Spatial variability in sediment phosphorus characteristics along a hydrological gradient upstream of Lake Rotorua, New Zealand

Abstract

This study examined particulate phosphorus (PP) characteristics of terrestrial, stream bed and lake bed sediments along a common hydrological pathway. The objective was to quantify spatial variability of PP in the catchment of a eutrophic lake, Rotorua (New Zealand). Median total phosphorus (TP; dry weight) concentration in sediments (‘terrestrial’) deposited from overland flow on farmland in the upper stream catchment was high (2,157 mg kg−1), greatly exceeding that of stream bed sediments (212 mg kg−1; Waiteti Stream) which exhibited downstream decline in TP concentrations. Lake bed sediments were enriched in TP (median = 2,661 mg kg−1) with relatively low Fe:P and Al:P mass ratios. Sequential extraction of three PP fractions showed that the mean proportion of HCl-extracted P (Ca-bound, denoting the refractory fraction) in extracted P was higher in terrestrial sediments (17 %) than stream bed sediments (4 %). Accordingly, TP concentrations of terrestrial sediments were highly (p < 0.01) correlated with Ca (R = 0.89), whereas TP was most closely correlated with Al (R = 0.83) in the stream bed sediments. Nonetheless, the majority of sequentially extracted P in both terrestrial and stream bed sediments comprised fractions (bicarbonate dithionate- and NaOH-extractable) that are potentially bioavailable in Lake Rotorua. Furthermore, phosphate buffering experiments indicated that terrestrial sediments desorbed P to be a source of PO4–P to receiving waters, whereas stream bed sediments buffered PO4–P concentrations in stream water. Such quantification of landscape-scale variability in PP characteristics can support strategic management of TP loading to the lake.