Measurement Of Water Quality

Estuarine water quality is dependent on a number of factors, such as loads of nutrients and sediments to the system, recycling of nutrients within the system, reworking of sediment and other integrating factors within the system (such as assimilation, flushing and light penetration). Water-quality parameters can be separated into those that are toxic to organisms at certain levels and those that have indirect effects on organisms by changing the nature of the system, such as nutrient overloading. Water quality can be determined using a variety of means, including direct measurement of specific variables, such as nutrients, or by measuring other variables, such as phytoplankton biomass or biodiversity. Phytoplankton biomass is a useful indicator because phytoplankton integrate many water-quality attributes over a variety of time scales and, although temporally and spatially variable, are less so than factors such as nutrients.

Water temperature (T), along with salinity (S), characterises the 'T-S signature' of water habitats (Box 4.4). The actual differences in T and S may be physiologically trivial, yet minute changes of just 0.1°C in temperature or 0.01 in salinity can be the planktonic equivalent of moving from a desert to a rainforest (see Figures 2.9, 2.10).

BOX 4.4 ELECTRONIC DETERMINATION OF SALINITY

Salinity used to be determined chemically, such as from the concentration of chlorine ions - which uniformly account for 55.0% of total ions. A kilogram, or nearly a litre, of seawater typically contains about 35 g of salts (or 3.5% weight for volume), and therefore has been expressed as 35 ppt. Today, one of the most common methods of estimating salinity is by its electrical conductivity. This modern method of salinity is a ratio of two electronic signals, so today there are no units for salinity ('the salinity was 35 last week'). For a given temperature, conductivity of water varies linearly with ion concentration - making measurement of electrical current between two submerged electrodes a convenient measurement (Figure 4.2a, b). Alternatively, salinity can be measured by inducing an electric field around the sensor, which is linearly proportional to the concentration of ions. Particular attention should be paid to this type of sensor as spuriously low readings will be recorded if it touches the side of the bucket, or even seagrass.

A simple, but coarser, measurement of salinity is the refractive index of water, which is measured with a portable refractometer using just a few drops (Figure 4.2c, d). The refractometer is calibrated for a direct read-out of S at 20oC. Salinity may be expressed in parts per thousand (ppt), or practical salinity units (psu), or usually without units (as the electrical method is actually a ratio). Unlike temperature, salinity is ecologically conservative parameter, and so it is an excellent indicator of circulation in an estuary. Together with water pressure, temperature and salinity determine the density of sea water. The density of pure water at 15°C is 1000 kg per m3 (that is 1 kg per litre), while warm sea water at 25°C and a typical oceanic salinity of 35 is about 1023.3 kg.m-3 (that is, 1.023 kg.L-1). The density is therefore expressed as rho (p = 1.0233). Oceanographers abbreviate this to sigma (in this case, o = 23.3; same units by convention).

Figure 4.2 a), b) Typical commercial CTD probes - (i) temperature, (ii) conductivity, (iii) dissolved oxygen and associated stirrer, (iv) pH and reference electrode (partially hidden), (v) turbidity, (vi) chlorophyll; c), d) using a refractometer; e), f) a Secchi disc and its deployment for measuring turbidity.

Figure 4.2 a), b) Typical commercial CTD probes - (i) temperature, (ii) conductivity, (iii) dissolved oxygen and associated stirrer, (iv) pH and reference electrode (partially hidden), (v) turbidity, (vi) chlorophyll; c), d) using a refractometer; e), f) a Secchi disc and its deployment for measuring turbidity.

Modern probes may have a chlorophyll fluorescence sensor (Figure 4.2a). This instrument shines a blue light into the water, which, in turn, causes the chlorophyll to fluoresce (that is, the chlorophyll molecule emits a photon). Once calibrated with actual extractions, as outlined above, the fluorescence is roughly proportional to the actual biomass of chlorophyll. The advantage of fluorescence over absorbance is that it only needs in situ concentrations - no extraction into solvents is necessary. The disadvantage is that many factors influence fluorescence, and the signal is at best ±50% precise. Other commercial fluorescence sensors make in-situ measurements of other pigments contained within phytoplankton cells. Examples include sensors that measure phycocyanin presence (that indicate the amount of cyanobacteria (blue-green algae) present in freshwater environments) or phycoerythrin (to determine cyanobacterial and cryptophyte presence in marine waters).

Turbidity refers to the interference of light by suspended matter, soluble coloured organic compounds or plankton in the water. The measurement of turbidity is used as an indirect indicator of the concentration of suspended matter, and also is important for evaluating the available light for photosyn-thetic use by aquatic plants and algae. One method of measuring turbidity is with an electronic transmissometer, which measures light attenuation in water optically, yielding a percentage transmittance. A much simpler, traditional method is to use a Secchi disc (Figure 4.2e, f). A Secchi disc is a black and white disc that is lowered in water to the point where it is just barely visible in order to measure the depth of light penetration (if you can see the bottom of the water body then it is not possible to measure a Secchi depth). Light penetration is progressively reduced by absorption with increasing water depth. Primary production is generally considered to take place to depths at which more than 1% of surface light is available.

Total suspended solids (TSS) refer to the concentration of suspended solid matter in water. TSS is measured by weighing the undissolved material trapped on a 0.45 ^m filter after filtration. The constituents that pass through the filter are designated total dissolved solids (TDS) and are comprised mainly of ions such as iron, chloride, sodium and sulfate. It should be noted that there is a direct proportional relationship between suspended solids and turbidity. The solids in suspension may include sediment or detrital particles and plankton.

Dissolved oxygen (DO) is the traditional and ubiquitous indicator of aquatic health. It determines the ability of aerobic organisms to survive and, in most cases, higher dissolved oxygen is better. The concentration of dissolved oxygen depends upon temperature (an inverse relationship), salinity, wind and water turbulence, atmospheric pressure, the presence of oxygen-demanding compounds and organisms, and photosynthesis. Of these, DO is introduced into the water column principally through re-aeration (simple mechanical agitation by wind) and through photosynthesis. DO is typically around 4 to 8 mg.L"1, or reported as percentage saturation, when 100% is in equilibrium with the air. Therefore high percentage saturation occurs during the day due to algal photosynthesis, and low (hypoxic, less than 1.5 mg.L"1 DO) or anoxic water (around 0 mg.L"1) occurs late at night due to respiration and decomposition. Even at 100% saturation, warm salty water holds less DO than cool fresh water. Dissolved oxygen deficit is the difference between the capacity of the water to hold oxygen and the actual amount of DO in the water (the converse of percentage saturation). A large deficit is an indicator of some oxygen demanding stress on natural waters, while a low deficit is an indicator of generally unstressed conditions (DO gives no indication of possible toxic contamination).

pH is a measure of acidity or alkalinity of the water. High pH indicates that the water is alkaline and low pH indicates that the water is acidic. Generally, pH exhibits low variability in coastal situations due to the high buffering capacity of seawater. Departures from the normal range of 7-9 are therefore especially significant (the pH scale is logarithmic). Low pH occurs following rainfall events on areas with exposed acid sulfate soils. The sulfuric acid run-off from these exposed soils can cause direct mortality of biota, as well as a variety of sub-lethal effects. Acid run-off also influences the chemistry of estuaries and can also damage infrastructure.

Biochemical oxygen demand (BOD) is an indirect measure of biodegradable organic compounds in water, and is determined by measuring the dissolved oxygen decrease in a controlled water sample over a 5-day period. During this 5-day period, aerobic (oxygen-consuming) bacteria decompose organic matter in the sample and consume dissolved oxygen in proportion to the amount of organic material that is present. In general, a high BOD reflects high concentrations of substances that can be biologically degraded, thereby consuming oxygen and potentially resulting in low dissolved oxygen in the receiving water. The BOD test was developed for samples dominated by oxygen-demanding pollutants such as sewage. While its merit as a pollution parameter continues to be debated, BOD has the advantage of being used over a long period.

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