Nutrient cycling analysis of anthropogenically influenced subtropical coastal ecosystems with a focus on pH-dependent phosphorus species compositions, distributions and transformations
Final Report Abstract
Coastal regions are increasingly subject to pollution-related phosphorus (P) deposition in nearshore sediments, which act as an efficient trap and vast reservoir for P. Despite known mechanisms of P accumulation, successive enrichment processes at a catchment scale are not well understood. This is mainly attributable to the lack of corresponding cross-ecosystem studies, diverse coastal ecosystems and low analytical specificity for individual sedimentary P phases. Therefore this investigation refined a conversion-extraction method (CONVEX) for the quantification of individual phosphorus species in sediment and subsequently used the method for a cross-ecosystem study of sediment P speciation and transformation at a nutrient-rich region of the Firth of Thames, New Zealand. For CONVEX method improvement, standard addition experiments were performed in order to confirm method specificity for particular calcium phosphate (Ca-P) species. At the same time, this standardization served to determine the solubility of used mineral standards. The subsequent cross-system study evaluated the pH and Eh influence on P fractions, speciation and transformation. To allow for regional diversity, crossregional transferability was verified by comparison with a contrasting P-poor tropical river delta. For CONVEX method standardization, carbonate fluorapatite (CFAP) specimens from different localities, fluorapatite (FAP), fish bone apatite, synthetic hydroxylapatite (HAP) and octacalcium phosphate (OCP) were characterized by XRD, Raman, FTIR and elemental analysis. Sediment samples were incubated with and without these reference minerals and subsequently sequentially extracted to quantify Ca-P species by their differential dissolution at pH values between 3 and 8. The quantification of solid-phase phosphates at varying pH revealed solubilities in the following order: OCP > HAP > CFAP (4.5% CO3) > CFAP (3.4% CO3) > CFAP (2.2% CO3) > FAP. Standard addition experiments together with simultaneous unspiked sediment analyses verified CONVEX method applicability for separate quantification of the most prevalent sedimentary Ca-P minerals as indicated by consistent differential dissolution of natural sedimentary species vs. added reference species. Surprisingly high OCP contents were found in the analyzed coastal sediments, which supports the hypothesis of apatite formation by an OCP precursor. The cross-system study and verification of cross-regional transferability revealed that the same factors affected fractions and species at different coastal systems and regions in the same way. Among these factors, pH was the most important variable for determining P fractions and speciation. A remarkable difference between the coastal zones was the accumulation of metastable calcium phosphates (Ca-Pmeta) at seaward sites of the P-rich region: The amount of Ca-Pmeta was six-fold increased compared to the P-poor region, whereas total Ca-P was just doubled. As shown by species contents, these elevated Ca-Pmeta levels were caused by species transformation. Pollution-related adsorbed P transformed into octacalcium phosphate and carbonate fluorapatite due to pH increase from landward to marine environments. These metastable and biologically available Ca-P species accumulated at the coastline, because their generation is faster than conversion into less soluble apatites or complete P removal by transport processes. Thus, anthropogenic P inputs are reflected by Ca-P species compositions. While insignificant transformation into less soluble Ca-P restricts an even larger accumulation by promoting P removal into the ocean, metastable species are simultaneously a potential source for detrimental effects at coasts.
