Application of short-term sediment dynamics and particle-bound phosphorus fractionation methods (SEDEX) to estimate the benthic nutrient loading potential in Upper Newport Estuary, California

Abstract

Estuaries act as sources, sinks, and biogeochemical transformation sites for natural and anthropogenically-derived nutrients. Sediment loading from watersheds provides an important source of particulate nutrients to estuaries often neglected when constructing nutrient budgets. Deposition and resuspension of these sediments are known to impact biogeochemical cycles in estuarine environments. Phosphorus (P) exists in many forms in aquatic environments, and increased P loading to coastal environments increases primary productivity potentially leading to eutrophication. Magnitude and variability of sediment deposition, resuspension, and sediment-bound P concentrations were evaluated in Upper Newport Bay (UNB), California. During 2004, seven push cores were collected from the intertidal and subtidal zones of three sites to evaluate recent sediment dynamics using 7Be and sediment-bound P fractions. Two sequential phosphorus extraction methods were used to determine the distribution of potentially bioavailable forms of P (labile, iron-bound, and organic), and refractory forms of P (calcium-bound and detrital) in sediment. A seasonal trend appears with greatest deposition occurring during the wet season (spring) when watershed runoff increases (0.1 to 0.4 g/cm2 d), and greatest sediment resuspension occurring during the dry season (summer) when minimal input from precipitation or stream runoff occurs. The average annual short-term sediment deposition rate calculated from the 7Be inventories indicated that deposition is 24.7cm/yr in the upper estuary. However, the long-term sediment deposition rate of 0.15 cm/year indicates that less than 1% of the annual sediment deposited in UNB is permanently buried. Sediment P concentrations revealed enrichment of P during the spring, corresponding to increased sediment input. During time periods when net sediment resuspension dominated over net deposition (e.g. summer), total-P concentrations decreased. This decrease was attributed to increased biological uptake of water column P during summer blooms, which lead to the desorption of P from particles. Nonetheless, most of the sequentially extracted P (~66%) was contained in the refractory phase. Based on the long term deposition rate in UNB, the burial rate for the refractory P was approximately 0.97 μmol P/cm2 yr. Sediment deposition and resuspension processes may act as important internal mechanisms for recycling phosphorus, particularly reactive P, and must be considered in estuarine biogeochemical cycles.