Applicability of Bioavailable Phosphorus in Sediments to Indicating Trophic Levels of Lakes and Reservoirs

Abstract

Twelve lakes and reservoirs with different water depths and different water residence times were studied to identify the applicability of bioavailable phosphorus of sediments in indicating trophic levels. Water and sediment samples were collected in these 12 lakes and reservoirs to analyze the relationship of nutrient levels between the sediment and the water column. Sodium hydroxide extracted phosphorus (NaOH-P) determined using the SMT classification method is defined as the bioavailable phosphorus of sediment. The results showed that total phosphorus levels in sediments in different lakes and reservoirs ranged from 225 to 760 mg·kg-1 (mean value 502 mg·kg-1); the NaOH-P levels in sediments ranged from 86 to 584 mg·kg-1 (mean value 263 mg·kg-1); the total phosphorus concentrations in the water was 0.02-0.35 mg·L-1 (mean value 0.11 mg·L-1), and the chlorophyll a concentrations in the water were 3-349 μg·L-1 (mean value 51 μg·L-1). It was found that NaOH-P was more effective than total phosphorus in indicating the trophic status of the lakes and reservoirs. However, the NaOH-P levels were significantly related to the phosphorus concentrations in the water column only in shallow water with a long residence time. It was revealed that water residence time and water depth are two key factors that affect the relationship of the phosphorus content between the sediment and the water column. In deep waters or waters with short residence time, the NaOH-P content in the sediment hardly influenced the phosphorus concentration in the water columns, even at high levels. However, in shallow waters with long residence time, the sediment acted as both sources and sinks and frequently exchanged nutrients with the overlying water, especially during bloom periods in summer. Thus NaOH-P could be a potential risk of eutrophication in such waters.