In native beds, consumption of sedimentary fine benthic organic matter by oligochaetes and nematodes made significant contributions to C flow, whereas, in exotic beds, consumption of epiphyton by grazers (e.g., snails, caddisflies, and chironomids) was an important pathway for C flow. Macrophytes made only small contributions to C flow in either bed type. The dominant native fish in Lake Wanaka, the bully Gobiomorphus cotidianus, was more abundant in exotic than in native beds, but bully predation rates on snails were significantly lower in exotic than in native beds. Invasion by adventive macrophyte species can cause significant shifts in lake productivity, species composition, and foodweb dynamics.