Sources of phosphorus in two subcatchments of the Manawatu River, and discussion of mitigation measures to reduce the phosphorus load

Figures & data

Figure 1  Manawatu Catchment above Teachers' College and the Upper Manawatu Catchment above Hopelands. Scale is 0–24 km. Major sampling sites are Palmerston North Teachers' College; Hopelands; Ta, Tamaki; T, Tiraumea; K, Mangatainoka; H, Mangahao; P, Pohangina; ○, waste water treatment plants.

Table 1  Land use (ha) from Horizons databases.

Land use	Manawatu catchment above Teachers' College (ha)	Upper Manawatu catchment above Hopelands (ha)
Sheep and/or beef	241,108	87,400
Dairy	51,891	20,300
Native forest	79,749	12,700
Exotic forest	13,044
Built up/parks	1909
Horticulture – vegetables	169
Horticulture – other	198
Cropping	727
Water body	761
Other	1841	6600
Total	391,397	127,000

Table 2  DRP and DOP concentrations of 21 samples collected at various flows at Teachers' College between 2004 and 2005.

	DRP	DOP
	(mg m^−3)	(mg m^−3)
Mean	17	10
Median	16	8
SEM	2	2
SEM, standard error of mean.

Table 3  Long-term average annual mass of sediment and PP load at Hopelands and Teachers' College modelled by NZeem®.

	Hopelands			Teachers' College
	Sediment (t)	PP (t)	P load (kg ha^−1)	Sediment (t)	PP (t)	P load (kg ha^−1)
Sheep/Beef	886,000	483	5.5	2,445,000	1332	5.5
Dairy	81,000	44	2.2	207,000	113	2.2
Forest	13,000	7	0.6	99,000	54	0.6
Total	981,000	535	4.4	2,750,000	1500	3.9
PP, phosphorus in suspended sediment particles.

Table 4  P contents of samples of sediment collected during six flood events at Teachers' College.

	Total P	Inorganic P	Organic P
	(mg kg^−1)	(mg kg^−1)	(mg kg^−1)
Mean	545	403	142
Median	535	395	143
SEM	21	18	7
SEM, standard error of mean.

Table 5  Length, width, area of wetted area of the river, and mean concentration of fine sediment on the river bed, and total amount of fine sediment on the river bed to a depth of 10 cm. Mean samples taken on 30 March and 22 April 2010.

Location		Length (km)	Width (m)	Area *10^3 (m^2)	Conc FS (kg m^−2)	Total FS (t)
Above Hopelands	Tamaki	26	9	235	0.27	67
	Above Tamaki Confluence	21	20	564	2.35	1336
	Tamaki Confluence—Hopelands	28	35	732	3.10	2268
	Total			1531		3662
Below Hopelands	Hopelands—Mangahao	18	45	806	3.10	2498
	Mangahao—Teachers' Coll	35	68	2350	1.73	4065
	Pohangina	46	24	1100	1.72	1897
	Mangahao	52	28	1470	0.50	739
	Mangatainoka	67	25	1645	0.84	1379
	Tiraumea	69	10	693	0.46	320
	Makuri	21	13	283	0.76	214
	Total			8351		1112
Total above Teachers' College		381		9882		14,774
FS, fine sediment.

Table 6  Mean total P concentration, kg P,% inorganic P and organic P in fine sediment in the river bed to a depth of 10 cm. Samples taken on 30 March and 22 April 2010.

		Total P		Inorganic P	Organic P
Location		(mg kg^−1)	(kg)	(%)	(%)
Above Hopelands	Tamaki	527	35	86	14
	Above Tamaki Confluence	470	623	77	23
	Tamaki Confluence—Hopelands	525	1191	70	30
	Total		1850
Below Hopelands	Hopelands—Mangahao	525	1311	70	30
	Mangahao—Teachers' College	543	2207	71	29
	Pohangina	400	759	84	16
	Mangahao	474	350	75	25
	Mangatainoka	675	930	53	47
	Tiraumea	660	211	76	24
	Makuri	389	83	48	52
	Total		5852
Total above Teachers' College			7702

Table 7  Bicarbonate-P and P extracted by 0.1M NaOH from the fine sediments in the river bed.

		BiCarbonate P		0.1 M NaOH P
Location		(mg kg^−1)	(kg)	(mg kg^−1)	( kg)
Above Hopelands	Tamaki	15	1	37	3
	Above Tamaki Confluence	47	62	94	125
	Tamaki Confluence—Hopelands	42	96	80	181
	Total		159		309
Below Hopelands	Hopelands—Mangahao	42	106	80	199
	Mangahao—Teachers' College	23	91	82	333
	Pohangina	22	42	59	111
	Mangahao	18	13	53	39
	Mangatainoka	41	56	171	236
	Tiraumea	42	14	107	34
	Makuri	18	4	35	7
	Total		326		960
Total above Teachers' College			485		1269

Table 8  Mean dissolved P, measured and estimated, from township waste water treatment plants (WWTP) and industries measured in 2010–12.

Location	DRP (t y^−1)	DOP^1 (t y^−1)	Total P (t y^−1)
Dannevirke	5.8	2.9	8.7
Eketahuna	0.2	0.1	0.3
Pahiatua	1.0	0.5	1.5
Woodville	0.2	0.1	0.3
Ashhurst^2	2.6	1.3	3.9
Industries	0.2	0.1	0.3
Total	10.0	5.0	15.0
DOP, dissolved organic P; DRP, dissolved reactive P.
^1 Estimated.
^2 Estimated from the census population.

Figure 2  Sources of dissolved P in Manawatu River at Hopelands (black) and Teachers' College (grey).

Table 9  Estimate sources of various forms of P at Hopelands per year from 1989–2009.

	P fraction	Sheep/Beef	Dairy	Forest	Point sources	From SS in bed of river	Total	Reference
Erosion storms (t) (long-term average)	SSP	483	44	7			535	NZeem®
Annual average (t)	DRP	10.4	6.2	0.4	5.8	0.3	23.0	Horizons data
Annual average (t)	DOP	5.2	3.1	0.4	2.9		11.6	50% of DRP
Dissolved P (t)	DRP + DOP	15.6	9.3	0.8	8.7	0.3	34.6	Our estimate
Dissolved P river (kg ha^−1)	DRP + DOP	0.18	0.46	0.06^1				Our estimate
Hectares		87,400	20,300	12,700
DOP, dissolved organic P; DRP, dissolved reactive P; SSP, suspended sediment P.
^1 McGroddy et al. (2008).

Table 10  Sources of various forms of P at Teachers' College per year from 1989 to 2009.

	P fraction	Sheep/Beef	Dairy	Forest	Point sources	From SS in bed of river	Total	Reference
Erosion storms (t)
(long-term average)	SSP	1332	113	54			1500	NZeem®
Annual average (t)	DRP	22.8	12.6	2.8	10	1.3	49.5	NIWA & Horizons data
Annual average (t)	DOP	11.4	6.3	2.8	5		25.5	50% of DRP
Dissolved P (t)	DRP + DOP	34.2	18.9	5.6	15	1.3	75.0	Our estimate
Dissolved P in river (kg ha^−1)	DRP + DOP	0.14	0.36	0.06^1				Our estimate
Hectares		241,000	51,900	92,800
DOP, dissolved organic P; DRP, dissolved reactive P; SSP, suspended sediment P.
^1 McGroddy et al. (2008)

Figure 3  Modelled estimates of sources of dissolved P (DRP + DOP) in the Manawatu River at Hopelands in 2010 (black), and loads achievable by 2020 (grey) if recommendations were implemented (t P yr⁻¹). Note: Some of the 511 t of PP remains on the bed of the river and generates a maximum of 0.3 t of dissolved P yr⁻¹.

Figure 4  Modelled estimates of sources of particulate P in Manawatu River at Hopelands in 2010 (black), and loads achievable by 2020 (grey) if recommendations were implemented on all affected land on day 1 (t P yr⁻¹).