Relationships between suspended particulate material, light attenuation and Secchi depth in UK marine waters

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Abstract

Measurements of sub-surface light attenuation (Kd), Secchi depth and suspended particulate material (SPM) were made at 382 locations in transitional, coastal and offshore waters around the United Kingdom (hereafter UK) between August 2004 and December 2005. Data were analysed statistically in relation to a marine water typology characterised by differences in tidal range, mixing and salinity. There was a strong statistically significant linear relationship between SPM and Kd for the full data set. We show that slightly better results are obtained by fitting separate models to data from transitional waters and coastal and offshore waters combined. These linear models were used to predict Kd from SPM. Using a statistic (D) to quantify the error of prediction of Kd from SPM, we found an overall prediction error rate of 23.1%. Statistically significant linear relationships were also evident between the log of Secchi depth and the log of Kd in waters around the UK. Again, statistically significant improvements were obtained by fitting separate models to estuarine and combined coastal/offshore data – however, the prediction error was improved only marginally, from 31.6% to 29.7%. Prediction was poor in transitional waters (D = 39.5%) but relatively good in coastal/offshore waters (D = 26.9%).

SPM data were extracted from long term monitoring data sites held by the UK Environment Agency. The appropriate linear models (estuarine or combined coastal/offshore) were applied to the SPM data to obtain representative Kd values from estuarine, coastal and offshore sites. Estuarine waters typically had higher concentrations of SPM (8.2–73.8 mg l−1) compared to coastal waters (3.0–24.1 mg l−1) and offshore waters (9.3 mg l−1). The higher SPM values in estuarine waters corresponded to higher values of Kd (0.8–5.6 m−1). Water types that were identified by large tidal ranges and exposure typically had the highest Kd ranges in both estuarine and coastal waters. In terms of susceptibility to eutrophication, large macrotidal, well mixed estuarine waters, such as the Thames embayment and the Humber estuary were identified at least risk from eutrophic conditions due to light-limiting conditions of the water type.

Introduction

Many recent European and US directives aimed at the assessment of eutrophication in marine waters include some measurement of nutrients and phytoplankton and look to describe the “risk” of undesirable biological response to nutrient enrichment (Tett et al., 2007). Our understanding of the process of nutrient enrichment and its causative influence on eutrophication symptoms is an important component of any eutrophication assessment of marine waters. Recent changes in our conceptual understanding of eutrophication (Cloern, 1999, Cloern, 2001, Costanza and Mageau,1999 Tett et al., 2007), suggest that there are complex direct and indirect responses to anthropogenic nutrient inputs (Nixon, 1995). In addition ‘filters’ play a role in determining the sensitivity to enrichment and, in marine waters, these include the light climate and advective loss (Cloern, 1987, Bricker and Stevenson, 1996). Given this complexity, the process of linking anthropogenic nutrient enrichment to biological response is not a trivial task.

A detailed assessment that would be required to achieve this and to cover all UK near-shore marine waters does not seem feasible or indeed scientifically justified. A more pragmatic approach is to first screen each water body to determine susceptibility to the impact of anthropogenic nutrient enrichment. Traditionally, identification of risk has relied on nutrient loading and/or observed winter nutrient concentration. While this approach is useful for initial screening, to identify those water bodies receiving high anthropogenic nutrient load, it ignores the question of susceptibility from a biological perspective, thus does not assess the limits of the system to sustain production. Nutrient enrichment alone does not diagnose eutrophication and consideration is needed of key physical characteristics of water bodies, which may modify the response of the dominant form of aquatic plant life (phytoplankton, macrophytes or angiosperms) and provide an additional level of screening.

For aquatic plants, the sub-surface light climate has a major influence on growth (Boynton et al., 1982, Bricker et al., 1999, Gallegos, 2001, May et al., 2003) particularly in inshore and near-shore environments where a high level of suspended particulate material may severely restrict the availability of light (Bowers et al., 2000, Mills et al., 2002, Painting et al., 2007). The amount of photosynthetically active radiation (PAR) in natural waters is of fundamental importance in determining the growth of aquatic plants. Primary production by phytoplankton is a light dependent process that provides the energy to drive the plankton and microbial food web that typically takes place down to depths to which about 1% of surface light penetrates (i.e., the euphotic zone). Absorption and refraction by water, and dissolved and suspended matter determine the quantity and the spectral quality of light at a given depth (Jerlov, 1968, Jerlov, 1976, Prieur and Sathyendranath, 1981), which in turn affects the photosynthesis of aquatic plants. Characterisation of the sub-surface light climate could therefore provide a means of screening water bodies for biological susceptibility to changes in ecosystem structure and function. A high level of detail has been used previously to characterise UK marine waters by the availability of light, with detailed studies on the interaction between inherent optical properties (absorption and adsorption coefficients and backscattering ratios) and apparent optical properties (diffuse attenuation coefficient and radiance reflectance; Bowers et al., 2000, McKee et al., 2003, Bowers and Binding, 2006, McKee and Cunningham, 2006). There has been considerable success in relating the optical properties inherent in the waters to characterise “water type”, including features such as ratios of particulate backscattering to non-water absorption (Bowers and Binding, 2006, Morel et al., 2006). Studies by McKee and Cunningham (2006) also characterise two optical water types differing in the ratio of particulate backscattering to non-water absorption at 676 nm, the ratio of non-water absorption coefficients and ratio of particulate scattering to non-water absorption. Further work on the difference between case 1 (light attenuation controlled primarily by phytoplankton) and case 2 waters (light attenuation controlled primarily by SPM and CDOM) showed variation in the empirical relationship between colour ratios and pigments and total suspended solids (Kratzer et al., 2000).

Detailed studies such as these provide far more detail to the ability of the marine water type to attenuate light through the water column and the potential of the system to limit or encourage the production of phytoplankton. However, there has been little systematic assessment of the sub-surface light regime across UK marine waters and it is difficult to characterise water types dependent on any part of their optical properties over such large variable marine areas. It would be almost impossible to characterise each water type using a full suite of optical properties, so a simpler method was investigated using statistical examination of in situ measurements of apparent optical properties and an optically significant constituent. This one-dimensional model was tested to observe if predictions of light attenuation could be derived from only one optical component.

Optically significant constituents which influence light attenuation include chromophoric dissolved organic matter (Kostoglidis et al., 2005, Foden et al., submitted for publication) suspended particulate matter (SPM) (Mills et al., 2002) and phytoplankton biomass (McMahon et al., 1992, Tett, 1992). The relative contributions of these different components to total light attenuation in estuaries have been studied in many marine waters (Gallegos et al., 1990, Kirk, 1994). Understanding the contributions by different constituents responsible for attenuation of PAR is important in predicting the underwater light climate from the constituent concentrations (Bowers et al., 2000, Kostoglidis et al., 2005). The primary light-attenuating constituent in near-shore marine waters may vary from CDOM (Kirk, 1976, Bowling, 1988, Kostoglidis et al., 2005) to phytoplankton (Dubinsky and Berman, 1979), to inorganic suspended matter (Mills et al., 2002) or some combination of these constituents (Heinermann and Ali, 1988). The shallow depth and the large tidal movement in most near-shore marine types in UK waters make these systems fundamentally different from deeper clearer marine waters. Coastal lagoons and shallow estuaries have high sediment surface area to water volume ratios, frequent wave resuspension of sediments, and low pelagic and high benthic primary productivity because most of the sediment surface is in the photic zone (Sand-Jensen and Borum, 1991). These features suggest that sediment resuspension, not increased pelagic productivity, may be the dominant control on light availability in UK coastal waters (Bowers, 2005). Studies in the Indian River Lagoon (Gallegos and Kenworthy, 1996), the Lagoon of Venice (Zharova et al., 2001) and Hog Island Bay, Virginia (Lawson et al., 2007) have also shown suspended sediment to control light availability in these coastal systems.

It is clear that the availability of good evidence regarding the light climate in the transitional and coastal waters of the UK is a limiting factor in the ability to undertake credible and effective risk assessments. This paper investigates the non-trivial relationship between suspended particulate matter and apparent optical properties as measured by diffuse attenuation coefficient. It presents the results of a spatially extensive survey of suspended particulate matter (SPM), light attenuation (Kd) and Secchi depth in UK and Irish waters (in the western Irish Sea). The focus is on a statistical examination of the empirical relationships between Kd and SPM and between Kd and Secchi depth. We demonstrate that the significant linear relationships between SPM and Kd for UK marine waters can be used to predict Kd from SPM. We also show that the significant relationship between Secchi depth and Kd in some types of marine waters can also be a useful predictor of light attenuation in UK waters.

Section snippets

Survey design

The study was carried out between August 2004 and December 2005. A total of 382 locations were visited (Fig. 1). Sampling took place from bridges for some estuarine sites and small boats for spatial sampling in inshore areas. In July 2005, additional sampling was undertaken in the Clyde sea area and Irish Sea (including Irish coastal waters of the western Irish Sea) during a Department of Agriculture and Rural Development (DARD) RV Corystes cruise.

Marine water types

Discrimination of marine water types have been

SPM and Kd data for model development

Over the time of this study 382 light and SPM profiles were taken in UK marine waters. The highest values of SPM are measured in transitional types. Calculated averages for each type for SPM and Kd are shown in Fig. 2.

Discussion

The aim of this paper was to present results from extensive measurements of the diffuse down-welling PAR attenuation coefficient (Kd), SPM and Secchi depth in UK transitional, coastal and offshore waters, and to analyse these results statistically in relation to marine water type (Rogers et al., 2003). Significant relationships were obtained between SPM and Kd, and between Secchi depth and Kd. The Kd–SPM relationship was used to augment the database of 382 Kd calculated from submarine

Acknowledgements

This work was funded by the Defra contract (E2202) Eutrophication Thematic Programme and by DARD. The authors would like to thank Environment Agency staff who assisted in field work, the Captain, Officers and Crew of the RV Corystes for their assistance during the Clyde Sea and Irish Sea survey in July 2005. The assistance of Mr B. Stewart (DARD) Dr. K. Kennington and Miss A Harrison (University of Liverpool) during the RV Corystes cruise is gratefully acknowledged. Thanks to David Bowers and

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    Present address: Fisheries and Aquatic Ecosystems Branch, AFESD, Agri-Food and Biosciences Institute, Newforge Lane, Belfast, BT9 5PX, Northern Ireland, UK.

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