Can Forel–Ule Index Act as a Proxy of Water Quality in Temperate Waters? Application of Plume Mapping in Liverpool Bay, UK

Abstract

The use of ocean colour classification algorithms, linked to water quality gradients, can be a useful tool for mapping river plumes in both tropical and temperate systems. This approach has been applied in operational water quality programs in the Great Barrier Reef to map river plumes and assess trends in marine water composition and ecosystem health during flood periods. In this study, we used the Forel–Ule colour classification algorithm for Sentinel-3 OLCI imagery in an automated process to map monthly, annual and long-term plume movement in the temperate coastal system of Liverpool Bay (UK). We compared monthly river plume extent to the river flow and in situ water quality data between 2017–2020. The results showed a strong positive correlation (Spearman’s rho = 0.68) between the river plume extent and the river flow and a strong link between the FUI defined waterbodies and nutrients, SPM, turbidity and salinity, hence the potential of the Forel–Ule index to act as a proxy for water quality in the temperate Liverpool Bay water. The paper discusses how the Forel–Ule index could be used in operational water quality programs to better understand river plumes and the land-based inputs to the coastal zones in UK waters, drawing parallels with methods that have been developed in the GBR and Citclops project. Overall, this paper provides the first insight into the systematic long-term river plume mapping in UK coastal waters using a fast, cost-effective, and reproducible workflow. The study created a novel water assessment typology based on the common physical, chemical and biological ocean colour properties captured in the Forel–Ule index, which could replace the more traditional eutrophication assessment regions centred around strict geographic and political boundaries. Additionally, the Forel–Ule assessment typology is particularly important since it identifies areas of the greatest impact from the land-based loads into the marine environment, and thus potential risks to vulnerable ecosystems.

Using Optical Water-Type Classification in Data-Poor Water Quality Assessment: A Case Study in the Torres Strait

“…However, the optical water type maps produced provided important information about water turbidity gradients in the study area, as well as information about their spatial and seasonal variability for the first time. The colour of waters is increasingly used as used an efficient optical feature to cluster OWTs in both inland and marine waters and to assess water quality patterns and trends (e.g., [51][52][53][54][55][56][57]). This study reiterated the potential of using a qualitative colour clustering approach with satellite images in large, complex and/or data-poor regions, where more classical quantitative remote sensing methods are limited by the lack of available field data.…”

“…Water colour is the result of the presence of marine constituents (SSC, chlorophyll-a [Chl-a] and coloured dissolved organic matter [CDOM]) and their interactions with solar irradiance [49]. Several studies have reported that there is a quantifiable relationship between water colour and water quality parameters, including a strong relationship with turbidity [50][51][52]. Accordingly, water colour is regarded as a simple yet efficient optical parameter for assessing water quality changes [14,53].…”

A Probabilistic Approach to Mapping the Contribution of Individual Riverine Discharges into Liverpool Bay Using Distance Accumulation Cost Methods on Satellite Derived Ocean-Colour Data

“…The presence of the river plume from the satellite-derived FUI values was determined using established techniques [29,30]. All the monthly median FUI layers were reclassified…”

“…In turn, the offshore assessment areas used to assess eutrophication under the OSPAR Comprehensive Procedure [18] can extend to larger areas, diluting the impact of these coastal waters. Recent work [29] has identified plume extents from Liverpool Bay that extend further than the WFD assessment areas, but are still relatively nearshore. The nutrient and plankton dynamics in these plume areas need to be considered in their own right, and not as the edge of the WFD or the initial zone in the OSPAR offshore area [18].…”

“…The nutrient and plankton dynamics in these plume areas need to be considered in their own right, and not as the edge of the WFD or the initial zone in the OSPAR offshore area [18]. Mapping plume waters using remote sensing and the water quality gradients within has been used successfully in monitoring programs, providing a tool to increase the data frequency and connect catchment activities to the transport of pollutants into the marine environment [29][30][31].…”