Extreme guanotrophication by phosphorus in contradiction with the productivity of alkaline soda pan ecosystems

Abstract

Waterbirds as nutrient vectors can cause high phosphorus loading in shallow inland aquatic ecosystems. The main goal of this study was to determine the causal relationships between the characteristic physico-chemical properties of intermittent (temporary) alkaline soda pan (playa) ecosystems and specific (surface and volume-related) P loading of waterbirds by in situ field investigation, estimation as well as laboratory experiments using standard methods. In addition, our aim was to estimate the contribution of groundwater and precipitation to the total phosphorus pool of soda pans in Hungary. The estimated high specific external P loading of waterbirds (mean: 185 mg P/m²/y, 3.32 mg P/L/year) can explain the majority of the hypertrophic TP pool (mean: 5.17 mg/L, 64%) in soda pans, which is mediated by large-bodied herbivorous (e.g. geese and ducks) and medium-bodied omnivorous (e.g. gulls) waterbirds, who are important external nutrient importers and major phosphorus source. The results also confirm the hypothesis that groundwater (3%) and precipitation (5%) together account for a smaller estimated (8% in this study) contribution to the hypertrophic TP pool in soda pans, while the contribution of waterbirds (64% in this study) to the TP is much higher (64-100%). In this study, the remaining part of TP (maximum 28%) pool can be explained by internal P sources. Soda pans are characterized by physical and chemical characteristics coupled with high densities of waterbirds, as biotic mediators of external P sources, which together cause the maintenance of high concentrations of P-forms. The extreme guanotrophication by high P loading of herbivorous waterbirds causing a hypertrophic state is in contradiction with the limited primary production of natural soda pans. This unique phenomenon can be explained by the multiple impact of prevailing extreme physico-chemical drivers (intermittent hydrological cycle, shallow water depth, high turbidity, salinity, alkalinity) and by the specific nutrient cycle of these alkaline soda ecosystems.

Keywords: Endorheic; Excrement; Groundwater; Hypertrophy; Intermittent; Waterbirds.