Phosphorus biogeochemistry and models in estuaries : case study of the Southampton Water system
Various factors may increase the supply of organic matter content (eutrophication status) of estuarine and coastal systems. The most common cause of eutrophication is considered to be nutrient enrichment. In this study data has been collected in monthly surveys from January 1998 to April 1999 in the Southampton Water system, and the major features of nutrients (nitrogen, phosphorus and silicon) and suspended particulate matter (SPM) thus obtained. Additionally phosphorus in porewaters, and the solid phase in sediments were measured. The forms of phosphorus in the solid phase of sediment (deposited and suspended) were also measured using a modified sequential phosphorus extraction protocol (SPEPs) method. The ranges of the dissolved inorganic phosphorus (DIP) and other nutrients are similar to the data from the SONUS investigations. The overall distribution of DIP are strongly influenced by the sewage inputs to the system, with a peak in concentration in the River Itchen from the sewage works followed by apparently conservative behaviour. The concentration of dissolved organic phosphorus (DOP) were often low (rarely exceeded 5 |iM) and irregularly distributed in the system. The concentrations of particulate phosphorus in the water column and in the particles decreased seaward. The P:Fe ratio in the bicarbonate-dithionite released phase from particles implied that the phosphorus "saturated" particles near the sewage outfall are likely buried before they reach the high salinity waters and the particles in the high salinity waters are mainly from sediment resuspension, the detritus of phytoplankton or from offshore. The N/P ratios obtained implies that the system has potential phosphorus limitation of phytoplankton growth. The concentrations of the forms of inorganic phosphorus in the sediment generally decreased seaward. The Ca-bound phosphorus appears to be the dominant form of inorganic phosphorus (about 55%-85% of total inorganic phosphorus) in most of the sediments in the system. Benthic flux measurements revealed that the sediments in the system generally act as a sink of dissolved inorganic phosphorus and a source of dissolved organic phosphorus. This is in contrast to the fluxes of DIP predicted from the pore-water concentration gradients and implied the presence of a ferric iron-rich oxidising cap on the sediment surface. The results of a Kd model applied on the freshwater-sea boundary revealed that within the range of the riverine DIP concentration only a small amount of removal or addition of DIP is likely to occur. The Kd model is useful in predicting the influence of resuspended particles on DIP in the water column after resuspension "events" caused by storm and dredging activities. The total phosphorus input to the system is estimated as 144 ± 23.7 tonnes per year. The riverine input is the major source of phosphorus (about 69% of the total inputs). The direct sewage input (30% of the total phosphorus inputs) is also an important source of phosphorus in the system, especially in summer. The atmospheric input (1% of the total phosphorus inputs) is small. A considerable exchange flux with adjacent coastal waters (about 40% of total phosphorus inputs) reveals that the system is a phosphorus source to the English Channel and adjacent areas. The particle burial is an important process and retains about 60% of the phosphorus entering the system. However, significant removal of phosphorus from the water column was not obviously observed, suggesting that there are other sources of phosphorus in the system or the estimate of internal loss is overestimated. Further study is needed to reaffirm the behaviour of all forms of phosphorus in the system, and assess their yearly variation. An investigation of the forms of particulate phosphorus and phosphorus in surfacial sediments on better temporal and spatial resolution is also needed together with more geological information on the origin and sizes of particles.