Phosphorus diagenesis in sediment of the Thau Lagoon

Abstract

We describe the depth distribution of phosphorus in relation to the distribution of major redox species (dissolved O₂, NO₃⁻, NH₄⁺, Mn, and Fe, and particulate S, organic C, reactive Mn- and Fe-oxides) in modern sediments of two stations in the Thau Lagoon. Sediments close to the oyster bank zone are enriched in organic carbon and are highly bioturbated, while those outside the bank are less bioturbated and organic carbon levels are lower. In all sites, early diagenesis follows the well-accepted depth sequence of redox reactions of organic matter mineralization. The upper sediments of the station enriched in organic carbon contain high amounts of reactive particulate organic phosphorus that arrives at the sediment surface through biodeposition. Only a part of this phosphorus is released to the bottom water after mineralization, since more than 50% of total P is buried as an authigenic phosphate mineral. In the middle of the lagoon, outside the oyster bank zone, organic matter seems to be much more refractory, but the distribution of the major redox compounds indicates that this organic matter is partially mineralized. A portion of the phosphate and ammonium released during mineralization does not escape the sediment, since the concentration gradient is close to zero between 1 and 25 cm depth below the sediment–water interface. Pore water N and P are likely fixed by biological uptake. The characterisation and magnitude of this process require further study.

Introduction

The oceanic phosphorus cycle depends on the inputs from rivers, and the ability of sediments to sequester and bury phosphorus (Froelich et al., 1982, Ruttenberg and Berner, 1993, Howarth et al., 1995). On shorter time scales, phosphorus sequestration by sediments affects the trophic state of lakes and lagoons, and the productivity of estuaries (Nixon, 1981, Caraco et al., 1990).

Coastal lagoons are a very common feature of coastal environments, occupying 13% of the world coastline. Because of their low water turnover and long residence time, many Mediterranean coastal lagoons are greatly affected by nutrient loads from their catchments (La Jeunesse and Elliott, 2004). The analysis of phosphorus compounds in sediments of the Thau Lagoon indicates that the upper 5 cm layer of sediment is enriched in phosphorus (Moutin et al., 1993, Mesnage and Picot, 1995). Direct benthic flux measurements indicated that some phosphorus is remobilized to the water column (Mazouni et al., 1996). It has been documented that the Thau Lagoon system has suffered natural disturbance, caused by anoxic conditions in the bottom waters during summer, while low winds and high temperature lead to oxygen depletion (Souchu et al., 1998). The highest benthic fluxes of P towards the water column have been recorded during these periods. The release of phosphate is believed to be caused by the reductive dissolution of iron oxides.

Both the amount and the form of the phosphorus sequestered by sediments are affected by diagenesis. Near the sediment–water interface, where most of the freshly deposited organic matter is decomposed, phosphate is rapidly remineralized and released to the sediment pore water from which it can readily escape to the overlying water. Consequently, only a portion of the particulate phosphorus that reaches the sediment–water interface is actually buried with the accumulating sediment (Krom and Berner, 1980, Krom and Berner, 1981, Balzer, 1986, Sundby et al., 1992). Deeper in the sediment, organic phosphorus may be converted to apatite without loss of phosphorus from the sediment (Ruttenberg and Berner, 1993). Fe-bound P may also act as an intermediate between organic P and apatite P (Slomp et al., 1996b). Phosphorus sequestration is particularly important in coastal zones, and early diagenesis may have a great effect on phosphorus burial (Sundby et al., 1992, Jensen et al., 1995, Anschutz et al., 1998).

The vertical degradation sequence of organic matter proceeds by a sequential consumption of electron acceptors during oxidation of organic carbon (C-org) according to the energy yield per unit of reactive organic matter. Reduction of oxygen near the sediment–water interface is followed by reduction of nitrate, manganese- and reactive iron oxides, sulphate and finally carbon dioxide (Froelich et al., 1979, Postma and Jakobsen, 1996). Early diagenesis yields reduced products and creates chemical gradients and fluxes. The processes involved in re-oxidation of reduced products, particularly of N, Mn, Fe compounds, are still a matter of debate (Hulth et al., 1999, Anschutz et al., 2000, Hyacinthe et al., 2001). Secondary reactions that change the chemical speciation of iron affect the distribution of phosphorus (Anschutz et al., 1998). In addition, bioturbation can redistribute chemical species in the sediment, by transporting phosphorus associated with particulate material across redox gradients (Schink and Guinasso, 1977, Slomp et al., 1998). Therefore, the redox stratification of sediments is not simply restricted to a surficial oxic and a deeper anoxic layer. In this study, we present vertical profiles of extractable and dissolved phosphorus as well as major components of the redox system.

One important factor controlling the burial efficiency of phosphorus appears to be the adsorption capacity of the sediment for phosphate which generally decreases rapidly with depth below the sediment surface. This decrease has been attributed to the progressive reductive dissolution of iron oxides upon burial (Krom and Berner, 1980, Anschutz et al., 1998). Therefore, Fe-bound P is often the initial sink of P supplied by organic matter, but not the major final sink. That means it is important to carry out extractions that differentiate between Fe-bound P, Ca-bound P and organic P and to assess how the distributions change with depth to evaluate the fate of P that reaches the sediment surface. For this reason, we have examined the distribution of several operationally defined reactive phases of both iron and phosphorus. The results obtained from sediments of a coastal lagoon environment can be compared to sediments collected in open sea systems, and analyzed using the same extraction processes.