Nutrient release and nitrogen transformations resulting from resuspension of Great Barrier Reef shelf sediments

Chongprasith, Pornsook (1992) Nutrient release and nitrogen transformations resulting from resuspension of Great Barrier Reef shelf sediments. PhD thesis, James Cook University of North Queensland.

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Abstract

The Great Barrier Reef (GBR) province is subjected to a range of episodic physical disturbances, the most dramatic of which are tropical cyclones. On average, two tropical cyclones cross the coast between 15° S and 21° S per year. These disturbances of the shelf system result in the elevation of the concentrations of suspended solids, particulate and dissolved nutrients in the water column. This study attempts to quantify the potential input of nutrients into GBR waters as a result of resuspension of shelf sediments by cyclones, to describe shelf area differences in the magnitude of this process, to assess the changes in nutrient concentrations within the water column following a sediment resuspension event and in particular the processes contributing to the elevation of dissolved inorganic nitrogen concentrations following a cyclone.

Experiments were carried out in the laboratory to investigate temporal trends in nutrient concentration and speciation after the initial prompt release of nutrients from resuspended sediment. Subsequent daily changes in the initially enhanced concentration were not large (< 2-fold range). The pattern of temporal changes in nutrient and chlorophyll concentrations differed between individual experiments. Concentrations of particulate nitrogen (PN) and phosphorus (PP) were largely constant for a few days during the experimental periods after being released from sediments. For the most part, dissolved organic nitrogen (DON) and phosphorus (DOP) concentrations in seawater with suspended sediment and controls were similar and stable through time. Ammonium (NH₄⁺), nitrate (NO₃⁻) and nitrite (NO₂⁻) concentrations fluctuated over time. Inorganic phosphate (P0₄³⁻) and silicate (Si(OH)₄) concentrations decreased after the initial release from the sediment. Chlorophyll a (chl a) released from sediment increased after about a week of the experimental period while phaeophytin concentrations were stable throughout the experimental period. Thus, the immediate effect of cyclonic sediment resuspension on nutrient concentrations and speciation can be assessed by the measurement of water samples taken up to several days after the passage of a cyclone.

Comparison of amounts of nutrient release from both the inner and outer shelf sediments was carried out. The amounts of suspended solids (r² = 0.8), PN (r² = 0.7), total dissolved nitrogen (TDN, r² = 0.6), DON (r² =0.4), PP (r² = 0.4), P0₄³⁻ (r² = Si(OH)₄ (r² = 0.4), phaeophytin (r² = 0.4,0.6) released were directly related to the mass of sediment resuspended into the water column. However, not all nutrient species followed this pattern, most likely due to vertical patchiness of NH₄⁺, NO₂⁻ and DOP in the sediment column and the high variability in these nutrient stocks between sediment sites.

Laboratory resuspension experiments showed that shelf sediments contributed significant amounts of total N, total P, PN, PP, NH₄⁺, total oxidized nitrogen (NO₃⁻ plus NO₂⁻), P0₄³⁻, Si(OH)₄, chl a and phaeophytin to the water column as a result of sediment resuspension. The amounts of total phosphorus, chl a and total pigments promptly released differed between sediment types (P < 0.05). Amounts of chl a and total pigments derived from resuspended inner shelf sediments were greater than from outer shelf sediments. More phosphorus was promptly released from outer shelf sediments. The prompt releases of the remaining nutrient species (total N, PN, DON, NH₄⁺, NO₃⁻ + NO₂⁻, PP, DOP, P0₄³⁻, Si(OH)₄, phaeophytin) were unrelated to sediment type.

DON, DOP, individual NO₃⁻ and NO₂⁻ concentrations in the water column, were not significantly increased as a result of simulated shelf sediment resuspension events. This was due to the difficulty of detecting small changes in concentrations of NO₃⁻ and NO₂⁻ which were low in sediment stocks and remained close to analytical detection limits and high existing DON and DOP concentrations in the water column.

A comparison was made of the amount of nutrients released promptly by resuspension of sediment from sites within inner, mid- and outer shelf areas. The estimates of nutrient release are presented in three groups, normalized to volume of wet weight of sediment, volume of water-free sediment and estimated porosity of the sediment. The calculated amounts of 10.5 - 28.0 μmol of total N, 0.1 - 0.6 μmol of total P and 0.2 - 2.6 μmol of Si(OH)₄ promptly released from cubic centimetre of wet sediment. The calculated amounts of 0.1 - 3.8 μg of chl a and 0.7 - 5.7 μg of phaeophytin were accumulated by stimulation of sediment resuspension.

Ammonium and nitrite oxidation rates in sediment-amended seawater were compared using two techniques: dark ¹⁴C-bicarbonate uptake with and without added nitrification inhibitors (Nitrapyrin, Allylthiourea, Hach2533) and the oxidation of ¹⁵N labelled tracers. Nitrapyrin (N-serve) was found to be the most efficient inhibitor of ammonium oxidation. Higher concentrations of nitrapyrin (50 mg 1⁻¹) were required as compared to previous studies, because sediment particles adsorbed nitrapyrin and also hindered the isotope measurements. The high concentration of nitrapyrin required may have affected nitrification rate, resulting in an apparent high rate of N-serve sensitive dark carbon bicarbonate uptake.

Ammonium oxidation rates measured by ¹⁵N techniques ranged from 0.4 to 6.0 nmol N 1⁻¹ h⁻¹. Ammonium oxidation rates in seawater with either freshly collected sediment (range 1.2-6.0 nmol N 1⁻¹ h⁻¹) or frozen-thawed sediment (0.9-4.7 nmol N 1⁻¹ h⁻¹) were significantly higher than rates in the control seawater (range 0.4-3.9 nmol N 1⁻¹ h⁻¹) ( P < 0.05). Ammonium oxidation rates in the seawater with frozen-thawed sediment did not differ from rates measured in seawater mixed with freshly collected sediment.

Nitrite oxidation rates were measured in experiments involving both ¹⁵NH₄⁺ and ¹⁵NO₂⁻ additions. The nitrite oxidation rates obtained from experiments with ¹⁵NO2 additions were always higher than the rates obtained with ¹⁵NH₄⁺ addition. The differences resulting from ¹⁵NO₂⁻ additions compared to ¹⁵NH₄⁺ additions were likely due to contamination of the nitrate samples by pre-existing nitrite. Nitrite oxidation rates measured in samples with freshly collected sediment added were 2- to 3-fold higher than rates in seawater with frozen-thawed sediment added where ¹⁵NO₂⁻ was used as the ¹⁵N source.

Nitrite oxidation rates measured in experiments with ¹⁵NH₄⁺ additions (range = 0.2 to 1.5 nmol N 1⁻¹ h⁻¹) in seawater with sediment added were significantly higher than rates in control seawater (0.2 to 1.2 nmol N 1⁻¹ h⁻¹). Nitrite oxidation rates (with ¹⁵NH₄⁺ addition) measured in seawater with freshly collected sediment differed from rates measured in seawater with frozen-thawed sediment (by a factor of 0.1-1.2).

Uptake rates of ammonium, nitrite and nitrate were measured by ¹⁵N techniques. Overall, ammonium uptake rates were higher (2.6-82.2 nmol N 1⁻¹ h⁻¹) than uptake of either nitrate (0.3-16.8 nmol N 1⁻¹ h⁻¹ ) or nitrite (0.1-9.8 nmol N 1⁻¹ h⁻¹). Ammonium uptake rates in seawater with freshly collected sediment added (37.7-82.2 nmol N 1⁻¹ h⁻¹ ) were significantly higher than rates measured in seawater with frozen-thawed sediment (4.3-57.0 nmol N 1⁻¹ h⁻¹) and in the control (2.6-39.1 nmol N 1⁻¹ h⁻¹) (P <0.05). Likewise, nitrate uptake rates in seawater with the freshly collected sediment added were significantly higher than rates measured in the control (0.3-1.0 nmol N 1⁻¹ h⁻¹). The nitrate uptake rates measured in seawater with freshly collected sediment (9.7-16.8 nmol N 1⁻¹ h⁻¹) did not differ from rates measured in seawater mixed with frozen-thawed sediment (8.7-16.5 nmol N 1⁻¹ h⁻¹). In contrast, nitrite uptake rates in seawater with frozen-thawed sediment added (0.3-9.8 nmol N 1⁻¹ h⁻¹ ) were significantly higher than control seawater (0.1-1.33 nmol N 1⁻¹ h⁻¹), but did not differ from rates measured in seawater with freshly collected sediment added (2.8-3.0 nmol N 1⁻¹ h⁻¹).

Measured anaplerotic ¹⁴C uptake/¹⁵N oxidation ratios (0.5-0.7) were high compared to ratios reported from previous studies (0.06-0.2). This was due to potential differences between tropical and temperate systems, the differences between behaviour of bacteria populations in culture media and natural seawater, or side effects of the nitrification inhibitor. Therefore, to estimate nitrification rates in any systems by ¹⁴C method, calibration of the C uptake/N oxidation ratio for that system is required.

Item ID: 33763
Item Type: Thesis (PhD)
Keywords: cyclones; GBR; Great Barrier Reef; nitrates; nitrogen; nutrients; phosphates; phosphorus; sediment resuspension; sediments
Date Deposited: 29 May 2015 04:06
FoR Codes: 04 EARTH SCIENCES > 0405 Oceanography > 040502 Chemical Oceanography @ 50%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 50%
SEO Codes: 96 ENVIRONMENT > 9611 Physical and Chemical Conditions of Water > 961104 Physical and Chemical Conditions of Water in Marine Environments @ 100%
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