Do tidal fluctuations affect microplastics distribution and composition in coastal waters?

The hydro-meteorological conditions in marine environments are recognized to have a major impact on the transport and dispersion of microplastics (MP), although their precise effects remain poorly understood. This study investigates the effects of tidal fluctuations on MP abundance and composition in a megatidal coastal water. Waters samples were collected every ninety minutes over the course of two complete tidal cycles - one during spring tide and another during neap tide. There were no significant disparities in term of abondance, size, and composition of MPs between the samples collected during the two tidal cycles. Nevertheless, MP abundance and characteristics (morphology, size and polymer types) can be influenced over the course of a complete tidal cycle due to the impact of tidal currents and water height. This study highlights the need to consider the fluctuations of the tidal cycle when planning in-situ surveys to better assess MP pollution in coastal environments.

Temporal dynamics of lateral carbon export from an onshore aquaculture farm

Many coastal areas are hotspots of aquaculture expansion, where the overuse of artificial feeds results in the accumulation of organic carbon in nearshore aquaculture ponds. In rural areas, wastewater from the aquaculture ponds is discharged to the nearshore waters through artificial ditches causing lateral carbon export from the land to the ocean. Such flux may be meaningful in coastal carbon budgets since aquaculture is the hotspot of carbon sequestration and storage. To quantify the magnitude and temporal dynamics of lateral carbon export from aquaculture ponds, we used high-frequency in-situ monitoring of turbidity, fluorescent dissolved organic matter, etc. across different temporal scales. We measured water levels and velocity profiles in a ditch cross-section to obtain year-round water exchange. Carbon export was integrated from water fluxes and organic carbon concentrations. Our results suggested that aquaculture ponds were a source of particular organic carbon (POC) and dissolved organic carbon (DOC). The net lateral flux of POC and DOC was 148 ± 38 kg yr^-1 and 296 ± 18 kg yr^-1. Temporally, the export of POC and DOC is influenced by both tides and wastewater discharge. Under the disturbance with aquaculture wastewater discharge, the mean DOC export in the ditch increased by 497 kg, which was 1.5 times that of the undisturbed; the mean POC export increased by 190 kg, which was 1.8 times that of the undisturbed. Thus, aquaculture activities can considerably disturb the coastal carbon balance by facilitating carbon-rich fluid exchange from onshore farms to nearshore estuaries. As aquaculture expands across Asia and the globe, this study provides important insights into the impacts of aquaculture on coastal carbon budgets.

Vertical distribution and river-sea transport of microplastics with tidal fluctuation in a subtropical estuary, China

The river-sea transport of microplastic with complex environmental conditions and diverse driving factors has received growing attention in the estuary. This research investigated the vertical distribution of microplastics in the water column and surface sediments and explored the effect of tidal variation on the transport of microplastics in Jiulong Estuary and Xiamen Bay, China. Results show that the microplastics in the estuary (630 ± 515 μm) was significantly larger than that in the bay (344 ± 420 μm, p < 0.01). Low-density microplastics are present in the whole water column, while high-density microplastics was apt to accumulate in the bottom water and surface sediment suggesting biofouling and material density of microplastics synergistic affect its vertical distribution. Every 1-2 h high-frequency samples collected in a whole tide found the increase of fine size (45-300 μm) and decrease of large size (>300 μm) in the flood tide, which implied fine microplastics were easily driven into the estuary from the bay at flood tide than large microplastics. The abundance of microplastics in the sediments decreased in the fast-rising and fast-falling period implies the tide influences the fragmentation and resuspension of microplastics in the estuary. Finally, the flux of microplastics entering Xiamen Bay was 53.5 t/month in the moderate flow month were estimated based on the abundance of different water layers instead of floating microplastics in the surface water.

Horizontal variation of microplastics with tidal fluctuation in the Chao Phraya River Estuary, Thailand

Microplastic (MP) pollution in estuarine environments is poorly characterized globally, although they are extensive buffer regions between terrestrial, freshwater and seawater environments. This research aims to investigate MP pollution levels and variations of MPs abundance with tidal fluctuation. Fourteen samples were collected from the surface water of the Chao Phraya River Estuary, Thailand using the Manta net at flood and ebb tides. The average abundance of microplastics at flood tide was 5.16 × 10⁵ particles/km² and at ebb tide was 3.11 × 10⁵ particles/km². The abundance of microplastics in the estuary was directly related to the tidal fluctuation, creating an accumulation of microplastics in the study area. Polypropylene, polyethylene, and polystyrene were the most common polymers. The findings provide important information on the pollution status of microplastics in the Chao Phraya River Estuary and the variation of suspended microplastics with tidal fluctuation should be considered in future estuarine microplastic studies.

Integrating culture and molecular quantification of microbial contaminants into a predictive modeling framework in a low-lying, tidally-influenced coastal watershed

Examinations of stormwater delivery in the context of tidal inundation are lacking. Along the coastal plains of the southeastern United States, tidal inundation is increasing in frequency and severity, often with dramatic adverse impacts on timely stormwater discharge, coastal flooding hazards, and even "sunny day flooding". Therefore, a comprehensive study was conducted to examine tidally-influenced stormwater outfalls discharging to Taylor's Creek, an estuary proximal to Beaufort, NC used regularly for recreation and tourism. Over a wide range of meteorological conditions, water samples were collected and analyzed for fecal indicator bacteria (FIB, used for water quality management) and previously published quantitative microbial source tracking (qMST) markers. Nineteen sampling events were conducted from July 2017-June 2018 with samples classified according to tidal state and defined as either inundated, receding, or transition. A first-of-its-kind multiple linear regression model was developed to predict concentrations of Enterococcus sp. by tidal cycle, salinity and antecedent rainfall. We demonstrated that the majority of variability associated with the concentration of Enterococcus sp. could be predicted by E. coli concentration and tidal phase. FIB concentrations were significantly (<0.05) influenced by tide with higher concentrations observed in samples collected during receding (low) tides (EC: log 3.12 MPN/100 mL; ENT: 2.67 MPN/100 mL) compared to those collected during inundated (high) (EC: log 2.62 MPN/100 mL; ENT: 2.11 MPN/100 mL) or transition (EC: log 2.74 MPN/100 mL; ENT: 2.53 MPN/100 mL) tidal periods. Salinity, was also found to significantly (<0.05) correlate with Enterococcus sp. concentrations during inundated (high) tidal conditions (sal: 17 ppt; ENT: 2.04 MPN/100 mL). Tide, not precipitation, was shown to be a significant driver in explaining the variability in Enterococcus sp. concentrations. Precipitation has previously been shown to be a driver of Enterococcus sp. concentrations, but our project demonstrates the need for tidal parameters to be included in the future development of water quality monitoring programs.

Tidal induced dynamics and geochemical reactions of trace metals (Fe, Mn, and Sr) in the salinity transition zone of an intertidal aquifer

Biogeochemical reactions in an intertidal aquifer influences the submarine groundwater discharge (SGD) associated trace metal flux to the ocean. Tidal fluctuation greatly affects the physical mixing, and biogeochemical transformation of trace metals in the intertidal aquifer. This study presents the dynamics of trace metals (Fe, Mn, and Sr) and the production of Fe²⁺ in the salinity transition zone is discovered. The variations of Fe²⁺ are led by the shifts of both physical mixing and biogeochemical reaction during tidal fluctuation. The transformation from amorphous Fe(OH)₃ to FeS is the main reason for the enrichment of Fe²⁺ in the zone with a salinity of 0.5-10. Mn behaves much less active than Fe in the intertidal aquifer due to the very limited Mn in the solid phase and the major driving force of Mn²⁺ variation is the physical mixing rather than geochemical reaction. Sr²⁺ behaves conservatively and shows a synchronous with salinity in the salinity transition zone. This study found that Fe²⁺ precipitates in a form not limited to Fe (hydro)oxides and the FeS minerals is the most possible form of precipitation in reduced aquifers. In that case, only a small part of Fe²⁺ discharges to the sea associated with SGD, but Mn²⁺ has a comparatively conservative property during the transport in the intertidal aquifer and majority of the Mn²⁺ originated from fresh groundwater will discharge with SGD in this study. The biogeochemical transformation pathways of Fe and Mn observed in this study provides insights into the cycles of Fe and Mn in an intertidal aquifer, which is of significance to accurately estimate the SGD derived Fe and Mn fluxes to the ocean.