Following the addition of Spirulina, there was a rapid increase in the flux rates of most analytes, which agrees with previous findings by Enoksson (1993) and Garber (1984). The fluxes for CO2 and alkalinity increased linearly with carbon loading, as
did the CRQ, however fluxes of ammonium and oxygen appeared to show a step change in the flux rate at additions of carbon >20.2 g C m-2. At this quantity of carbon loading, the flux rate whilst still increasing, slowed significantly. The fluxes of nitrite and phosphate showed little correlation with increasing carbon loading.
It is proposed from these findings that sediments became saturated with carbon at the surface and the ability of the sediments to decompose more carbon aerobically was limited due to the rapid consumption of oxygen. Oxygen penetration into the sediments at this point was less than 2 mm. This may suggest that at the point at which the step change in oxygen flux rates occurred, sediment metabolism was switching from predominantly aerobic respiration to predominantly anaerobic respiration.
This may explain why CO2 fluxes did not display a step change in flux rate, whilst
oxygen fluxes did. Both aerobic and anaerobic respiration produce CO2 as end
products and therefore CO2 should continue to increase at a linear rate as carbon
loading increases regardless of the type of respiration occurring. The CRQ ratios indicate that as carbon loading increased so did the proportion of anaerobic respiration and it therefore follows that CO2 fluxes should continue to increase as
well. Alkalinity fluxes also increased at a linear rate, with sulphate reduction (a form of anaerobic respiration) the most likely source of the alkalinity (Berelson et al., 1998), which also provides extra support to the proposition that anaerobic respiration became the dominant form of metabolism as carbon loadings increased.
Unexpectedly however, ammonium fluxes did not increase linearly with CO2 fluxes.
If the production of CO2 increases linearly with carbon loading, than it could be
expected that ammonium fluxes would increase linearly as well, because ammonium is produced through both aerobic and anaerobic forms of respiration. However, the results from this experiment showed ammonium fluxes initially increase linearly with
carbon loading, but that the flux decreases at the point that metabolism switches to anaerobic respiration.
One possible reason why ammonium flux rates slow down at this point would be anaerobic ammonium oxidation (ANAMMOX), whereby ammonium is oxidised under anaerobic conditions. According to ANAMMOX stoichiometry, ammonium is oxidised using nitrite as the electron acceptor and inorganic carbon as the carbon source.
NH4+ + 1.32NO2- + 0.066HCO3- + 0.13H+ → 1.02N2 + 0.26NO3- + 0.066CH2O0.5
N0.15 + 2.03H2O (Reginatto et al, 2005)
The stoichiometry for ANAMMOX suggests that only a small amount of inorganic carbon is used for every ammonium ion oxidised (0.066:1). This could explain why CO2 continued to increase linearly while ammonium did not. At the point where the
step change in the ammonium flux rate occurs, the sediments have become almost anaerobic providing an environment potentially conducive to ANAMMOX. Therefore it is possible that at this point, ANAMMOX is responsible for consuming some of the ammonium produced, and because the ANAMMOX reaction uses only a small amount of inorganic carbon, ANAMMOX should not greatly affect the CO2 flux rate,
but should cause a decline in the ammonium flux rate. It is therefore proposed that ANAMMOX is responsible for the step change in ammonium flux rate at the point where the sediment is switching from aerobic to anaerobic respiration.
In table 5-2 below, all results have been aggregated into four discrete categories: zero, low (3.4 – 6.8), medium (10.1 – 27) and high (33.8 – 67.6) rates of carbon loading to summarise the changes in biogeochemical processes under different carbon loading scenarios. With zero carbon loading, the sediment behaved in much the same way as was observed in intact sediment cores from the field study. The sediments were characterised by a relatively deep oxygen penetration depth of 5.2mm and low CRQ ratio of 0.6 indicating the dominance of aerobic respiration and the likely
consumer of oxygen in marine sediments and could also be responsible for the low CRQ ratio in this case. Modelled oxygen consumption profiles in chapter 3 often showed a peak of oxygen consumption occurring at the oxic/anoxic interface and is possibly due to the oxidation of sulphides. The consumption of alkalinity by the sediments in this scenario further supports nitrification and possible sulphide oxidation, as both these processes consume alkalinity.
The addition of carbon causes a rapid response from the sediments in which bacteria begin to remineralise the added carbon. Because bacteria consume oxygen during the decomposition of organic matter, the depth of oxygen penetration was reduced. Under low carbon loading (< 6.8g m-2) the oxic zone was reduced from 5.2 mm to 3.7mm. This reduction in oxic zone has an impact on both carbon and nitrogen cycling pathways. The CRQ ratio has now risen to 0.8, and while this indicates that aerobic respiration is still the dominant carbon remineralisation pathway, the influence of anaerobic respiration is increasing or it could be that nitrification is decreasing. It is also possible that there is a lag time between the addition of carbon and the
stimulation of anaerobic respiration. The smaller oxic zone also begins to reduce nitrification, with the result that nitrification/denitrification becomes uncoupled and direct denitrification becomes more important. That is, denitrification supported by the uptake of nitrate by the sediments. Nitrite has also switched from being taken up by the sediments to being released.
This trend increases going into the medium-loading scenario. Under this scenario, aerobic conditions have further degraded as observed through the increased depletion of the oxic zone to 1.9mm and the CRQ ratio continuing to rise to 1.1. Nitrate fluxes into the sediment increased and ammonium fluxes out of the sediments increased rapidly. This suggests that the demand for nitrate is now considerably exceeding supply. The demand could either becoming from direct denitrification or dissimilatory nitrate reduction to ammonium (DNRA). Studies by Macleod et al. (2004) and
Christensen et al. (2000) have shown that both direct denitrification and DNRA become more important as sediments become enriched with organic carbon. However the efficiency of denitrification decreases as organic carbon increases (Macleod et al., 2004). Alkalinity fluxes also rapidly increase and were directed out of the sediment into the water column, compared to the low scenario where alkalinity was consumed
by the sediment. The increase in alkalinity is most likely due to sulphate reduction, which is an anaerobic process, and therefore supports the suggestion that anaerobic respiration becomes the dominant form of benthic metabolism as carbon loading increases.
Under the high loading scenario in Table 5-2, anaerobic respiration is now the dominant metabolic process (CRQ = 1.6), with the oxic zone almost depleted and ammonium and phosphate fluxes at their highest levels. Alkalinity fluxes out of the sediment have also continued to increase, indicating increased activity by sulphate reducers.
Table 5-2. The effect of increasing carbon loading on sediment biogeochemical processes. The flux rates are in µmols m-2 h-1.
g C M-2 O2 Pen O2 CO2 Alk CRQ NH4+ NO2- NO3- DIN PO43 - CO2:DIN Zero 5.2 -483 259 -319 0.6 24 -0.2 0.8 24 0.1 10.6 Low 3.7 -807 650 -172 0.8 183 0.5 -9.5 174 4.7 3.7 Medium 1.9 -1640 1719 514 1.1 425 0.2 -15.6 410 2.7 4.2 High 0.3 -2177 3404 1049 1.6 602 1.6 -33.6 570 10.7 6.0
Low = 3.4 - 6.8; medium 10.1 – 27.0; High 33.8 – 67.6 g C m-2.