Within-lake detection of the effects of tourist activities in the littoral zone of oligotrophic dune lakes

Abstract

Intensive recreational use of oligotrophic lakes can lead to increases in epilimnetic nutrient concentrations (through direct inputs from urine or re-suspension of sediments) and the development of undesirable algal blooms. Despite these adverse ecological responses to tourist activities, many lake monitoring programs do not address tourist nutrient inputs at appropriate spatial and temporal scales. This paper presents results of investigations aimed at detecting the effects of nutrient inputs to perched dune lakes on Fraser Island, principally through within-lake comparisons of nutrient and algal variables. Nutrient concentrations and algal biomass were measured in heavily visited (disturbed) and inaccessible (reference) sites within five perched dune lakes on Fraser Island, Australia, during the summer of 1999/2000. Whilst nutrient and phytoplankton chlorophyll a concentrations did not differ between sites, periphyton chlorophyll a concentrations were occasionally significantly higher in disturbed sites than in reference sites, particularly in the very popular clear lakes, suggesting that algal growth may be enhanced by tourist activities. Experimental manipulations of nitrogen and phosphorus concentrations in algal (phytoplankton and periphyton) bioassays were undertaken in each lake over the 2000/2001 summer, to assess algal responses to nutrient additions. The response of phytoplankton communities to nutrient additions varied greatly between lakes, with evidence of limitation or co-limitation by nitrogen and phosphorus in all systems. Periphyton biomass showed similar trends to phytoplankton in some lakes, but these were not significant. Nutrients added to lakes by tourists are likely to be rapidly assimilated by littoral zone periphyton communities in these oligotrophic lakes. As a result, impacts of tourism are not likely to be detected by traditional measurements of open water nutrient and phytoplankton chlorophyll a concentrations. Instead, measurement of periphyton growth and/or biomass (chlorophyll a) in the littoral zone might be the most spatially and temporally relevant indicator of tourist impacts in these lakes.