Phosphate adsorption by riverborne clay sediments in a southern-Italy Mediterranean reservoir: Insights from a "natural geo-engineering" experiment

Abstract

This paper reports data from a southern-Italy reservoir (Lake Occhito) characterized by a strong riverborne sediment transport. Main hydrochemical, trophic, and nutrient variables were measured (over a twelve-month period) in both lake and tributaries. Lacustrine sediments were subjected to mineralogical characterization and to phosphorus fractioning, while a 6-day long batch experiment was carried out to evaluate the lake sediment orthophosphate adsorption capacity. A set of algal growth potential tests was also undertaken on the lake and its tributaries. Results highlight the presence of a strong gradient in nutrient availability among the inflows. Most of the nutrient loads were from the main tributary (20.3 t P a^-1, ~83 %), that showed the highest trophic potential (average: 56.8 mg L^-1) and was nitrogen/phosphorus co-limited. The other inflows were phosphorus limited and characterized by a higher sediment transport. The lake showed the lowest nutrient concentrations (average total phosphorus: 21 μg P L^-1) and was strongly phosphorus limited. Clays were the principal minerals in the lake sediments (~51 %), while the main phosphorus fraction was apatite (~78 %). The batch experiment demonstrated the capability of the lake sediments to reduce orthophosphate concentrations in phosphorus-rich waters (initial orthophosphate: 320 μg P L^-1; ~80 % reduction). The lake sediment orthophosphate kinetics of abatement was similar to that of a commercially available phosphorus sorbent (lanthanum modified bentonite), although the stability of phosphorus binding was higher for the commercial product. Theoretical average in-lake total phosphorus, chlorophyll-a, and transparency values, estimated through Vollenweider models, were approximately double of the average values measured in the lake. Therefore, the massive presence of riverborne clay sediments seems to markedly reduce the in-lake orthophosphate concentrations (and light penetration), inducing an overall lowering of the lake trophic state, as if the lake ecosystem were permanently subjected to a geo-engineering phosphorus sorbent treatment.

Keywords: Algal growth test; Mediterranean reservoirs; Nutrient limitation; Phosphorus adsorption; Phosphorus fractioning; Riverborne clay sediments.