Response of periphyton to nutrient addition in a tufa-depositing environment

Abstract

We examined the effects of added nutrients on primary producers, protozoans and meiofauna — as well as on tufa deposition — in a short-term in situ experiment using nutrient- diffusing substrata. The study was carried out in the tufa-depositing barrage lake system National Park Plitvice Lakes (Croatia). To address the question of whether the extent of tufa deposition and the flow velocity modify the response of periphyton to nutrient addition, we selected 2 sites — fast flow (FF) and slow flow (SF) — on each of the 2 barriers, one barrier having a low rate of tufa depo- sition (LTD) and the other having a high rate of tufa deposition (HTD). The nutrients examined were nitrogen (N), phosphorus (P), and their combination (N+P). Both N and P were limiting nutri- ents for autotrophs in periphyton, and the response to nutrient enrichment was lower at HTD sites. A combination of HTD and FF was the most limiting factor for periphyton. Nutrient enrichment did not impede calcite deposition; P enrichment even enhanced calcite deposition — probably as a result of a denser autotrophic periphyton matrix. HTD supported a lower taxa richness of het- erotrophs in the periphyton, and flow velocity had taxa-specific effects. The addition of nutrients led to a decrease in the number of heterotrophic taxa at FF sites, while there was no effect at SF sites. The response of ciliates to flow velocity was taxa-specific. The effect of nutrients on the abundance of heterotrophs was rather weak, except for omnivorous and bacterivorous ciliates. Our results showed a direct effect of N and P on autotrophic periphyton in this tufa-depositing sys- tem, and only a weak effect on protozoans and micro-metazoans.