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

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.


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

Study Location
Port Curtis estuary situated on the east coast of Australia (Figure 1), is a macrotidal estuary covering an area of approximately 200 km 2 . 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 × 10 6 m 3 , 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]. 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].

Field Data Collection
Six survey days were completed in Port Curtis estuary between 15th September to 6th October 2010 during neap, transitional, and spring tide conditions (Table 1). 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: 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].

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). 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.

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 logspaced 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].

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.
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 depthaveraged 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). When comparing zones across all tidal conditions, the mid-estuary depthaveraged 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 lowerestuary 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 surfacewaters compartment. However, although maximum concentrations were observed in the surface waters, total profiles typically demonstrated increased VC within the bottomwaters at the mid-and upper-estuary sites. This is supported by the greater 95th percentile (P 95 ) 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 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 surfacewaters 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 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), depthaveraged 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). 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-, midand 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].

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.
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 bottomwater). 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 surfacewaters 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), 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.     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. 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).

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.