Journal of the Royal Society of Western Australia, 83: 275-284, 2001 Sediments of Leschenault Inlet: a comparison with other estuaries in south-western Australia A J McComb, S Qiu, E I Paling & N A Hill School of Environmental Science, Murdoch University, Perth WA 6150 email: [email protected] Abstract This paper describes the properties of sediments from Leschenault Inlet, south-western Australia, and compares them with two other estuarine systems in the South West, the Swan-Canning and the Peel-Harvey. Surface sediments of Leschenault Inlet contained a large proportion of fine material, with a particle size finer than in Peel-Harvey sediments. Organic enrichment in surface sediments was higher than that in Peel Inlet, and comparable with part of the eutrophic Harvey Estuary. It was lower than central Harvey Estuary and the Swan-Canning Estuary. Sediments contained a substantial proportion of apatite P, distinctly higher than in Peel-Harvey and Swan-Canning. Concentrations of sediment total phosphorus were relatively low, but higher than in Peel Inlet. The rate of phosphorus release was low compared with the Peel-Harvey and the Swan-Canning. Keywords: Leschenault Inlet, south-western Australia, estuary, sediments, nutrients. Introduction consists largely of sandy soils with low nutrient retention capacities (Anon 1983). Nevertheless the Inlet has been seen Leschenault Inlet (Fig 1) is located near Bunbury, 180 as a healthy, biologically-productive waterway with km south of Perth, Western Australia. It is a long, narrow, relatively little algal growth. There were no significant shallow (< 2 m) interdunal lagoon, approximately 11 km water quality problems revealed in the Collie River during long and 2 km wide. The estuary is open to the Indian monitoring in 1993-95. It has been considered one of few Ocean through a channel dredged in 1951. estuaries with a low risk of nutrient enrichment, according to the US EPA risk assessment model for estuarine The system is fed by two main rivers, the Collie and eutrophication. However, the estuarine reaches of the the Preston. The Collie, with a catchment of 3 500 km2, is Collie River have shown physio-chemical and biological fed by the Brunswick, Wellesley and Lunenburgh Rivers signs of nutrient enrichment in recent years. Phytoplankton from the east of the catchment. The Preston, with a catchment of 1 400 km2, is fed by several tributaries; blooms of Cryptomonas sp and Heterosigma akashiwo occurred in the Collie River near its confluence with the Ferguson River, Joshua Brook and Crooked Brook. Several Brunswick River in April and May 1994 (Anon 1995). There dams on the two main rivers provide water for irrigation were high levels of plant biomass, comparable (on a unit and domestic supply. Wellington Dam is on the Collie, area basis) with those of Peel-Harvey. Seagrass and brown south of Collie township, and the Glen Mervyn dam is on algae are dominant, and appear not to be limited by nutrient the upper reaches of the Preston. availability (Lukatelich 1989; Hillman et al. 2000). The climate is Mediterranean with an annual rainfall There are limited data on nutrient enrichment and of about 1000 mm, mainly concentrated (87%) between May water quality in the Inlet. Data from well-documented and August. The average maximum temperature in winter estuaries in south-western Australia suggest that excessive in Bunbury is 16.8 °C (July), while in summer the average nutrient loads, and especially phosphorus and nitrogen daily maximum is 27.6 °C and the average minimum is 15.1 from agricultural and urban catchments, can be a major °C. Easterly winds prevail in summer, but a strong south¬ threat to estuarine water quality and ecological health. west breeze occurs on most afternoons. Compared with the rural catchment, nutrient loads from The Inlet has significant environmental and economic the Bunbury urban area and from groundwater are value to the region, and the waterway and its surrounds relatively small. Nutrient loads entering the estuary vary are a hub of recreational activity. It has safe, protected from year to year corresponding to rainfall and stream flow, waters, most of which are freely navigable by small pleasure but it has been estimated that an annual average of about craft. Much of its popularity is due to its quality as a 51 tonnes of phosphorus and 610 tonnes of nitrogen enter recreational fishing and crabbing area. the estuary though surface run-off from the rural catchment The Inlet has a diversity of ecologically-important (Anon 1995). habitats including seagrass beds, tidal mud, sand flats, salt Sediment is important in nutrient cycling in estuarine marshes, fringing sedgelands, heathlands and Melaleuca systems because it stores large amounts of nutrients, and woodland, with their associated biodiversity. It contains may release them when the concentration in the overlying small remnant stands of the grey mangrove, Avicennia water is low (Thornton et al. 1995). In many cases sediment marina. The areas of aquatic vegetation and mangroves are can be significant, either as a source or sink, adjusting nursery areas for fish and invertebrates, many of which nutrient concentrations in the water column, and so are important to the recreational fishing industry. controlling primary production and the possibility of algal blooms. It is therefore important to understand the The coastal plain catchment of Leschenault Inlet sediment properties of the estuary, and especially physico¬ © Royal Society of Western Australia, 2000 chemical properties of the surface sediment. These matters 275 Journal of the Royal Society of Western Australia, 83 (4), December 2000 276 McComb, Qui, Paling & Hill : Sediments of Leschenault Inlet ¥ STATION! 83 STATION 3 1.000 0.600 0.300 0.180 0.150 0.106 0.075 0.020 PARTICLE SIZE (mm) Figure 2. Particle size distribution for sediments from Leschenalut Inlet. are addressed in this paper, which compares the sediments interface, sealed with rubber bungs, transported to the of Leschenault Inlet with those of others, more intensively laboratory, covered with black polythene, and incubated studied estuarine systems in south-west Australia, the at 20 °C. Similar intact cores were collected from site 7 in Swan-Canning and Peel-Harvey. Peel Inlet, site 1 in Harvey Estuary, sites 1, 3 and 27 in Leschenault Inlet and sites 9, 16 and 23A in the Swan- Materials and Methods Canning Estuary. Data recording and analysis Sampling Wet to dry ratio (W/D) was determined by drying a Sediment monitoring commenced in September 1988 known weight of wet sediment [water content = 1-(D/ W)]. and continued until July 1990. Samples were taken for Percentage organic matter was determined by loss on physical and chemical analysis from 11 sites in Collie River ignition (550 °C, 1 hr). in July 1990, and monthly at 3 sites in Leschenault Inlet, 5 in Peel Inlet, 7 in Harvey Estuary and 6 in Swan-Canning Additional sediment samples were collected for Estuary (Fig 1). particle size analysis, These were dried overnight (105- 110 °C), weighed, and immersed in an aqueous solution of Samples were collected from the top few centimeters the dispersant sodium hexametaphosphate (2 g L '). The by SCUBA diving, returned to the laboratory and stored samples were periodically agitated until all sediment was overnight at 4 °C. They were dried (105 °C), ground and dispersed. The suspension was passed through a nest of stored until analysis. To obtain mixed samples of surface sieves which divided the particles into size classes: < 75 sediment from a larger area at each site, ten sediment cores 4 m, 75-150 4m, 150-355 4m, 355-710 4m and > 710 4m. were taken from each site by pressing a perspex coring These fractions were dried, weighed and the cumulative tube gently into the sediment. The upper 5 mm of sediment frequency of particle size classes calculated according to was extruded from the cores, placed in a bucket and well Buller & McManus (1979). mixed. Sub-samples were stored on ice in the field and at 4 °C in the laboratory. During toxic Nodularia blooms in Total phosphorus was determined by digesting 1 g of the Peel-Harvey Estuary between November and February dried sediment with concentrated perchloric acid. Nitric it was not safe to dive. On three such occasions cores were acid was added first to digest volatile organic substances therefore collected remotely using a perspex cylinder which may form explosive substances in the presence of attached to a long pole operated from the boat. On these perchloric acid. The released phosphate, as soluble reactive occasions only one core was taken at each site. phosphorus (SRP), was measured colorimetrically with the acid molybdate/ascorbic acid reagent described by Intact cores were collected by SCUBA diving for Strickland & Parsons (1972) and measured with a Varian metabolic work in the laboratory. A perspex cylinder, 50 634 or DMS 90 spectrophotometer. cm by 9.5 cm (internal diameter), was gently pressed approximately 15 cm into the sediment. Tire intact sediment The method for phosphorus fractionation was similar and cylinder were removed with the associated water to that of Williams et al. (1976), but without citrate- column, taking care not to disturb the sediment/water dithionite-bicarbonate (CDB) extraction. Sediment CaC03, 277 Journal of the Royal Society of Western Australia, 83 (4), December 2000 Table 1. Particle size distribution of sediments from Leschenault Inlet and Collie River*. See Fig 1 for site locations. Location Site Qi Median Q3 QD Skewness Leschenault Inlet 1 5.8 6.0 6.2 0.2 0.00 3 2.6 5.9 6.2 1.8 0.01 27 2.9 4.4 6.0 1.55 -0.29 Collie River 5 (North) 3.8 4.1 6.6 1.4 -0.01 5 (South) 0.7 1.8 5.2 2.2 0.03 5 (Centre) 0.3 0.5 4.5 2.1 -0.28 10 (West) 2.2 4.6 6.3 2.0 0.07 10 (East) 1.3 4.0 6.1 2.4 0.15 10 (Centre) 3.6 5.0 6.5 1.4 0.03 11 (North) 0.6 1.3 2.5 0.9 0.02 11 (South) 0.7 3.0 5.8 2.5 0.19 11 (Centre) 1.1 2.4 5.5 2.2 0.05 Footnote: Q1 = 1st quartile (first 25% cumulative frequency as weight percentage), Median (first 50% of cumulative frequency as weight percentage), Q3 = 3rd quartile (75% of cumulative frequency as weight percentage), QD (quartile deviation); and skewness of distribution frequency. Ca, Fe and A1 content were analysed by the Chemistry Centre of Western Australia. Ammonia was measured by the isocyanurate method (Dal Pont et aJ. 1974). Phosphate release rates were determined by measuring concentrations of total and soluble reactive phosphorus in the water column above the sediment in intact cores collected monthly from each estuary. Samples were withdrawn from each core on five occasions over approximately 10 days. On each occasion measurements were made of pH (Beckman Model 021), temperature, dissolved oxygen (Xertex Delta Model 4010 or Yeokal) and redox potential (Orion Model 20 meter with platinum and reference electrodes). Results and Discussion Sediment particle size Particle size fractions finer than 75 h m ranged from 40.4 to 87.0% at the three sites in Leschenault Inlet. Most particles were less than 20 Pm, particularly at site 1 (Fig 2). Sediments from sites 3 and 27 had a relatively higher percentage of coarse particles and a wider range of values, reflected in the variation of the median particle size, and the quartile deviation, suggesting these sediments were poorly sorted (Table 1). The skewness values for sediments from sites 1 and 3 were near zero but a value of -0.29 for site 27 sediments suggested that the cumulative frequency of sizes was skewed towards coarse particles. There was a wide variation in median particle size (MD) for sediments from Collie River (Table 1). The average MD (in 0) from site 10 was significantly higher than other sites, reflecting a higher proportion of finer particles with distance from the point of discharge into the estuary. The quartile deviation in particle size distribution ranged from 0.9 at site 11 (north) to 2.5 at site 11 (south), indicating a significant difference in the level of particle-sorting force in transects across the Collie River (Table 1). This can be attributed to changing velocity of the flow around river Figure 3. Cumulative weight frequency and particle bends, across banks and in depressions on the river bed. diameter (0) for sediments from Collie River Site 11, north Seasonal flooding of the Brunswick River, carrying a large (N); south (S); and centre (C). 0 = - log2 (particle size mm). volume of silty water, enters the Collie River about 500 m 278 McComb, Qui, Paling & Hill : Sediments of Leschenault Inlet upstream of site 11. The merging of the two streams increases deposition environment and the degree of the particle mixing scouring of the riverbed downstream, and forms 'high (Sly 1978). There was little skewness in the distribution pattern energy' sites which favour only sedimentation of larger and at the centre and western side of site 10 and northern side of coarser sediments; and some 'low energy' sites which favour site 11; the distribution of particle size in these sediments was sedimentation of finer particles. There would thus be relatively symmetrical around the mean value. significant differences in flow velocity and "energy" levels across the river, and the sediments would be poorly-sorted W/D ratio and organic content in some regions and well-sorted in others. The wet to dry ratios (W/ D) from sites 1 and 3 averaged The particle size distributions of Collie River Site 11 are from 2.11 to 3.08 (Table 2). The range was highest at site 3, illustrated in Fig 3. The third quartile values were relatively near the middle of the inlet adjacent to the SCM pipeline. high at all sites, with the exception of site 11 (north). This There was a greater depth gradient at this site. The water suggests that finer particles accumulated in most sampling depth of the sites ranged from 0.5 m (site 1) to approximately areas of the river bed of the Collie River. 2.0 m (site 3). Tire skewness of particle size distribution indicates, in Mean organic content was 8.6 to 12.1% (Table 2). comparison to a sample considered completely mixed, the Sediment organic content in Leschenault was higher than presence of size classes over- or under-represented. The in Peel Inlet, comparable with some sites in Harvey Estuary, skewness values varied significantly at all sites, a positive and clearly lower than in the Swan River. Distribution of value indicating higher accumulation of finer particles, a sediment A1 and Fe in these four estuaries followed the negative value that the distribution was more on the coarse pattern of organic content. In Leschenault, concentrations side, an effect of higher flow affecting the riverbed. The were highest at site 1 and lowest at site 27, which was skewness therefore indicates the "energy level" of the Table 2. Physical and chemical properties of the sediments from Leschenault Inlet, Harvey Estuary, Peel Inlet and Swan-Canning Estuary. Estuary Site W/D* Organic matter CaCCh A1 Fe Ca (%) (%) (%) (%) (%) Leschenault Inlet 1 2.83 (2.79-3.97) 11.17 (9-8-15.2) 13.0 3.6 4.4 13.0 3 3.08 (1.85-4.50) 12.08 (5.7-14.9) 8.0 3.4 4.0 2.5 27 2.11 (1.72-3.41) 8.60 (3.9-15.6) 24.0 1.5 1.9 2.4 Harvey Estuary 1 4.88 (2.72-6.29) 17.72 (12.2-22.0) 9.0 6.0 3.8 2.9 28 2.69 (2.20-3.56) 8.73 (1.0-11.9) 6.0 2.6 1.8 2.1 29 3.55 (2.30-5.05) 10.84 (8.0-17.6) 35.0 4.2 2.6 14.4 30 1.30 (1.28-1.58) 2.30 (1.0-3.5) 2.0 0.8 0.5 0.6 31 2.21 (1.51-6.00) 5.38 (1.8-19.0) 3.0 1.0 0.6 1.0 37 3.41 (1.73-6.25) 9.49 (3.0-18.4) 1.0 1.8 1.2 0.2 P59 4.60 (3.14-5.65) 18.63 (11.7-22.0) 9.0 7.6 5.5 1.6 Peel Inlet 4 1.87 (1.47-2.11) 4.5 (1.4-6.3) 4.0 0.5 0.5 1.8 5 1.55 (1.44-1.85) 3.1 (0.9-4.6) 1.0 0.2 0.2 0.2 6 1.45 (1.33-1.95) 2.7 ((0.6-4.9) 1.0 0.2 0.1 0.4 7 1.65 (1.60-1.92) 6.0 (3.3-7.6) 20.0 1.4 1.2 8.2 8 2.47 (1.71-3.59) 8.4 (4.5-14.5) 5.0 2.8 2.0 1.8 Swan-Canning 5 3.92 (2.27-5.57) 14.1 (8.6-20.6) 4.0 9.1 6.2 0.5 Estuary 9 5.00 (4.41-5.50) 19.2 (16.0-21.6) 5.0 9.3 7.0 0.7 13 4.33 (3.61-5.40) 18.5 (14.6-19.9) 14.0 8.1 5.0 5.0 16 4.17 (3.41-5.05) 19.5 (15.3-22.6) 7.0 9.7 5.2 1.7 23A 4.67 (4.25-5.74) 19.2 (16.6-21.8) 15.0 6.4 5.8 4.7 *W/D: Wet to dry ratio by weight; range in parentheses Table 3. Properties of surface sediments from Collie River. Site Depth (m) W/D ratio Total phosphorus (gg1) Collie 5 North 1.6 3.82 1106 Collie 5 South 2.7 3.24 881 Collie 5 Centre 2.7 1.84 221 Collie 10 West 4.1 3.46 666 Collie 10 East 2.0 4.37 856 Collie 10 Centre 4.1 4.76 1084 Collie 11 North 1.5 2.11 381 Collie 11 South 1.5 1.64 194 Collie 11 Centre 1.5 1.79 267 Collie 13 5.0 6.42 1102 279 Journal of the Royal Society of Western Australia, 83 (4), December 2000 apparently more affected by exchange of water between attributable to the small site numbers in Leschenault Inlet, the inlet and ocean. Sediments from Leschenault Inlet and the large between-site variations in Harvey Estuary. showed similar trends to those of Peel Inlet and Harvey Sediment total P in the Swan-Canning Estuary was Estuary, where the concentration of iron and aluminium significantly higher than in the Peel Inlet and Harvey contents were correlated with % organic matter (r2 = 0.84 Estuary (p = 0.050 and 0.005 respectively). and 0.86 for A1 and Fe, respectively). Organic phosphorus in the Leschenault sediments The central sites (sites 5 and 11) of the Collie River had accounted for 19 to 31 % of total P, with an average similar to lower W/D ratios than those on the north and south banks, Harvey Estuary (25%). This proportion was higher than in although the ratio at the central site (site 10) remained high the Swan-Canning (19.6%). There was no significant (Table 3). The W/ D ratios at site 5 (north) and site 10 (east) difference in organic phosphorus coTitent between were relatively high although the water was relatively Leschenault and Peel Inlet, and between Leschenault Inlet shallow. Each of these sites was inside a bend in the river, and Harvey Estuary. Organic phosphorus in Leschenault where water flow is reduced and fine particles accumulate, Inlet was significantly lower compared with the Swan- whereas water velocity at the outside of bends discriminates Canning Estuary (p = 0.006). Apatite phosphorus averaged against the settlement of fine particles. Only one sample about 37% of the total P in Leschenault sediments, the highest was collected from site 13, upstream of the confluence with among these estuaries. This may reflect a strong marine the Brunswick River where the river narrowed considerably. influence, as apatite phosphorus can be 5 times higher in ocean sediments than in the Peel-Harvey (Lukatelich & Phosphorus forms McComb 1986). The concentration of apatite-P was higher There were high variations in concentrations of sediment in Leschenault than in Peel, but lower than in the Swan- phosphorus between months, sites and estuaries (Table 4). Canning Estuary (p = 0.036 and p = 0.020, respectively). The level of the total phosphorus in Leschenault Inlet was Non-apatite phosphorus averaged about 36% of total P in higher than in Peel Inlet (p = 0.013), but lower than in the Leschenault Inlet, comparable with Peel Inlet (36.8%), but Swan-Canning Estuary (p = 0.002). Although mean total P lower than in Harvey Estuary (46.1%) and Swan Canning appeared low when compared with Harvey Estuary, it was Estuary (56.7%). The concentration of non-apatite P was not statistically significant (p = 0.093) at 95% confidence level. also lower in Leschenault than in Harvey Estuary and the The low confidence level of the differences was partly Swan Canning Estuary (P = 0.032 and 0.021 respectively). Month Figure 4. Sediment phosphorus release from intact cores collected from several estuaries south-western Australian. Sampling sites are shown in Fig.l. 280 McComb, Qui, Paling & Hill : Sediments of Leschenault Inlet p t-h co © CO LO ON NO r"j GO rt 04 LO p 04 IN ON t-h t-h CO NO ON 04 Gn IN d Lf) ON iri t—t nO t—t CO ON NO 00 rf IN O LO rf rH NO rt 04 ON 00 NO 00 ND rt CO (N cn co ro ro co m LO IN LO CO LO LO LO 00 ^ LO O IN r-1 04 CO NO tN rt N O', (ON ON NO ^ oo ,-^t-h O GcoN LoOi GoOi N OLOn O^ G^N I^N h OrNt ^C—Ov CNO 0N0 nnss bC 0tvLN-4T" U ^Lt—OO( rt(o—NOiI nor^oot , o^ccoo COC(NOO0i NHG(NNi GC^LNOO' | I0N0 0CO4 OLON SCOI <OLO? 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ON 04 04 CO 04 rt r—t rH_ Op rH, LO t—1 LO LO be on co O rH IN ON rH tN On NO NO O ON O O IN LO rH crHo O0N0 04 ^H N^O rV~" ' In 1 CO r ON co 04 LO rt r— CO O IN o nO CO IN IN rt* co O no 0s r—1 on ^o In •—- CO CO 04 CO 04 t—1 t—t 04 04 CM NO rf rt T-* 04 rt LO) rt —h 00 04 NO N- CO CVnS Gnast ^ON nQGsJ < < Gs CD CO 0IN4 <*h rt LO) NO IN 00 S C(NOJ O04N OCO CHO CNO PIfh) 4t LO ON crHo rrHt NrHO C0O4 'i bO G G G PO- : ns A u n o>> G SHni ¥ Wp f00t-»» XH CD 281 Journal of the Royal Society of Western Australia, 83 (4), December 2000 Potential for phosphorus release The laboratory incubations of intact cores suggested that the potential for sediment phosphorus release was relatively small in Leschenault Inlet (Fig 4). In some cases phosphate concentrations in the water column fell during incubation. This occurred in the sediment cores from Site 1 and Site 27 in February to March. Rates of phosphate and total phosphorus release were low in most months compared with other estuaries. There was little change in water column pH from all three sites in Leschenault Inlet, but DO and redox potential decreased with time (Figs 5, 6). There was no oxygen depletion during incubation, as there was in organic-rich sediments, such as those from Harvey Estuary. Paired cores from Harvey Estuary in the same period showed significant oxygen depletion during incubation. Conditions developed in the water column may favour nutrient release from sediment, but changes in water column concentrations were relatively small. The possibility remains that phosphate in the water column may have been in part transferred to the particulate phase, for example, attached to particle surfaces or taken up by phytoplankton and other microorganisms. The increase in total phosphorus at the end of incubation suggests that such activity could have been involved in altering phosphate concentrations in the water column. Sediment release rates for phosphate were -15.8 (i.e. the water concentrations fell) to 5.6 mg nr2 d*1 in Leschenault DO concentrations (mgD1) in intact cores collected in March 1990 Inlet (Table 5), and the calculated annual average would from Leschenault Inlet. Site 1; Site 3; Site 27. . <+ pH; be -0.11 mg nr2 d*1. This is extremely low compared with --O-DO; ¥ SRP;_¥ ... TP. the estimated annual rate for Harvey Estuary during 1982- 1990 (Table 5). Table 6 also shows that phosphate release from the sediments studied is always coupled with ammonium release, though this information is not directly available for Leschenault Inlet. Interrelations between components There was a significant correlation between W/D ratio and total phosphorus in Collie River sediments (Fig 7). Similar correlations exist in many other estuaries in south¬ western Australia (Fig 8), despite difference in trophic status and sediment phosphorus concentrations. The slope of the relationship, however, may differ between systems according to sediment properties and the degree of nutrient enrichment (Hill ef al. 1991; McComb ct al 1998). VV/D ratio was also correlated with the proportion of particles less than 75 Fm in Leschenault, Peel-Harvey and Swan-Canning estuaries (r2 = 0.74, p < 0.01). There was no correlation between water depth and the other parameters measured in Collie River. This may suggest that water flow, rather than local morphology, is the dominant factor in the distribution of sediment components. Surface sediments from Leschenault Inlet had a large proportion of fine material, with a particle size finer than in the Peel-Harvey Estuary. Consistent with this, organic enrichment of surface sediment was relatively high. It was higher than in Peel Inlet, comparable with part of Harvey Estuary, but lower than in central Harvey Estuary and the Figure 6. Phosphate (SRP) release and redox potential in intact Swan-Canning Estuary. cores collected in March 1990 from Leschenault Inlet. Site 1; Site 3; Site 27.-¥-Redox;-¥-SRP. This paper focused on the redox-related P release. 282 McComb, Qui, Paling & Hill : Sediments of Leschenault Inlet ¥ HARVEY ESTUARY Figure 7. W/D ratio and total phosphorus content in sediments Figure 8. W/D ratio and total phosphorus concentrations in from Collie River. CU Site 5; 0 Site 10; A Site 11. sediments from estuaries of south-west Australia, (y=105.29 + 193.78x; r=0.65). which was usually driven by bacterial activity and triggered for the difference in P release. The proportion of apatite-P by favorable conditions including high respirable C. to total P, however, appears to be consistent with the P Several factors are usually associated with this type of P release data. The Leschenault sediments contain a release, mainly a) the level of organic respiratory C; b) the substantial proportion of apatite P, distinctly higher than composition of the sediment P form, and c) their absolute in Peel, Harvey and Swan-Canning. amounts. The organic content in Leschenault Inlet was At least in part for this reason, the rate of P sediment higher than in Peel Inlet and similar to that of the Harvey release remained low in Leschenault Inlet compared with Estuary. Sediment oxygen demand associated with the the other estuaries. This low P release may be also related degradability of organic matter was similar to those in the to the overall effects of other factors, such as the relatively Peel Inlet and Swan-Canning Estuary, though it was lower low amount of sediment P forms, though this was not compared with those in Harvey Estuary (Qiu & McComb strongly supported by the limited sediment data. Likewise, unpublished data). Therefore it is difficult to interpret the the field conditions should not be neglected, as this type of different P release rates between estuaries by the amount P release is usually triggered under favorable and the nature of organic matter alone. On the other hand, environmental conditions, such as an increased water, there was no clear indication that the level of 'absolute which stimulates microbial development near the sediment- amount' of any sediment P forms was predominantly water interface. higher or lower in Leschenault, which may be responsible Table 5. Phosphate release rate (mg nr2 day1) over 10 days' incubation from Leschenault Inlet sediments, between November 1988 and October 1989. Station Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct * 1 1.5 3.6 1.6 -5.2 -0.2 -15.8 5.3 3.6 3.4 1.0 0.7 * 3 5.2 1.2 2.3 1.5 -0.2 -15.4 - - - - - "k 27 -0.1 2.4 2.1 0.3 - 0.8 1.6 1.6 5.6 1.8 0.3 ^Samples were not collected because of poor weather conditions. Table 6. Phosphate and ammonium release rates (± se) of sediments from Station 1 in Harvey Estuary. Harvey Estuary Peel Inlet Year Phosphate Ammonium Phosphate Ammonium (mg m'2 day1) (mg m2 day1) (mg m"2 day1) (mg m'2 day1) 1982 24.3 ±9.6 - - - 1983 28.4 ±4.4 98.3 ±13.3 - - 1984 20.6 ±3.3 194.9 ±33.8 5.2 ±2.1 102.4 ±26.4 1985 19.8 ±4.0 216.2 ±51.5 4.5 ±0.9 128.2 ±33.0 1986 20.5 ±4.6 212.5 ±40.5 4.9 ±1.7 75.8 ±16.3 1987 13.1 ±5.5 202.2 ±49.0 2.7 ±1.2 86.2 ±11.7 1988 5.2 ±2.4 75.8 ±14.1 3.0 ±3.5 63.5 ±15.2 1989 14.3 ±4.4 252.8 ±67.3 8.5 ±1.7 252.8 ±67.3 1990 16.7 ± 5.4 - 7.2 ±2.8 - 283 Journal of the Royal Society of Western Australia, 83 (4), December 2000 References Lukatelich R J 1989 Leschenauit Inlet - macrophyte abundance and distribution. Report 15. Waterways Commission, Perth. Anon 1983 Peel-Harvey Estuarine System Study - Symposium - Lukatelich R J & McComb A J 1986 Nutrient recycling and the Prospects for Management. Bulletin 136. 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