Food web responses to low-level nutrient and ¹⁵ N-tracer additions in the littoral zone of an oligotrophic dune lake

Abstract

We used natural abundance stable isotopes to establish the structure of the littoral zone food web of an oligotrophic, perched dune lake on Fraser Island, Australia. Mixing model analyses incorporating riparian vegetation, seston, and periphyton sources indicated that periphyton carbon was the most significant food resource for aquatic consumers, despite the abundance of allochthonous carbon sources. In order to examine the consequences of nutrient inputs from tourists visiting this remote lake, repeated additions of low levels of phosphate and 15 N-enriched ammonium nitrate were made to three littoral zone sites. Additions led to significant increases in periphyton chlorophyll a (Chl a) concentrations in enriched sites but had no measurable effect on phytoplankton Chl a concentrations. Periphyton collected 5 h after the first nutrient addition had substantially enriched ␦ 15 N signatures, suggesting that periphyton rapidly assimilated the added nutrients (and 15 N-tracer). After 10 d of additions, all other primary food sources for consumers also became 15 N-enriched, indicating that ongoing nutrient inputs are likely to lead to increased primary production and detrital processing. Substantially enriched consumer ␦ 15 N signatures were also measured, indicating that the added nutrients were assimilated and passed through multiple trophic levels. Our results indicate that ongoing low-level nutrient additions by tourists to oligotrophic lakes could lead to increased primary (periphyton) and secondary (consumer) production. However, increases in periphyton production and biomass accrual could eventually escape control by grazers, leading to adverse ecological and aesthetic effects.

Temporal variability of benthic algal δ13C signatures influences assessments of carbon flows in stream food webs

“…The rate of change in isotope signatures in response to these environmental factors is also variable, but given that benthic algae can have fast tissue turnover rates under favourable conditions, algal isotope signatures can often track changes in local environmental conditions over relatively short timeframes (i.e. less than 2 h) (Lajtha & Michener, 1994;Vymazal, 1995;Hadwen & Bunn, 2005). These fluctuations in stable isotope signatures can have significant implications for the interpretation of carbon flows in food web studies, as the timing of changes (and of sampling) mediates the degree to which the collected algal samples are directly relevant (as a basal resource) to the sampled consumers (Boon & Bunn, 1994;Vander Zanden & Rasmussen, 2001).…”

The trophic importance of epiphytic algae in a freshwater macrophyte system (Potamogeton perfoliatus L.): stable isotope and fatty acid analyses

“…Past research suggests that epiphytic algae (mostly small filamentous green algae and diatoms) may be important carbon sources in these communities (Underwood and Thomas 1990;James et al 2000a;Jones and Waldron 2003;Hadwen and Bunn 2005). Macrophytes make the greatest contribution to the organic carbon pool in the littoral zones of these lakes, but may not be direct food sources for animals (Keough et al 1996;Hecky and Hesslein 1995).…”

“…Analytical tools, such as stable carbon and nitrogen isotopes, and fatty acid analysis, may add detail to these generalisations (Desvilettes et al 1997;James et al 2000b;Jones and Waldron 2003;Hadwen and Bunn 2005;Jaschinski et al 2008a). The fractionation of d 13 C is thought to be low-maximally 1% per trophic level-and d 13 C is therefore useful to identify different carbon sources.…”

Longitudinal trends in river functioning: Patterns of nutrient and carbon processing in three Australian rivers

“…All consumer groups were stored in labelled zip-lock bags and immediately placed on ice. This procedure allowed the specimens to void their guts, removing unassimilated material and thereby aiding laboratory processing (sensu Hadwen and Bunn, 2005). Samples were frozen upon return to the labortaory prior to further processing.…”