Investigation into the dynamics of dissolved organic phosphorus concentrations over an annual period in a river and estuary system

Billinge, Jack (2018) Investigation into the dynamics of dissolved organic phosphorus concentrations over an annual period in a river and estuary system. University of Southampton, Doctoral Thesis, 217pp.

Record type: Thesis (Doctoral)

Abstract

Dissolved organic phosphorus (DOP) may play an important role in the control of primary production in aquatic and coastal systems. However, whereas dissolved inorganic phosphorus is routinely measured in most UK rivers and estuaries by the Environment Agency, DOP is not determined and there are few high frequency temporal records of how DOP concentrations vary seasonally and in relation to changes in river flow. The aim of research presented in this thesis was to investigate the DOP dynamics in the lower reaches of two major UK south coast rivers (the Hampshire Avon and Dorset Stour) and the receiving waters of the Christchurch Harbour estuary over an annual period at a high (i.e. weekly) sampling frequency. Water samples were collected from 1 site on the Hampshire Avon river (Knapp Mill), two sites on Dorset Stour river (Throop and Iford Bridge), and one site at Christchurch Harbour estuary (Mudeford Quay) on a weekly basis from April 2013 to April 2014.

In order to investigate the concentrations and fluxes of DOP in the two rivers and the estuary, a new method of analysis of total dissolved phosphorus (TDP) using inductively coupled mass spectrometry (ICP-MS) was implemented, in addition to a well described spectrophotometric technique to measure soluble reactive phosphorus (SRP). The DOP concentration was then assumed to be the difference between the TDP concentration and the SRP concentration. ICP-MS analysis of certified reference materials containing known concentrations of TDP indicated that the technique was detecting TDP to within 5% of the expected concentration.

DOP concentrations were highest in the River Stour, with the highest concentrations detected at Throop (up to 12.1μM) and Iford Bridge (up to 21.0μM) on the River Stour. The River Avon had the lowest DOP concentration of the four sites (up to 1.45μM). The estuarine site at Mudeford Quay was high in DOP concentration in the summer (ranging between 5.7μM and 18.8μM), with lower concentrations in the winter (ranging from undetectable concentrations to 2.8μM). The DOP concentration at all of the sites decreased during periods of high river flow, most likely due to dilution of DOP by rainfall.

During periods of high river flow increased fluxes of TDP, SRP and DOP occurred in both rivers flowing into the Christchurch Harbour estuary, with 45.2% of the annual SRP flux at Knapp Mill and 49.5% at Throop occurring during an extended period of high river flow from 16/12/13 to 27/3/14. Riverine fluxes of DOP at Knapp Mill and Throop were highest over periods of high river flow during January and February 2014 when river flows were sustained at peak values over 69m3/s and 50m3/s respectively for several weeks. At Knapp Mill, 50% of the annual flux of DOP occurred over a 9-week period of high river flow in early 2014.

During summer 2014 several transect surveys of Christchurch Harbour were conducted to investigate the relationship between TDP, SRP and DOP with salinity. On most dates TDP and SRP showed non-conservative removal of phosphate between the riverine and seawater end member concentrations. DOP concentrations on some dates showed a non-conservative removal of phosphate at low salinities whereas on other dates DOP showed a small increasing linear relationship with salinity.

In addition to TDP, SRP and derived DOP concentrations, weekly measurements of nitrate and silicate concentrations were analysed on samples collected from each site as well as measurements of water quality parameters including temperature, conductivity/salinity, chlorophyll a concentration and turbidity. Mean dissolved inorganic nitrate concentrations over the sampling period were 391.6μM at Knapp Mill, 508.9μM at Throop, 545.6μM at Iford Bridge and 319.3μM at Mudeford Quay. Mean silicate concentrations over the sampling period were 135.4μM at Knapp Mill, 104.3μM at Throop, 112.8μM at Iford Bridge and 84.1μM at Mudeford Quay. Highest concentrations of inorganic nutrients were seen over summer, with the lowest seen in winter.

The results from this study have provided new data on the seasonal changes in DOP in comparison to SRP measured at weekly intervals in two contrasting south coast UK rivers that feed into the small shallow eutrophic estuary of Christchurch Harbour. These high frequency measurements have provided new insight into the processes influencing the flux of dissolved phosphate from temperate rivers into estuaries highlighting the importance of including analysis of organic forms of nutrients when assessing the nutrient status of natural waters.