Turbidity

Entry to the marine environment

Recorded levels in the marine environment

Effects on the marine environment

Potential effects on interest features of European marine sites

The issues surrounding the potential effects of turbidity on nature conservation interests in the marine environment have been recently reviewed by Parr et al 1998. The reader is referred to this document for further details.

Entry to the marine environment

Turbidity is a measure of the attentuation of light in the water column and can be caused by the light adsorption properties of the water, plankton, suspended particulate organic and inorganic matter and dissolved colour.

Turbidity can be measured in a variety of ways, the results of which are not usually inter-convertible:

  • light transmission (transmissometers),
  • light scattering (nephelometry),
  • scattering and transmission (ratiometric turbidimeters),
  • secchi disk, total suspended solids (sample filtered and dried at 105 C),
  • ash-free suspended solids (sample filtered and dried at 500 C) and remote sensing.

In addition to the above, Parr et al (1998) also discussed the options for measuring the depth of macroalgal colonisation as a turbidity-monitoring tool, in combination with a measure of epiphyte density (which could itself be used as a measure of trophic status).

Turbidity is due primarily to suspended particulate matter, but a relatively minor contribution to light adsorption in the water column (less than in freshwaters) may be made by dissolved/colloidal humic substances, often referred to as gilvin (Kirk 1994).

Particulate matter can be derived from (Parr et al 1998):

  • soil (land) erosion;
  • coastal erosion;
  • sediment resuspension (including dredging);
  • suspended solids in discharge effluents;
  • phytoplankton standing crop; and
  • chemical flocculation at the freshwater/saltwater interface in estuaries.

Of the above, sediment resuspension represents the major source in most cases. The building of flood defence structures, such as sea walls, while preventing coastal erosion at the site itself, may transfer wave energy further along the coast, leading to erosion in another place. Dredging may make a large contribution to suspended solids, depending on tidal cycle. For example, in the Loire estuary during spring tides, dredging contributes about 5% of suspended particulate matter, but during neap tides, dredging may account for 20% (Sauriau et al 1994).

Recorded levels in the environment

Parr et al (1998) quoted annual mean suspended solids (105 C) and ash-free suspended solids (levels 500 C) around the English and Welsh coast of 1 - 327 and 1 - 227 mg l-1 respectively. However, the extreme upper values disguise the fact that, at the majority of sites, annual mean levels are more typically 1 - 110 mg l-1 for both parameters. Higher levels are found in the mid/inner regions of the Firth of Severn (maximum values in the inner firth), around the Solway Firth (up to 20 mg l-1) and along the East coast from Flamborough Head south to east Kent (30 - 100 mg l-1 at 13 sites and >100 mg l-1 at 2 sites).

In estuaries, turbidity levels are usually much higher than those in adjacent coastal waters, with peak levels confined to a discrete region (the turbidity maximum), usually in the upper-middle reaches, which moves up and down the estuary with the tidal ebb and flow. The level of suspended solids depends on a variety of factors including: substrate type, river flow, tidal height, water velocity, wind reach/speed and depth of water mixing (Parr et al 1998). The level of suspended solids can be enhanced by anthropogenic activities in the river catchment as well as within the river and the estuary. Changes in river flow as a result of abstraction can influence suspended solids concentrations reaching estuaries.

Effects in the marine environment

The effects of non-toxic substances, such as turbidity, on the marine environment can be sub-divided into direct effects (those organisms directly affected by changes in turbidity) and secondary effects (those arising in the ecosystem as a result of changes in the organisms directly affected).

Direct effects

The direct effects of high levels of turbidity in the water column include:

  • a reduction in phytoplankton biomass in the presence of high concentrations of suspended particulate matter (especially in estuaries) because of the reduction of available light;
  • a reduction in growth rates, areal coverage and depth of colonisation of macrophytes and macroalgae (including kelp) where turbidity is high;
  • some evidence of adverse effects on zooplankton which can become associated with turbidity maxima in estuaries as a result of physical processes governing the movement of particles;
  • turbidity or suspended solids can affect benthic invertebrate communities both when the particles are in suspension and when they are deposited;
  • turbidity or suspended solids can directly affect fish populations.

In many coastal plain estuaries, light attentuation confines the photic zone to a fraction of the water column such that light availability is the limiting factor on phytoplankton productivity despite high nutrient levels in many cases (Parr et al 1998). However, high turbidity levels do not necessarily preclude high phytoplankton standing crops. Where rapid and complete mixing of the water column allows algal cells to have some exposure to light at the surface, phytoplankton biomass can increase where turbidity is high (Parr et al 1998).

Parr et al (1998) identified the effects of high turbidity on macrophytes, including Zostera, and on macroalgae, including Laminaria spp. and Fucus vesiculosus. Reduced growth rates, standing crop, areal coverage and depth of colonisation have been reported to be related to turbidity. Parr et al (1998) stressed the importance of the role of periphyton responding to increased nutrient concentrations in reducing the available light for these plants when attributing the observed effects to turbidity levels in the water column. The depth of colonisation of macroalgae is proposed as a biological measure of turbidity.

High zooplankton densities have been reported from areas of high turbidity in estuaries but it is unclear whether this is a natural physical phenomenon that benefits or harms zooplankton communities.

The effects of high turbidity or suspended solids on the benthos can occur through the concentration of particles in suspension (especially in the boundary layer between sediments and the water column) and through the deposition of particles onto sediments or hard surfaces.

Filter-feeding organisms entrain particles from the water column using a variety of feeding appendages. An increase in the concentration of suspended organic particles in the lower layers of the water column represents an increase in food supply and filter-feeding animals generally benefit. However, many toxic substances are associated with organic particles and an increase in supply of the latter may result in an increased exposure to the former. However, an increase in the concentration of inorganic particles could be detrimental because the organisms have to expend energy dealing with more particles of low nutritional value. Large increases in organic or inorganic particles tend to have detrimental effects by overloading feeding processes, damaging feeding structures or smothering organisms. The result is generally a shift in community structure away from filter-feeding animals in favour of deposit feeding animals.

Deposition of particles onto hard surfaces or sediments changes the physical nature of substratum for benthic organisms. Hard surfaces coated with fine particles are generally not as attractive to colonising organisms as clean surfaces and changes in community structure can occur. Deposition of organic particles onto sediments can change the particle size distribution of the sediment and therefore its physical properties and the composition of the benthic community. Perhaps more importantly, the biodegradation of the organic particles exerts an oxygen demand on the sediment reducing available oxygen to infaunal animals and changing many of the chemical processes within the sediment. Deposition of organic particles may also increase the load of toxic substances to the sediment because many substances are associated with organic particles. Deposition of inorganic particles changes the physical characteristics of the sediment and therefore the associated benthic fauna. In extreme cases, the deposition of organic and inorganic particles can result in the eradication of the benthic fauna. SOAEFD (1996) described the effects on the benthos of the deposition of dredged material at licensed disposal sites in the UK which included the effects of the deposition of inorganic particles.

There is some evidence to suggest that fish populations may be affected by changes in turbidity, the deleterious effects being reduced food availability for most fish species, and clogging of gillrakers and gill filaments by particulate matter. However, moderate turbidity levels may provide protection from predators (other fish and birds), and estuarine turbidity gradients may provide a navigational aid (Bruton 1985). Indeed, fish distribution in estuaries appears to be strongly linked to turbidity gradients, with different fish species favouring different turbidity waters (Cyrus and Blaber 1987a,b), and available evidence suggesting that the alteration in light availability has a greater effect on fish distribution than the concentration of suspended particulate matter (Cyrus 1983). However, most studies relating estuarine fish populations to turbidity have been undertaken in warmer climates than the UK (see Parr et al 1998), so few data are available for UK fish species.

Indirect effects

The indirect effects of sustained increases in turbidity in the water column include:

  • reduction in habitat complexity due to restrictions or removal of macroalgae/seagrass;
  • resuspension of sediments results in associated effects of increased oxygen demand, release of nutrients and potentially toxic substances;
  • fish feeding on benthic invertebrates may be adversely affected by a shift in the distribution and composition of benthic invertebrate communities;
  • birds and sea mammals may be affected by a change in the supply of food organisms.

Macroalgae and other aquatic plant communities in the intertidal and the subtidal provide a very important habitat for invertebrate and fish communities. A reduction in the extent or the complete removal of these communities as a result of increased turbidity represents a significant impact on a European marine site.

Parr et al (1998) identified a prime cause of turbidity to be the resuspension of sediments. In estuaries, in particular, this can be exacerbated by encroachment of development onto the intertidal and increasing channelisation which prevents deposition of suspended material, maintains current speeds and increases resuspension. The associated effects of increased oxygen demand, release of nutrients and toxic substances are described in Appendix B respectively.

Benthic invertebrate communities can provide a significant proportion of the diet of some benthic fish with common prey items, including crustacea, siphons of suspension and surface deposit feeding bivalves and annelids. Changes in the community composition due to the deposition of organic and inorganic particles can result in a reduction in biodiversity and increasing dominance by annelid species. Such a change could adversely affect benthic feeding fish communities.

The combined effects of sustained increases in turbidity or an increase in the frequency episodes of increased turbidity have the potential to adversely affect communities of birds and sea mammals using the affected system.

Potential effects on interest features of European marine sites

Potential effects include:

  • a reduction in phytoplankton biomass in the presence of high concentrations of suspended particulate matter (especially in estuaries) because of the reduction of available light;
  • a reduction in growth rates, areal coverage and depth of colonisation of macrophytes and macroalgae (including kelp) where turbidity is high;
  • some evidence for adverse effects on zooplankton which can become associated with turbidity maxima in estuaries as a result of physical processes governing the movement of particles;
  • turbidity or suspended solids can affect benthic invertebrate communities both when the particles are in suspension and when they are deposited;
  • turbidity or suspended solids can directly affect fish populations;
  • reduction in habitat complexity due to restrictions on or removal of communities of macroalgae and other aquatic plants;
  • turbidity caused by resuspension of sediments results in associated effects of increased oxygen demand, release of nutrients and potentially toxic substances;
  • fish feeding on benthic invertebrates may be adversely affected by a shift in the distribution and composition of benthic invertebrate communities;
  • birds and sea mammals may be affected by a change in the supply of food organisms.

References