Nitrate additions
Two cores from Station 1 were treated with 5 or 10 mg L-1 NO3-N (Figure 2). Five 5 mg L-1 NO3-N markedly reduced SRP concentration in the water column. The control core from this station had a relatively low oxygen demand, but SRP concentrations came to exceed 160 µg L-1. The reduction in phosphate concentration coincided with a small rise in DO, though not as marked as in station 28 cores. DO in the control core averaged 5.0 mg L-1, while in the core with 5 mg L-1 NO3-N addition DO averaged 5.4 mg L-1 (Figure 2).
Five Station 27 cores received 5 - 100 mg L-1 NO3-N (Figure 3). Marked decreases in SRP concentration occurred only at NO3-N additions of 75 mg L-1 and over. The reduction in SRP occurred at a higher DO concentration, similar to that which occurred in station 28 cores. At higher nitrate additions SRP concentration still reached more than 250 µg L-1, but this concentration was equivalent to that observed from these sediments under aerated incubation, on a similar time scale of incubation (McAuliffe 1991).
Station 28 cores received 5 - 50 mg L-1 NO3-N. Applications in the range 5 - 40 mg L-1 NO3-N had little effect on water column phosphate concentration. Application of 50 mg L-1 NO3-N, however, showed a marked effect on phosphate concentration, which became almost undetectable in the water column.
The addition of nitrate to the reconstituted cores affected the redox potential of surface sediment. In cores from Sediment 2, Station-27, from the experiment shown in Figure 3 addition of 75 or 100 mg L-1 NO3-N, which reduced SRP concentrations, coincided with positive Eh in the surface sediment (Figure 4). For the 5- and 1- mg L-1 NO3-N additions, Eh values were between those of control and 50 mg L-1 NO3-N additions.
Seasonal effects
Station 1 sediment #1, collected during February 1987 immediately after the collapse of a large bloom of Nodularia spumigena, received similar nitrate applications. The response of the control core to the incubation (Figure 5a) was different to that shown in sediment #2 (Figure 2) which was collected in May when the estuary had low nutrient concentrations in the water column and low oxygen demand in the sediment. DO fell quickly to near zero, and the concentration of SRP in the water column increased to a maximum of 622 µg L-1 in the control core after 17 days of incubation (Figure 5).
Applications of 5 and 10 mg L-1 NO3-N had no apparent effect on SRP concentration, but 50 and 100 mg L-1 NO3-N greatly reduced SRP concentrations in the water column (Figure 5). These additions also maintained DO at about 4 mg L-1 and limited SRP concentrations to that found under aerated conditions (McAuliffe 1991). In general, sediment #1 required about 10 times the dose of nitrate required by sediment #2 to reduce SRP in the water column.
Long-term effectiveness
The cores from station 1 sediment #1 receiving 50 and 100 mg L-1 NO3-N applications were incubated for over 155 days to examine the long-term effect of the treatments. The 50 mg L-1 NO3-N treatment was effective for only about 30 days (Figure 6b). After day 30, DO levels fell and SRP concentrations rose, eventually exceeding 2 mg L-1 (a value similar to that in the control core at the time). Depletion of DO occurred when nitrate concentration fell to below 20 mg N L-1.
In cores receiving 100 mg L-1 NO3-N, DO concentrations were similar to those receiving 50 mg L-1 NO3-N, initially maintained at about 4 mg L-1 for 30 days, and then fell. A resurgence in DO concentration occurred after day 90, with a relatively small increase of SRP in the water column.
Resuspension
Harvey estuary has a high rate of sediment resuspension which was considered relevant to the nutrient exchange between sediment and water (Gabrielson and Lukatelich 1985). The effects of resuspension on phosphate release during the nitrate application were examined, by using Station 1 sediment #2 cores, amended with 20 mg L-1 NO3-N and 0.3 g of sucrose (to stimulate the oxygen demand), and artificially suspended for more than 20 days (Figure 7). Addition of sucrose significantly increased oxygen demand of the sediment core, compared with its state without sucrose addition (Figure 2). The addition of 20 mg L-1 NO3-N to the non-resuspended core resulted in a more rapid recovery of DO concentration and reduced phosphate release. Frequent resuspension (at 3-day intervals) caused initially a more rapid fall in DO concentration; an earlier recovery of DO; and a stronger reduction in SRP concentration during incubation.
Method of nitrate application
A 10 mg L-1 NO3-N addition to either sediment or water column decreased phosphate release for about 7 days. There was little difference in SRP concentrations between the two methods after day 7, though DO appeared to recover more quickly after water application. Both cores eventually reached SRP concentrations similar to that of the control.
Application of 25 mg L-1 NO3-N to the water column suppressed phosphate release for the entire incubation period (21 days), but an equivalent application to the sediment suppressed phosphate release for only 12 to 14 days.
Application of 35 mg L-1 NO3-N reduced phosphate release regardless of the method of application (Figure 8). Dissolved oxygen rose earlier in the core which received application to the water column, which also appeared to have lower concentrations of SRP than the core receiving sediment application.