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.

Distribution of iron porphyrin like complexes along the land sea continuum of the Iroise Sea

<p>The aim of FeLINE project (Fer Ligands In the aulNe Estuary) was to determine the distribution of iron and associated ligands concentrations along the land sea continuum of the Iroise Sea (Bay of Brest, France). Iron porphyrin like ligands (Fe-Py) such as heme and hemoproteins are relevant complexes in iron biogeochemical cycling as they can persist in seawater and on marine particulates. This work reveals for the first time the distribution of Fe-Py concentrations (dissolved plus reactive particulate) along a temperate macrotidal estuary. Unfiltered samples were collected in October 2019 across a transect of the Aulne river and estuary / Rade of Brest / Iroise Sea during low tidal coefficient (39). Fe-Py concentrations were determined using flow injection analysis with chemiluminescence detection adapted from Vong et al. (2007). Various interferences (organic, metallic, pH and salinity) were tested. The detection limit attained was 11 pmol.l<sup>-1</sup> and the time of analysis 1min30s per sample. The Fe-Py concentrations varied from 0.007 ±0.002 nmol.l<sup>-1 </sup>for S=33.98 and 1.177 ±0.007 nmol.l<sup>-1 </sup>for S = 0.92. The Fe-Py concentrations clearly showed a non-conservative behavior due to various processes other than simple mixing of natural and seawater. The highest values revealing a Fe-Py enrichment were observed in the Estuarine Turbidity Maximum (ETM) for which concentrations varied between 1.177 ±0.007, S = 5.2 and 0.738 ±0.004 nmol.l<sup>-1</sup> S = 8.59. This positive anomaly of Fe-Py concentrations (40%) also corresponded to the lowest pH values (pH =7.27-7.32). The distal part of the transect displayed a negative anomaly for salinities comprised between 15 and 25 (loss of 37%). The four last points geographically corresponding to the Bay of Brest (S>35) exhibited low and stable Fe-Py concentrations of 0.007±0.002 and 0.024 ± 0.003 nmol.l<sup>-1</sup>. The supply and removal fluxes were respectively estimated at 2.4±0.2g/d and 8.1 ± 0.8g/d, revealing an average Fe-Py removal of 39.8% that is probably due to particle flocculation.</p>