Observations of Suspended Particulate Matter Concentrations and Particle Size Distributions within a Macrotidal Estuary (Port Curtis Estuary, Australia)

Abstract

An understanding of suspended particulate matter (SPM) dynamics is of great importance to design awareness and management strategies of estuaries. Using a Laser In Situ Scattering and Transmissiometry (LISST) instrument, variations in suspended particle size volumetric concentrations (VC) and particle size distributions (PSD) were measured at six sites within Port Curtis estuary (Australia). The port is a macrotidal estuary with significant economic and environmental importance. Observed VC and SPM sizes demonstrated spatial and temporal trends strongly controlled by the variable energy conditions operating on the neap and spring cycle timescale, with a clear trend towards increasing concentrations and decreasing SPM sizes with increasing tidal ranges. Mid-estuary sites were characterized by the greatest depth-averaged VC under transitional and spring conditions. Estuary-wide mean spring tide total water profile concentrations revealed a near 300% increase in comparison to neap tide condition concentrations. In the upper-estuary sites the mean contribution of the combined 2.5–35 µm size classes to the total profile PSDs was greatest during all tidal conditions, whilst within the lower-estuary site the combined 35–130 µm size classes were greatest. Mean contributions of the largest size class (300–500 µm) dominated surface-waters throughout the estuary during the neap tide period, which when compared with the transitional and spring tide conditions, demonstrated changes of −82% to −48% and −82% to −40%, respectively. Overall, the results from this case study provides further evidence of the important influence of neap and spring tidal regimes on SPM dynamics within estuarine settings and the need to observe parameter dynamics on such timescales.

1. Introduction

Mechanisms controlling concentrations and size distributions of suspended particulate matter (SPM) within estuaries are complex and dynamic [1,2,3,4]. Vertical and spatial distributions, size, and composition of estuarine SPM are constantly changing in response to high and low frequency processes [5]. Forces influencing SPM concentrations and particle size distributions (PSD) include deterministic (tidal cycles and tidal range) and stochastic (river flow, wind, waves, turbulence) components [6,7,8], anthropologic and biological influences [9], and the physical, chemical, and biological characteristics of SPM [10,11,12,13]. The delivery and resuspension of bottom sediments (e.g., current- and/or wave-induced) are important process influencing SPM. An understanding of the dynamic SPM concentrations and PSD is important since they play critical roles in the function and health of coastal environments. For example, knowledge of SPM characteristics is essential for quantifying fluxes of substances and determining the fate of pollutants [12,14], whilst the resuspension of sediments has been shown to influence productivity in shallow-water environments [15,16] through water column nutrient enrichment [17,18] and impacting light availability [19,20]. Additionally, resuspension events may impact water quality through release of trace metals and organic contaminants [21], while also influencing biogeochemical cycles [10]. As such, an understanding of SPM dynamics in estuaries is fundamentally important regarding the management of these environments.

Suspended size distributions are traditionally determined from water samples, photographic imaging techniques [22], and optical or acoustic backscattering or transmission sensors [23]. While these techniques can provide an accurate measure of the underlying grain size distribution, they are prone to disturbing the fragile aggregates and flocculates that exist in the marine environment [24,25]. Additionally, these techniques are typically lengthy in procedure and unsuited to measuring the concentration of a suspension if the size distribution changes [26]. In order to avoid these problems in determining suspended PSD, the use of LISST (Laser In Situ Scattering and Transmissiometry) instruments have been adopted [27,28]. When estimating size distribution of suspended particles, a major benefit of field-deployable LISST instruments compared to traditional approaches is the ability to make measurements in situ without the need to collect, store, transport, or otherwise handle water samples. High resolution LISST measurements of SPM volume concentration (VC) and PSD in marine and estuarine environments have been well documented, e.g., [10,12,23,29].

Port Curtis, located between Keppel Bay and Rodds Bay in central Queensland (Australia), is an estuary of significant economic and environmental importance adjacent the World Heritage listed Great Barrier Reef Marine Park. Whilst being the largest multi-commodity port in Queensland and the fifth largest coal port in the world, the estuary is ecologically important with large areas of intertidal habitats, including seagrass meadows and coral reefs [30]. Additionally, the estuary also supports vulnerable dugong (Dugong dugon) and endangered green turtle (Chelonia mydas) [31,32].

The purpose of the study was to contribute to the understanding of tidal influences on SPM dynamics in macrotidal estuaries, using Port Curtis estuary as a case study. Suspended particulate matter concentrations and PSD datasets obtained through LISST instrumentation profiles during neap, transitional, and spring tide surveys were compared to investigate tidal-induced temporal and spatial variations within the estuary.

2. Materials and Methods

2.1. Study Location

Port Curtis estuary situated on the east coast of Australia (Figure 1), is a macrotidal estuary covering an area of approximately 200 km². Depths range from <1 m on the intertidal flats to >20 m within the central channel. Tides are semi-diurnal, with a maximum tidal range of 4.69 m at Gladstone and up to 6.00 m within the Narrows [33]. The large tides ensure that the water column is vertically well-mixed and are also responsible for significant re-suspension of fine sediment. The tide propagates into the estuary through the straits separating Facing Island from the mainland (Gatcombe Channel) and Curtis Island (North Channel), and through the Narrows via Keppel Bay in the north (Figure 1). Tides undergo a spring–neap cycle with a period of approximately 14 days, with maximum ranges during spring tides and ~1 m during neap tides. The strongest tidal currents are focused in the main tidal channels. Former point measurements show maximum current velocities of up to 0.85 and 0.72 m s⁻¹ in the surface and near-seabed layers during neap tide conditions, respectively, and 1.0 and 0.85 m s⁻¹ in the surface and near-seabed layers during spring tide conditions [34]. Though a less frequent physical forcing mechanism compared to tidal forcing, freshwater inputs from the nearby Fitzroy and Calliope Rivers, and Auckland Creek discharging into the estuary are occasionally important during periods of high rainfall (i.e., summer months). Total monthly freshwater summer peak discharges from the Calliope River typically range between approximately 5 and 50 × 10⁶ m³, which can result in rapid reductions in salinity throughout the marine dominated waterway [35]. Salinity typically ranges between 32.7 and 36.7 psu with mean winter and autumn salinities being 35.8 ± 1 and 32.8 ± 0.7 psu [36], respectively, reflecting the summer-dominated rainfall inputs. No significant spatial salinity trends are apparent within the estuary during ‘dry’ quiescent periods [37,38]. Additionally, Herzfeld et al. [39] report that freshwater inputs do not appear to contribute to residual thermohaline circulation, owing to the strong tidal mixing preventing the establishment of large baroclinic pressure gradients within the estuary. Furthermore, the modeled e-folding time for flushing of the estuarine and adjoining coastal system is approximately 19 days, whilst the mid-estuary e-folding time is approximately 30 days (i.e., 2 × neap–spring tidal cycles) [39].

The estuary is also influenced by local wind-driven waves, which contribute to turbulences and sediment suspension events. Wave activity is greatest during periods of prolonged south-easterly trade winds. Port Curtis estuary sediments consist primarily of silts and clays within the shallow intertidal banks, while fine and coarse sand can be predominantly found in the more tidally energetic deeper regions of the estuary [34]. Sediment provenance within the estuary is discussed by Jackson et al. [40]. Modeled approximate estimates of sediment transport and alluvial sediment supply from the northern Fitzroy River estuary and Calliope River are 25 and 40 Ktpa, respectively, whilst, estimated net sediment exports and inputs at the sea boundary are approximately 13 and 660 Ktpa, respectively [40].

2.2. Field Data Collection

Six survey days were completed in Port Curtis estuary between 15 September to 6 October 2010 during neap, transitional, and spring tide conditions (

Table 1. Meteorological and hydrodynamic conditions in the Port Curtis estuary during the survey days in 2010.

	15–16 September	21–22 September	5–6 October
Sample sites	6	6	6
LISST profiles per site	6	6	7
Total LISST profiles (n)	36	36	42
Tidal phase 1	Neap	Transitional	Spring
Tidal ranges 1 (m)	1.12–2.02	2.84–3.00	3.19–3.69
Maximum wind Speed and direction(09:00 and 15:00) (km/h) 2	8 and 9 (WSW)	21 and 30 (ESE)	14 and 13 (ENE)
Recorded rainfall 3 (mm)	0	40.8	0
Days since last rainfall and amount 3 (mm)	3 (1 mm)	>5 (0 mm)	4 (22.4 mm)

¹ Port Curtis: Data sourced Flinders National Tidal Facility. ² Gladstone city: Data sourced World Weather Online. ³ Gladstone Airport (BOM Site number: 039326): Data sourced Australian Bureau of Meteorology.

). During all surveys, conditions were typically calm (wind speed) with wave heights < 1 m. During surveys, measurements were taken at six sites, across three designated estuary zones: lower-estuary (Site 1), mid-estuary (Sites 3, 4, and 9), and upper-estuary (Sites 5 and 7) zones (Figure 1). Profile measurements taken at each site during longitudinal surveys consisted of depth, water temperature, SPM VC, and PSD using a Laser In-Situ Scattering and Transmissometer (LISST-100X C Type, Sequoia Scientific Inc., measurement range of 2.5–500 μm). Stations were profiled approximately 20 min apart. Surface to bed profiles collecting real time high temporal data were made by slowly lowering the LISST through the water column from the rear of a stationary vessel. Cables used to lower the instrument contained within a housing frame (measuring 1.2 × 0.5 × 0.4 m; see [23]) contained depth indicator markings to aid determination of depth and importantly how far the instrument was from the bottom sediments in relation to readings taken off the vessel depth sounder. As the instrument housing frame touched the bottom sediment, it was returned to the surface. Only a single profile was taken at each site to avoid sampling where sediment from the bottom had been resuspended into the water column following the LISST housing frame contacting the seabed. Temperature profiles demonstrated small decreases in temperature with increasing depth with mean temperature ranges for individual profiles of 0.6, 0.7, and 0.5 °C in the lower-, mid-, and upper-estuary, respectively, reflecting a well-mixed system. Mean surface-water temperatures within the lower-, mid-, and upper-estuary were 23.3, 24.0, and 23.9 °C, respectively.

2.3. LISST-100X—Volumetric Concentrations and Particle Size Data

The LISST-100X uses laser diffraction technology to measure the PSD of the SPM. The instrument uses a series of concentric ring detectors and works by measuring the angular distribution of forward scattered light energy. The intensities of light gathered by the ring detectors are inverted to estimate the particle area concentration for 32 log-spaced size categories ranging from 2.5 to 500 μm. Applying the principle that the larger the particle, the narrower the angular band into which light will be scattered, enables the particle diameter to be calculated [27,28]. The particle area concentration estimates, using an empirical volume conversion constant, then provide a volume concentration distribution (VD) over this same size range [12]. The total VC [μL L⁻¹] is computed by simple summation of VD over the 32 classes [28]. The LISSTs’ transmissometer detector is located in the center of the ring detectors in order to measure the light which is not scattered or absorbed [12]. Extensive descriptions of the LISST instrument and its operation principles can be found in Agrawal and Pottsmith [27,41].

2.4. Data Processing and Analysis of LISST Data

Upon retrieval of the instrument following each survey day, raw data were offloaded and analyzed using the LISST data processing program version 4.60 where the data were converted to volumetric PSDs using the manufacturer’s instructions and accounting for the presence of random shaped particles as described in Agrawal et al. [42]. Multiplication of the VD with a volume conversion coefficient that yields the absolute volume concentration in each size bin was performed. The volume conversion coefficient is obtained by a factory calibration whereby the scattering pattern of particles with known sizes and VC are measured. Distributions were used to estimate the mean ± standard deviation, 95th percentile and maximum contribution to selected particle size classes: 2.5–7, 7–35, 35–75, 75–130, 130–300, and 300–500 µm of the SPM using standard statistical measures. Reported VC and PSD are based on measurements collected at 0.17 m intervals from the water surface to bottom sediments. Reporting profile depths ranged between 5.5 and 14 m according to the tide phase at time of measurement. Results are presented for relative bottom-, mid-, and surface-water column compartments (depths), as well as total profiles (i.e., depth-averaged).

The recommended manufacturer’s limit for valid measurements regarding optical transmission is ≥15%, when the optical transmission is higher than 30% the effect of the multiple scattering is negligible [27]. During all profiles, all percentage transmission readings never fell below 30%, as such the influence of the multiple scattering and low optical transmission can be excluded [10].

Due to various influencing factors [10] it is purported that LISST-derived measurements are best used to reflect and interpret the relative change in VC and PSD of SPM, rather than relying on their exact values [12,43]. Nonetheless, LISST instruments have been demonstrated to work well in examining aggregate size distributions [15,44] and this study will provide an important dataset that will aid further advancements in our understanding of the SPM variability within macrotidal estuaries and in particular the economically and environmentally significant Port Curtis estuary.

3. Results and Discussion

3.1. Volumetric Concentration

Depth-averaged SPM VC ranged between 4.8 ± 5.0 µL L⁻¹ (Site 5 neap conditions; Figure 2) and 45.1 ± 37.5 µL L⁻¹ (Site 3 spring conditions; Figure 2) and exhibited spatial and temporal trends.

The upper-estuary sites revealed the lowest depth-averaged VC under all conditions, whilst the lower-estuary site demonstrated the greatest depth-averaged VC during neap tide conditions. Additionally, mid-estuary sites were characterized by the greatest depth-averaged VC under transitional and spring conditions (Figure 2).

Accounting for all sample conditions, the depth-averaged VC within the low-, mid-, and upper-estuary sites were 22.2 ± 29.7, 24.5 ± 33.8, and 18.3 ± 31.5 µL L⁻¹, respectively (

Table 2. Depth compartment and depth-averaged (M) ± standard deviation (SD), median (Md), 95th percentile (P₉₅), and maximum (Max) volumetric concentrations [μL L⁻¹] within the lower- (Site 1), mid- (Sites 3, 4, and 9), and upper-estuary (Sites 5 and 7) site/s based on all sample conditions.

Survey depth	Lower-estuary site
	M ± SD	Md	P95	Max
Surface	25.7 ± 49.0	18.0	62.1	765.8
Mid	19.8 ± 10.2	19.3	38.9	50.3
Bottom	21.3 ± 13.7	19.2	46.6	67.0
Depth-averaged	22.2 ± 29.7	18.8	46.0	765.8
	Mid-estuary sites
	M ± SD	Md	P95	Max
Surface	22.1 ± 47.3	14.7	47.3	738.4
Mid	22.6 ± 22.0	17.7	62.1	144.6
Bottom	28.4 ± 26.6	24.0	79.9	155.3
Depth-averaged	24.5 ± 33.8	17.3	70.3	738.4
	Upper-estuary sites
	M ± SD	Md	P95	Max
Surface	16.0 ± 37.9	9.8	31.8	595.7
Mid	15.5 ± 14.1	9.8	42.0	71.1
Bottom	23.0 ± 35.9	12.6	83.4	272.2
Depth-averaged	18.3 ± 31.5	11.0	53.0	595.7

). When comparing zones across all tidal conditions, the mid-estuary depth-averaged VC was 10.4% and 33.9% greater than the lower- and upper-estuary sites, respectively. Additionally, the depth-averaged VC for the lower-estuary site was 21.3% greater compared to the upper-estuary sites. The mean surface-waters VC ranged between 16.0 ± 37.9 and 25.7 ± 49.0 µL L⁻¹ with the greatest mean VC occurring at the lower-estuary site. Furthermore, the mean mid-water and bottom-waters VC ranged between 15.5 ± 14.1–22.6 ± 22.0 µL L⁻¹ and 21.3 ± 13.7–28.4 ± 26.6 µL L⁻¹, respectively, with the greatest mean VC observed at the mid-estuary sites (Table 2). Mean mid-water VC within the mid-estuary zone was 14.4% and 45.8% greater when compared to the lower- and upper-estuary VC, respectively, whilst the bottom-water VC was 33.3% and 23.5% greater in comparison to the lower- and upper-estuary VC, respectively.

The greatest variability of VC was observed within the surface-water compartment with % relative standard deviation (%RSD) values of 191, 214, and 237%RSD within the lower-, mid-, and upper-estuary surface waters, respectively (Table 2). Additionally, single occurring maximum VC observed in all estuary-zones were observed in the surface-waters compartment. However, although maximum concentrations were observed in the surface waters, total profiles typically demonstrated increased VC within the bottom-waters at the mid- and upper-estuary sites. This is supported by the greater 95th percentile (P₉₅) VC in the bottom-waters within the mid- and upper-estuary sites, which was most evident during spring tide conditions (Table 2), due to increased resuspension of bottom sediments resulting from increased current speeds. Resuspension and transport are strongly influenced by factors such as current speed and direction, wave action, particle size, and associated settling velocity, and water depth. Larger particles typically located within the deeper more energetic regions would tend to settle more quickly once re-suspended, while smaller particles that are re-suspended are transported before settling during decreased current speeds and wave action.

The observed low maximum concentrations during neap tide conditions are believed to be due to particle aggregation and macrofloc formations persisting during these conditions and organic matter in the form of detritus and planktonic cells [45,46]. Previous surveys of the system [47] have reported organic matter contributions to surface-water SPM to be >40% within the lower- and mid-estuary sites and >50% within the upper-estuary sites. In addition to the formation of flocs during neap tides, reduced resuspension by low current speeds during neap tides (compared to spring tides) contribute much to the observed lower VC within the estuary.

Despite greater wind speeds during the transitional sampling period (Table 1), depth-averaged VC demonstrated a clear trend towards increasing concentrations with increasing tidal ranges (velocities) (i.e., spring tides VC > transitional tides VC > neap tides VC) (

Table 3. Depth compartment and depth-averaged (M) ± standard deviation (SD), median (Md), 95th percentile (P₉₅), and maximum (Max) volumetric concentrations [μL L⁻¹] during neap, transitional, and spring tide conditions based on all sample sites.

Survey depth	Neap tide conditions
	M ± SD	Md	P95	Max
Surface	12.9 ± 29.9	6.0	42.2	420.9
Mid	6.2 ± 4.4	5.1	14.9	35.3
Bottom	6.0 ± 3.3	5.3	11.4	22.3
Depth-averaged	8.3 ± 17.6	5.5	19.0	420.9
	Transitional tide conditions
	M ± SD	Md	P95	Max
Surface	24.7 ± 60.2	17.2	39.5	738.4
Mid	19.8 ± 9.7	19.8	35.9	60.0
Bottom	23.1 ± 11.8	21.6	41.0	78.0
Depth-averaged	22.6 ± 35.5	19.2	39.1	738.4
	Spring tide conditions
	M ± SD	Md	P95	Max
Surface	23.3 ± 36.1	19.2	49.3	765.8
Mid	31.8 ± 22.5	26.8	69.1	144.6
Bottom	45.6 ± 39.9	31.7	119.4	272.2
Depth-averaged	33.9 ± 35.1	25.8	87.8	765.8

). The estuary-wide depth-averaged VC under neap, transitional and spring tide periods were 8.3 ± 17.6, 22.6 ± 35.5, and 33.9 ± 35.1 µL L⁻¹, respectively, representing an increase of 172.3% from neap to transitional conditions and a further 50.0% from transitional to spring conditions (Table 3). Comparison of estuary-wide depth-averaged VC during neap to spring tide conditions show an increase of 298.8%. Additionally, surface-, mid- and bottom-water mean VC increased, 80.6%, 412.9%, and 660.0%, respectively, when comparing spring tide conditions to neap tides. Elevated SPM VC during the spring tide conditions decrease light availability, which can potentially affect seagrass meadow and coral reef health survivability, whilst also potentially remobilizing sediment-bound contaminants [14,20].

3.2. Particle Size Distribution

In addition to the observed VC dynamics, resuspension and flocculation processes influenced by tidal dynamics throughout the estuary caused variations in the SPM sizes. The in situ PSD of SPM during the neap, transitional, and spring tide conditions within the surface-, mid-, and bottom-water compartments are presented in Figure 3 and

Table A1. Depth compartment-averaged mean (M) ± standard deviation (SD), 95th percentile (P₉₅), and maximum (Max) particle size class contribution during neap, transitional, and spring tide conditions within the surface (Surf), mid, and bottom (Bott) depth compartments.

Zone	Site	Survey Depth	Contribution (%)		Neap					Transitional						Spring
				Particle Size Class (µm)
				2.5–7	7–35	35–75	75–130	130–300	300–500	2.5–7	7–35	35–75	75–130	130–300	300–500	2.5–7	7–35	35–75	75–130	130–300	300–500
Lower-estuary	1	Surf	M ± SD	5±2	16 ± 9	13 ± 9	9 ± 7	17 ± 7	40 ± 29	4 ± 2	24 ± 9	27 ± 9	16 ± 5	17 ± 9	21 ± 15	5 ± 2	25 ± 8	28 ± 8	15 ± 4	17 ± 8	10 ± 11
			P95	10	33	27	19	27	82	6	41	38	23	30	45	8	35	37	20	30	35
			Max	12	39	29	31	36	89	8	43	38	25	42	60	11	40	39	21	45	53
		Mid	M ± SD	6 ± 2	21 ± 6	19 ± 5	16 ± 5	21 ± 5	17 ± 17	4 ± 1	24 ± 7	30 ± 6	19 ± 3	18 ± 8	5 ± 6	4 ± 1	22 ± 5	29 ± 5	19 ± 2	20 ± 6	6 ± 5
			P95	9	30	26	21	30	57	5	34	38	24	32	17	5	32	36	23	30	15
			Max	10	33	27	23	35	72	6	36	39	26	35	45	5	34	38	24	34	24
		Bott	M ± SD	6 ± 1	24 ± 4	23 ± 3	19 ± 2	21 ± 5	7 ± 6	3 ± 1	22 ± 7	29 ± 6	20 ± 3	20 ± 8	6±7	3±1	19 ± 5	27 ± 5	19 ± 2	24 ± 7	8 ± 5
			P95	8	30	28	22	29	19	5	32	36	24	30	15	5	29	33	22	35	19
			Max	10	32	30	24	32	28	6	37	38	26	31	48	6	34	35	25	38	25
Mid-estuary	3	Surf	M ± SD	8 ± 5	23 ± 9	11 ± 7	10 ± 5	14 ± 7	33 ± 21	5 ± 2	26 ± 7	28 ± 7	17 ± 4	18 ± 8	6 ± 7	7 ± 2	30 ± 7	28 ± 6	15 ± 5	13 ± 6	8 ± 10
			P95	16	41	23	18	28	68	7	35	35	23	32	20	11	41	35	21	21	35
			Max	26	56	25	28	39	73	15	36	36	26	56	38	15	43	36	23	49	49
		Mid	M ± SD	9 ± 3	30 ± 7	17 ± 6	16 ± 4	15 ± 6	11 ± 11	4 ± 1	21 ± 5	26 ± 6	20 ± 3	23 ± 6	6 ± 5	5 ± 2	22 ± 6	27 ± 5	19 ± 2	21 ± 7	6 ± 4
			P95	15	42	25	25	24	35	6	30	34	24	33	15	7	33	35	22	30	13
			Max	19	51	26	27	30	70	7	32	35	27	35	28	10	37	37	23	36	23
		Bott	M ± SD	8 ± 2	28 ± 5	20 ± 5	18 ± 3	18 ± 5	8 ± 7	3 ± 1	17 ± 5	24 ± 6	19 ± 3	26 ± 8	9 ± 6	4 ± 2	19 ± 6	25 ± 5	19 ± 2	25 ± 7	9 ± 5
			P95	11	37	26	23	25	24	6	28	34	24	37	22	7	32	32	22	34	18
			Max	13	24	29	26	29	33	8	30	35	24	39	28	8	35	33	24	37	22
	4	Surf	M ± SD	5 ± 5	14 ± 8	7 ± 6	4 ± 5	14 ± 9	55 ± 24	6 ± 4	27 ± 9	24 ± 7	15 ± 5	17 ± 9	11 ± 6	7 ± 3	30 ± 9	27 ± 9	16 ± 8	13 ± 9	10 ± 15
			P95	16	40	18	12	31	85	14	45	34	23	28	39	13	41	36	35	29	36
			Max	20	47	30	26	44	87	15	52	35	25	54	59	17	45	37	36	31	84
		Mid	M ± SD	9 ± 4	25 ± 9	14 ± 6	7±3	12±7	33±21	5±3	24±8	26±7	19±4	19±9	6±5	5±3	24 ± 8	25 ± 9	17 ± 9	17 ± 9	12 ± 10
			P95	16	42	23	13	26	69	13	40	35	23	37	18	10	36	37	35	33	43
			Max	17	44	27	16	36	79	14	42	36	24	39	22	13	37	38	36	34	76
		Bott	M ± SD	9 ± 4	28 ± 8	17 ± 5	11 ± 4	15 ± 7	18 ± 16	4 ± 2	21 ± 6	25 ± 6	20 ± 4	22 ± 7	6 ± 5	5 ± 3	21 ± 7	23 ± 7	18 ± 5	23 ± 8	11 ± 8
			P95	16	44	25	17	27	48	9	33	35	25	33	15	12	34	35	29	34	27
			Max	20	48	26	20	37	59	11	37	36	26	37	22	14	39	37	32	36	29
	9	Surf	M ± SD	6 ± 4	22 ± 9	17 ± 8	12 ± 6	17 ± 10	25 ± 22	5 ± 2	27 ± 7	29 ± 9	16 ± 4	15 ± 7	8 ± 10	6 ± 3	24 ± 9	24 ± 6	14 ± 4	17 ± 7	15 ± 14
			P95	11	38	28	20	34	66	10	38	38	24	30	32	13	39	35	19	30	44
			Max	15	71	31	23	85	82	13	43	40	25	44	59	16	41	36	22	35	75
		Mid	M ± SD	7 ± 3	25 ± 7	22 ± 5	17 ± 4	19 ± 8	10 ± 9	4 ± 1	22 ± 6	28 ± 8	20 ± 4	20 ± 8	6 ± 7	5 ± 2	22 ± 8	25 ± 6	17 ± 2	22 ± 8	10 ± 7
			P95	12	38	29	22	31	30	7	34	37	26	32	23	8	34	35	21	33	21
			Max	17	48	31	23	34	48	8	39	39	27	39	28	11	39	36	22	36	27
		Bott	M ± SD	7 ± 3	26 ± 7	24 ± 4	19 ± 3	18 ± 8	5 ± 4	3 ± 1	19 ± 5	26 ± 8	20 ± 4	23 ± 8	8 ± 8	4 ± 1	20 ± 6	25 ± 6	18 ± 2	24 ± 8	10 ± 7
			P95	12	37	30	23	32	12	5	29	37	25	36	26	6	30	33	21	36	25
			Max	16	45	32	25	46	29	7	32	39	25	39	37	7	32	35	22	38	27
Upper-estuary	5	Surf	M ± SD	9 ± 6	24 ± 9	10 ± 6	7 ± 5	13 ± 9	36 ± 25	7 ± 3	27 ± 9	23 ± 8	15 ± 4	17 ± 7	11 ± 13	8 ± 5	26 ± 8	24 ± 9	14 ± 6	15 ± 8	13 ± 17
			P95	21	48	20	14	33	75	13	42	33	21	32	42	20	44	37	22	20	53
			Max	27	53	24	31	37	90	16	50	36	23	39	60	23	48	38	23	34	60
		Mid	M ± SD	8 ± 4	31 ± 9	18 ± 4	12 ± 4	14 ± 7	13 ± 14	6 ± 3	25 ± 6	24 ± 6	19 ± 4	20 ± 7	6 ± 5	6 ± 4	21 ± 8	22 ± 8	17 ± 5	22 ± 9	12 ± 10
			P95	19	46	24	17	27	39	11	38	33	24	34	16	16	36	36	23	35	32
			Max	23	53	26	19	35	69	13	41	35	26	39	23	21	46	38	24	37	52
		Bott	M ± SD	9 ± 3	33 ± 7	21 ± 4	15 ± 3	15 ± 8	5 ± 6	5 ± 2	23 ± 5	24 ± 6	20 ± 5	22 ± 6	6 ± 5	5 ± 4	20 ± 8	22 ± 6	17 ± 4	24 ± 9	11 ± 7
			P95	17	46	26	19	29	15	9	34	34	26	31	16	15	39	32	22	35	22
			Max	19	52	29	23	40	31	11	37	36	27	35	36	17	42	34	24	37	38
	7	Surf	M ± SD	9 ± 5	21 ± 9	10 ± 5	6 ± 4	13 ± 7	41 ± 6	7 ± 5	21 ± 8	20 ± 6	16 ± 6	21 ± 9	11 ± 13	9 ± 4	28 ± 9	23 ± 5	15 ± 4	17 ± 7	8 ± 6
			P95	17	38	18	13	24	80	18	51	31	25	33	43	17	45	32	21	30	22
			Max	18	42	23	27	40	90	20	57	34	27	48	51	19	49	35	23	34	30
		Mid	M ± SD	9 ± 4	33 ± 9	16 ± 4	10 ± 3	12 ± 7	16 ± 14	6 ± 4	23 ±	21 ± 3	18 ± 5	24 ± 9	8 ± 5	6 ± 3	22 ± 8	20 ± 4	15 ± 4	24 ± 7	13 ± 8
			P95	19	46	22	14	25	42	14	43	25	24	36	17	13	37	26	20	34	29
			Max	20	50	25	17	31	58	14	46	29	26	38	21	17	43	28	23	36	37
		Bott	M ± SD	9 ± 3	35 ± 8	19 ± 4	12 ± 3	12 ± 7	9 ± 12	5 ± 3	21 ± 9	20 ± 3	18 ± 4	27 ± 8	9 ± 5	6 ± 2	21 ± 7	21 ± 4	16 ± 3	25 ± 7	11 ± 7
			P95	18	46	24	17	22	35	13	42	24	23	37	18	11	35	27	20	36	27
			Max	20	50	27	21	42	72	14	45	26	25	42	32	13	41	29	21	39	32

.

During the neap tide conditions, the 7–35 µm particle size class typically demonstrated the greatest contribution to the overall PSD. This was exclusively the case within the mid- and bottom-waters with the upper-estuary sites being characterized by the greatest contribution of 7–35 µm sizes compared to the lower-estuary site (Table A1). Contributions of the 7–35 µm size class ranged from 21 ± 6% (Site 1 mid-water) to 35 ± 8% (Site 7 bottom-water). Site combined mean contributions of 2.5–35 µm size classes to the total profile PSDs were greatest during neap conditions in the bottom- and mid-water compartments with contributions of 38% and 37%, respectively (Figure 3). Alternatively, the surface-waters throughout the estuary during the neap tide conditions demonstrated the largest observed contribution of any given size class throughout the estuary, being 300–500 µm, with contributions ranging between 25 ± 22% (Site 9) and 55 ± 24% (Site 4, Table A1). During the neap conditions the contribution of the 300–500 µm size class at all sites within the surface waters was greatest and decreased with depth (Table A1). Additionally, the site combined mean contribution of the 130–500 µm size classes to the total profile PSDs were also greatest in the surface-waters (53%) and reduced with depth during neap conditions (Figure 3). The SPM with the larger size class contribution in the near-surface layer during neap conditions did not originate from the seabed but is the result of sediment flocculation. The aggregation of particles is influenced by biological processes, including algae growth and organic gelling, playing an important role in the aggregation and flocculation [4,45,46].

Increased contributions of the larger size class (flocs) in the surface-waters coincided with the (relative) low energy neap tide conditions, characterized by slower maximum current velocities and/or turbulence. Such conditions sustained fragile sediment aggregation due to flocculation at all sites that would be otherwise destroyed (disaggregate) under higher stresses [45,46,48,49] associated with the greater tidal ranges of the transitional and spring tides. Observed SPM sizes (and VC) were strongly controlled by the variable energy conditions operating on the neap and spring cycle timescales, which would also occur during individual tidal cycles [4,50,51].

The destruction of the flocs during greater tidal ranges is supported by the largely reduced mean contributions of the 300–500 µm class to the surface-water PSD during transitional and spring tides (

Table 4. Surface-water mean percentage contribution of the 300–500 µm class to the PSD within the lower- (Site 1), mid- (Sites 3, 4, and 9), and upper-estuary (Sites 5 and 7) sites based on the neap sample conditions and percentage change (∆%) of the size class contribution based on transitional and spring sample conditions in comparison to neap condition contributions.

Zone	Site	300–500 µm Class
		Neap (% Contribution to PSD)	Transitional (∆% vs. Neap)	Spring (∆% vs. Neap)
Lower-estuary	1	40	−48	−75
Mid-estuary	3	33	−82	−76
	4	55	−80	−82
	9	25	−68	−40
Upper-estuary	5	36	−69	−64
	7	41	−73	−80

), which demonstrated changes of −82% to −48% and −82% to −40% based on transitional and spring mean contributions, respectively, compared to the neap condition mean contributions (Table 4).

With changing tidal ranges from neap to spring tides, the tidal influence on the occurrence of larger size classes was apparent. During the neap and transitional conditions, across all sites, the 35–75 µm size class typically dominated the PSD through the water column. Contributions within the low-, mid-, and upper-estuary sites during transitional and spring tide conditions ranged from 27 ± 5% to 30 ± 6%, 23 ± 7% to 29 ± 9%, and 10 ± 6% to 24 ± 6%, respectively (Table A1).

When comparing the depth-averaged contributions across the estuary zones (i.e., zone-based) during the tidal conditions, temporal and spatial patterns were also apparent (Figure 4). During the neap tide conditions, the size classes of 2.5–35 µm collectively contributed the greatest proportion compared to the PSD classes during transitional and spring tide conditions. Additionally, the 130–500 µm classes were also greatest during the neap tide compared to other tidal conditions (Figure 4). During the transitional and spring tide conditions, the 35–130 size classes collectively contributed 47%, 45%, and 39% within the lower-, mid-, and upper-estuary, respectively, during transitional conditions, and 46%, 43%, and 38% within the lower-, mid-, and upper-estuary, respectively, during spring conditions to the total profile PSDs.

The contribution to the total profile PSD during spring tide conditions of the 35–130 µm size classes were reduced during the neap conditions by 28.2%, 32.6% and 31.6%, within the lower-, mid-, and upper-estuary sites, respectively.

In the upper-estuary sites, the mean contribution of the combined 2.5–35 µm size classes were greatest during all sampled tide conditions, accounting for 41%, 30%, and 30% of the total profile PSDs during neap, transitional, and spring tides, respectively. Such contributions were 36.6%, 10.0%, and 13.3% greater when compared to the lower-estuary sites for the corresponding neap, transitional, and spring tide conditions, respectively. Alternatively, within the lower-estuary site the mean contribution of the combined 35–130 µm size classes to the total profile PSD was greatest during all sampled tide conditions (Figure 4). Specifically, during the neap, transitional, and spring tides the corresponding zone-based mean contributions to the total profile were 33%, 47%, and 46%, respectively. In comparison, the zone-based mean contribution of the same classes to the total profile PSD within the upper-estuary sites during the neap, transitional, and spring tide conditions were 21.2%, 17.0%, and 17.4% lower for the corresponding tidal conditions, respectively. In addition, the greatest zone-based mean contribution of the 130–500 µm classes was also observed at the lower-estuary site (44%; neap tide conditions).

4. Conclusions

This technical note presents in situ observations of spatial and temporal variations in suspended particle VC and PSDs measurements using a LISST within the macrotidal estuary of Port Curtis. The results from the field surveys further document the influence of tidal conditions on SPM dynamics and relative changes in concentrations and PSDs as a result of varying tidal current conditions operating on the neap and spring cycle timescale.

Mid-estuary sites were characterized by the greatest depth-averaged VC under transitional and spring conditions, whilst depth-averaged VC demonstrated a clear trend towards increasing concentrations with increasing tidal ranges (i.e., spring tides VC > transitional tides VC > neap tides VC). Estuary-wide mean spring tide total profile concentrations during peak current speeds demonstrated a near 300% increase in comparison to concentrations observed during the neap tide (minimum current speeds) conditions.

Particle size distributions attributable to resuspension and flocculation processes were also shown to demonstrate spatial and temporal patterns operating on the neap and spring cycle timescale. In the upper-estuary sites, the mean contribution of the 2.5–35 µm size classes to the total profile PSDs were greatest during all sampled tide conditions. Alternatively, within the lower-estuary site the mean contribution of the 35–130 µm size classes were greatest during all sampled tide conditions. Surface-waters throughout the estuary during the neap tide conditions demonstrated the largest observed contribution of any given size class throughout the estuary, 300–500 µm. Comparisons of the mean contributions of the 300–500 µm class to the surface-water PSD during transitional and spring tides, compared with the neap tide conditions, demonstrated changes of −82% to −48% and −82% to −40%, respectively. Such variations in SPM size resulted from differences in aggregation and disaggregation behaviors of SPM particles, which in turn, is in-part influenced by current velocities.

Although temporal and spatial trends were observed during this study, it is important to note that the identified tidally induced patterns operating on the neap and spring cycle timescale, will presumably be complicated according to event-based influences such as sustained strong wind speeds increasing wave action, large freshwater input during periods of high rainfall, and/or influences from adjoining coastal processes. The occurrence of rainfall induced longitudinal or lateral salinity gradients during large summer-time rainfall events, in addition to increased SPM inputs, will expectedly influence SPM dynamics, however, such investigation was beyond the scope of this study. Such events should be considered additionally to provide a more robust approach in understanding potential PSD and VC dynamics within estuaries, especially due to the importance of these events in the suspension, transportation, and delivery of SPM, and the influence of salinity regimes on VC and PSD sizes [4,23,50].

Overall, the results from this case study provide further evidence of the important influence of neap and spring tidal regimes on SPM dynamics within estuarine settings and the need to observe/acknowledge parameter dynamics on such timescales. Future like-minded studies within this, and other, estuaries are encouraged to investigate how much of observed SPM dynamics are generated within the estuary in comparison to the adjoining coastal zone.