The littoral of Lake Dunstan, Central Otago, New Zealand, is dominated by the introduced submersed macrophyte Lagarosiphon major (hereafter Lagarosiphon) which effectively excludes native macrophytes from most of the littoral. Lake Dunstan is a large hydro lake that was - after reaching its operational water level in 1993 - rapidly colonized by Lagarosiphon from upstream sources. Because of its extensive biomass and growth, it is believed the macrophyte plays an important part in the lake ecosystem but also that it requires frequent management to allow for recreational activities, to restore aesthetic values of beaches and especially to prevent further spread into unaffected waterways. To assess the integration of Lagarosiphon into the lake ecosystem and possible impacts of extensive macrophyte removal a 3 year study was initiated focusing on invertebrate and fish distribution and diet in relation to Lagarosiphon. A stable isotope analysis was used to reconstruct carbon flows and trophic structure within the littoral of Lake Dunstan. Additionally, a small scale weed management experiment was carried out to assess possible impacts of macrophyte removal on invertebrates and fish.
Lagarosiphon harboured a high density and diversity of macroinvertebrates. The invertebrate community was dominated by mollusc species in terms of biomass and abundance. There was little temporal and spatial variation in invertebrate distribution and communities on a large scale (lake-wide variation), but there were differences in invertebrate biomass on a smaller scale in relation to the location of samples within the macrophyte belt. Invertebrate biomass was lowest at the bottom of the dense macrophyte beds.
The positive association of common bully (Gobiomorphus cotidianus: Eleotridae) with the dense Lagarosiphon stands and a diet - mainly consisting of invertebrates common on Lagarosiphon - suggested the important role of this macrophyte as a provider of food and shelter for small fish species. Brown and rainbow trout (Salmo trutta, Oncorhynchus mykiss: Salmonidae) diet was dominated by invertebrate species common on this macrophyte as well.
Results of the stable isotope analysis suggested a secondary role of Lagarosiphon in the lake food web as a surface provider for the growth of epiphytic algae that appeared to be a major primary producer in the littoral of Lake Dunstan. Detritus (FPOM), assumed to consist partly of decaying Lagarosiphon, played a less important role as a base for the food chain. The main carbon flow pathway was from the primary producers (epiphyton - FPOM) through primary consumers (Mollusca, Chironomidae, Trichoptera) through to the top predators (tertiary consumers: trout and bullies) in the system.
A small scale experiment on the influence of macrophyte harvest, i.e. the cutting of channels into dense macrophyte beds, showed that a partial removal of macrophyte biomass could help achieve several management goals: it not only reduced macrophyte biomass to enable recreational activities but also had no negative impacts on invertebrate biomass and diversity. The channels even increased invertebrate abundance and biomass in the treated compared to the untreated control plots along the cut edges. Regrowth of Lagarosiphon was rapid, with the cut channels being completely overgrown within 4 months.
All these results taken together suggested a major role of Lagarosiphon in the littoral of Lake Dunstan. Lagarosiphon supported a species rich invertebrate community and was the main provider of food and shelter for fish in this lake. A partial removal of macrophyte biomass might be a more suitable management option than large scale eradication to avoid negative impacts on the lake ecosystem. The most important aspect in management of this weed remains the prevention of further spread into pristine waters.
| Identifer | oai:union.ndltd.org:ADTP/217498 |
| Date | January 2007 |
| Creators | Bickel, Tobias O., n/a |
| Publisher | University of Otago. Department of Zoology |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://policy01.otago.ac.nz/policies/FMPro?-db=policies.fm&-format=viewpolicy.html&-lay=viewpolicy&-sortfield=Title&Type=Academic&-recid=33025&-find), Copyright Tobias O. Bickel |
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