Discussion

The classical theories of Mortimer (1941) proposed that sediment phosphorus release under reduced redox potential is related to iron (III) reduction at the sediment-water interface. More recent understanding recognised the ligand exchange of hydroxyl iron compounds (Schindler and Stumm 1987) and microbial cell contributions to phosphorus release from aquatic sediments (Gächter et al. 1988). The phosphorus release, measured in this paper as the increase in SRP concentration in the water column, may be considered to include both biotic and abiotic processes that all rely heavily on redox potential and oxygen concentrations in the system (Boström et al. 1988). The SRP concentrations in the water column were found in most cases, to increase with the decrease in DO concentrations. This relation has been reported in numerous sediment studies, and is generally true for the Harvey estuarine sediments studied here.

Nitrate additions

In presence of high nitrate concentration, the ease of oxygen depletion was related to sediment properties, in particular its oxygen demand, so a high level of variation may be expected for sediments which differ in properties affecting oxygen demand. It has been reported in field studies that phosphorus release to hypolimnetic water was inhibited when the nitrate concentration exceeded 1 mg L-1, an effect attributed to stabilising redox potential to reduce iron reduction (Andersen 1982). In this paper, adding 5-40 mg L-1 of NO3-N to Station 28 Sediment 2 had little effect in reducing phosphate release, but 50 mg L-1 of NO3-N was effective. For Sediment 2 from in Station 1, however, 5 mg L-1 of NO3-N was sufficient to reduce phosphate release (Figure 2). In Station 27 sediment 2, nitrate demand was even higher and only a dose of about 75 mg L-1 of NO3-N changed the SRP concentration in the water column.

Both oxygen demand and nitrate demand are related to microbial respiration, and so is the amount of bioavailable carbon in the cores. There were marked differences in organic content (as LOI%) between the sediments of the three sites, with the highest content (17.5%) in station 1 and lowest (2.7%) in station 27 (Table 1). In the case of Station 1 sediment 1 collected after the collapse of a Nodularia bloom, when the sediment demonstrated high oxygen demand due to increased detrital organic matter, the amount of nitrate needed to reduce phosphate release exceeded 50 mg N L-1 (Figure 5). This effect was demonstrated by adding sucrose to sediment cores during a resuspension experiment (Figure 7). Previous work (Pettersson and Boström 1980; Tiren 1985) also showed that increased carbon supply reduced the effectiveness of nitrate additions to control phosphate release. As they suggested, the balance between the redox stabilisation of nitrate and the increased oxygen consumption due to added glucose was delicate and the system turned to and fro from anaerobic phosphorus release to nitrate induced phosphorus binding.

In fact, the application of 5 mg L-1 to 100 mg L-1 NO3-N increased redox potential at the sediment-water interface from below -200 mV to over 200 mV. This would theoretically lower the iron (III) reduction. The existence of iron (III) oxides favours maintenance of an oxidised layer at the sediment-water interface, which is critical in reducing oxygen consumption of sediment and minimising phosphate flux across the sediment-water interface to the overlying water (Wunchen 1981).

Coinciding with the net decrease in SRP concentration in the water column, there was always a reduction in oxygen demand and stabilised Eh. As the tested cores were open to air, the DO readings should be considered as a dynamic balance between oxygen consumption and supply. The decreased DO is therefore an indication that the rate of consumption is more rapid than the rate of supply from the air. From the perspective of Mortimer's theory, the decrease in SRP can be interpreted as resulting from increased phosphate binding to iron oxides. However, the processes involved would be more complicated. Behind the net effects of decreased SRP after nitrate application there would also be stimulation of denitrification near the sediment-water interface, in response to the high nitrate concentrations (Boström et al. 1988). A large population of denitrifying bacteria may therefore be expected to contribute, directly or indirectly, to the changes in phosphate concentrations through microbial uptake and release.

Nitrate-reducing bacteria may also use iron (III) as alternative electron acceptor when nitrate concentrations fall to a critical level (Söresen 1982). The consequence is that, as the enzymatic reduction of iron (III) continues, phosphate ions previously bound (adsorbed) to them are released. This release of phosphate may contribute significantly to an increase in SRP concentration when the initial population of nitrate reducing bacteria was high (Jansson 1986). However, there was little evidence in this study that addition of nitrate may eventually increase SRP concentrations in the water column. Among the 26 cores which received nitrate additions, most showed a considerable reduction in nitrate over the incubation period. In only two cores did water column SRP concentrations eventually exceed those of the controls (by 10-20%). In these cores the initial nitrate addition was relatively small and did not cause any early reduction in water column SRP.

Most cores were incubated over 20-30 days, a time generally appropriate for the effects of nitrate on DO, Eh and SRP to be expressed. The long-term (155 day) incubation with a 50 mg L-1 NO3-N addition showed that the major drop in DO and nitrate occurred in 20-30 days, followed by a rapid increase in phosphate release (Figure 6). There was a stage between 20-30 days of incubation in which both nitrate and DO concentrations decreased, suggesting a coexistence of two respiration mechanisms in the system, using either oxygen or nitrate as electron acceptors (Kuenen and Robertson 1988; Kerner 1993).

The 100 mg L-1 core received more nitrate than was initially required to stabilise oxygen demand, but this "overdose" appears to have only caused more bioavailable carbon in the sediment to be consumed through microbial oxidation. That is, if there is no carbon limitation in the sediment, the high concentration of added nitrate will eventually be consumed by microbial activity in the sediment, after which a stimulation of phosphate release is still possible.

Resuspension

The reduction of SRP by high concentrations of nitrate appears effective under sediment resuspension, a process which often occurs in shallow estuaries. Nitrate application under frequent resuspension (3 days) appeared to be most effective in SRP reduction, coinciding with a lower rate of nitrate consumption and more rapid DO recovery (Figure 7).

Kelderman (1984) also reported that resuspension reduced phosphorus concentration in the water column and suggested two reasons for this: (1) resuspension of sediments "washed out" inhibitory substances on the surface of sediment particles, resulting in an increase in adsorption sites; (2) anaerobic sediment may contact aerobic overlying water, resulting in iron (II) oxidation and formation of insoluble iron-phosphate complexes.

A further consideration is the rate of surface contact between adsorbent particles and phosphate ions. This rate is apparently higher under suspension than under static conditions, when phosphate movement is largely limited by rate of diffusion. The reduction in SRP concentration in resuspended systems may therefore be considered as a combined effect of the processes involved.

In this study, however, an increase in DO concentration during resuspension may have contributed to SRP reduction. The reduction in water column SRP coincided with increased water column DO, which may partly result from increased mixing at the air-water interface. Thus the resuspension may also increase chemical oxidation of ferrous ions and allow readsorption of phosphate, for example through the formation of insoluble iron (III)-phosphate complexes.

Method of nitrate application

The application of nitrate through either water column or sediment reduced SRP concentration in the water column when nitrate concentration was sufficiently high. However, nitrate concentrations decreased more rapidly after sediment application than in the water application. Thus nitrate application through the water column appeared to have a better potential to control water column SRP at lower nitrate dose rates.

In flooded soils and sediments, nitrate in the overlying water and aerobic surface sediment diffuses into anaerobic layers as a result of a downward concentration gradient created by denitrification in the deeper, anaerobic sediment (Reddy and Patrick 1984). Application of nitrate directly into sediment may increase the rate of downward nitrate penetration, and thereby reduce the effectiveness of nitrate in buffering oxygen demand and controlling redox in the overlying water. In contrast, application of the same amount of nitrate into the water column would reduce the rate of downward diffusion due to the dilution effect, thereby more effectively maintaining an oxidised zone near the sediment-water interface.

The effective nitrate dosages used in this paper were 50-100 mg NO3-N L-1 (equivalent to 25-50 g m-2), which are significantly lower than those used by Ripl (141g NO3-N m-2). Beside the difference in sediment composition, the method of treatment, ie. adding nitrate to sediment or water column appears to be an important factor affecting the results.

Riple (1985) added nitrate to the sediment, and the nitrate concentration in the epilimnion never exceeded 5 mg L-1; but there was a high concentration of nitrate (460 mg NO3-N L-1) in the interstitial water. This caused marked denitrification (3.2g N m-2 day-1). In this paper, in which application was to the water, nitrate concentrations in the water column were much higher than in Ripl’s method, even at lower dosages. The higher concentration of nitrate in the water column rather than in sediment is considered beneficial in reducing nitrate loss through sediment denitrification, so maintaining a higher redox status at the sediment water interface, and effecting better control of sediment phosphorus release.