Research Article: Towards Cost-Effective Operational Monitoring Systems for Complex Waters: Analyzing Small-Scale Coastal Processes with Optical Transmissometry

Date Published: January 20, 2017

Publisher: Public Library of Science

Author(s): Marta Ramírez-Pérez, Rafael Gonçalves-Araujo, Sonja Wiegmann, Elena Torrecilla, Raul Bardaji, Rüdiger Röttgers, Astrid Bracher, Jaume Piera, Francois G. Schmitt.


The detection and prediction of changes in coastal ecosystems require a better understanding of the complex physical, chemical and biological interactions, which involves that observations should be performed continuously. For this reason, there is an increasing demand for small, simple and cost-effective in situ sensors to analyze complex coastal waters at a broad range of scales. In this context, this study seeks to explore the potential of beam attenuation spectra, c(λ), measured in situ with an advanced-technology optical transmissometer, for assessing temporal and spatial patterns in the complex estuarine waters of Alfacs Bay (NW Mediterranean) as a test site. In particular, the information contained in the spectral beam attenuation coefficient was assessed and linked with different biogeochemical variables. The attenuation at λ = 710 nm was used as a proxy for particle concentration, TSM, whereas a novel parameter was adopted as an optical indicator for chlorophyll a (Chl-a) concentration, based on the local maximum of c(λ) observed at the long-wavelength side of the red band Chl-a absorption peak. In addition, since coloured dissolved organic matter (CDOM) has an important influence on the beam attenuation spectral shape and complementary measurements of particle size distribution were available, the beam attenuation spectral slope was used to analyze the CDOM content. Results were successfully compared with optical and biogeochemical variables from laboratory analysis of collocated water samples, and statistically significant correlations were found between the attenuation proxies and the biogeochemical variables TSM, Chl-a and CDOM. This outcome depicted the potential of high-frequency beam attenuation measurements as a simple, continuous and cost-effective approach for rapid detection of changes and patterns in biogeochemical properties in complex coastal environments.

Partial Text

Coastal regions are highly dynamic and productive ecosystems, with high ecological and economic values [1]. They are also vulnerable areas subjected to considerable anthropogenic pressures through urban and industrial development, pollution, fisheries, agriculture and aquaculture, recreation, etc. These pressures have caused, in many cases, habitat degradation carrying serious environmental and economic consequences, such as harmful algal blooms (HABs), anoxia, accumulation of pollutants and toxins or over-exploited fisheries [2]. Added to this, the effects of climate change and natural hazards are threatening the capability of coastal ecosystems to support goods and valuable services [3]. For these reasons, increasing national and international efforts have been addressed over the last decades to establish and implement environmental strategies for preservation, conservation and sustainable use of these ecosystems. In accordance with the requirements of the European Water Framework Directive (WFD, 2000/60/EC) and the Marine Strategy Framework Directive, new interdisciplinary research programs are successfully being carried out such as the coastal module of the Global Ocean Observing System (Coastal GOOS), the Coastal Observing System for Northern and Arctic Seas (COSYNA), the Global Earth Observations System of Systems (GEOSS) or the Marine Geological and Biological Habitat Mapping (GEOHAB). All those programs have been conceived to monitor, forecast and assess the state of coastal ecosystems, which involve integrated, multidisciplinary and multiscale observing systems. Coastal environments are governed by complex physical and biogeochemical processes and thus, undergo changes over a broad range of time-space scales. Continuous and routine provision of data is therefore required to assess the states of these ecosystems, detect changes in these states and evaluate their impacts [4]. In addition, there is a demand for compact, inexpensive, stable and low power in situ sensors to enable sustainable and long-term monitoring.

At first, the results from validating the above-mentioned beam attenuation-based proxies with laboratory-measured biogeochemical variables are presented. Secondly, the temporal and spatial variability and patterns of these optical and biogeochemical parameters in Alfacs Bay are shown.

Continuous measurements of spectral beam attenuation coefficient collected in situ with an advanced-technology transmissometer have been proven as a powerful tool to better understand the existing interactions between physical and biogeochemical variables in the complex estuarine waters of Alfacs Bay (NW Mediterranean). In particular, this approach allowed the detection of qualitative changes in the major biogeochemical variables (i.e. Chl-a, TSM and CDOM) at high temporal and spatial scales in this microtidal estuary. Spatial patterns observed in the biogeochemical properties were driven by the vertical stratification of the water column. Accordingly, surface and bottom water layers were characterized by a different relative contribution of the major biogeochemical variables to the bulk beam attenuation. Meanwhile, observations along the 48 hours time series revealed a coupling between physical (meteorological and hydrodynamic conditions) and biogeochemical properties, since the prevailing hydrodynamic regimes determined the variations in water composition. The temporal and spatial patterns were obtained based on the spectral features of the total non-water beam attenuation coefficient and validated with laboratory results of discrete water samples (i.e. biogeochemical variables and partitioned absorption coefficients). Significant linear relationships were found between the non-water beam attenuation proxies and the biogeochemical variables. However, for future studies, it is highly recommended to include in situ beam attenuation measurements of 0.2 μm-filtered seawater for better CDOM characterization. The proposed proxies are subject to numerous uncertainties due to several factors affecting the attenuation signal (CDOM absorption and particle characteristics such as size, shape, composition, etc., which determine their absorption and scattering properties). For this reason, the collection of discrete water samples for laboratory analysis of biogeochemical variables is required for validation purposes.