Carbon flow in the littoral food web of an oligotrophic lake

Benthic food web dynamics and carbon flow were examined in the littoral zon e of Lake Coleridge, a large deep oligotrophic lake, using radioactive and stable isotope techniques in conjunction with analyses of stomach contents of the fauna. We specifically address two hypotheses: (1) that macrophytes only contribute to the carbon flow to higher trophic levels when they have decayed; and (2) that epiphytic algae is the major source of carbon for mac roinvertebrates, and thus fish, with only minor contributions from phytopla nkton or terrestrial sources. Epiphytic diatoms were a major component of t he stomach contents of the gastropod snail Potamopyrgus antipodarum, and of chironomids. Animal remains were also common in the diet of some chironomi ds, while amorphous organic matter predominated in the stomachs of oligocha etes. A variety of epiphytic algal taxa was found in trichopteran larvae. F eeding rate of P. antipodarum measured with radioactive tracers increased b y 10x on decayed macrophytes (Elodea) compared with live material, while fe eding rates on characean algae increased by a factor of 3 when decayed mate rial was presented. However, assimilation rates were less than 20% on decay ed material compared with 48-52% on live material. Potential carbon sources were easily distinguished based on their delta C-13 values, although isoto pic ratios showed significant variation among sites. Epiphytic algae showed less variation among sites than macrophytes and were depleted by 4-5 parts per thousand compared with macrophytes. Detrital material, organic matter in the sediments and plankton were significantly depleted in delta C-13 rel ative to macrophytes and slightly depleted relative to epiphytic algae. Mos t macroinvertebrate taxa showed a similar pattern among sites to macrophyte s and epiphytic algae. P. antipodarum and chironomids were slightly enriche d compared with epiphytic algae. Ratios for the common bully (Gobiomorphus cotidianus) were generally consistent with a diet dominated by chironomids, while there was some evidence for terrestrial inputs for koaro (Galaxias b revipinnis) and juvenile brown trout. Epiphytic algae appear to underpin mu ch of the production in the littoral zone of this oligotrophic lake, with t richopteran and chironomid larvae mediating carbon flows from algae to fish . Macrophytes do not make a major contribution directly to carbon flow to h igher trophic levels even when decayed. The lack of a direct link between m acrophytes and higher trophic levels is due to the faunal composition, incl uding a lack of large herbivores.