Water quality data

The statistical properties for water quality data are different from those of existing hydrological data due to the smaller number of samples available, both spatially and temporally (Helsel and Cohn, 1988). In climate databases, the data are available with differing frequencies varying from yearly, monthly, daily, hourly to even shorter frequencies; however, for a water quality database, a typical frequency is once per month. Also, the spatial distribution of data is more limited for water quality data. The small spatiotemporal resolution of water quality data may not produce correct statistic metrics (Mayer and Butler,1993), so an expert interpretation of the results is necessary (Arhonditsis and Brett,

2004).

To have the best choice of water quality parameters, we can look at the input data requirements for CE-QUAL-W2, which is a comprehensive water quality model. The CE-QUAL-W2 model uses data from several groups of inorganic and organic nutrients, phytoplankton and zooplankton. The CE-QUAL-W2 has 28 state variables, 23 derived water quality variables and 73 water quality fluxes (Cole and Wells, 2015a). Most of these variables are directly available from water quality databases, but there are a number of variables that are calculated from a series of equations or based on the correlations with other variables.

The CE-QUAL-W2 model needs all 28 state variables, as water quality input data, for a complete analysis of the system (Chapter 6). In the water quality database for Lake Diefenbaker, there were no data available for the state variables iron, carbonaceous biochemical oxygen demand (CBOD), particulate silica and zooplankton. The remaining 24 variables were taken directly from the database or derived based on relationships with the other water quality variables. Below is a short description of the most important water

quality variables and their effects on model results.

Phosphorus

Inorganic phosphorus (PO4), an essential component of algal production (Pote and Daniel,

2000), is the most common limiting nutrient for algal production in freshwaters; hence many restoration programs limit the discharge of this nutrient to freshwaters (Ortolani,

2014). However, because of the strong absorptive characteristics of phosphorus, the potential influences of buried phosphorus in sediment can remain for some time; hence many restoration programs suggest using total phosphorus (TP) instead or in addition to PO4 for

long-term rehabilitation (Havens and Walker Jr, 2002). Total dissolved phosphorus (TDP) is the sum of organic phosphorus compounds which come from algal excretion (Kuenzler,

1970)) and inorganic phosphorus (PO4). Water quality models require the inorganic fraction

(PO4) for simulating algal behaviors; however, because PO4 has strong correlations with

TDP, when the observations are not available, TDP measurements can be used instead of PO4.

Nitrogen

The CE-QUAL-W2 model uses nitrate (NO3) plus nitrite (NO2) as a single state variable

(Cole and Wells,2015a). There are strict drinking water guidelines for NO3 concentrations

because of severe health problems associated with elevated levels of NO3 in drinking water

(Metzler and Stoltenberg, 1950). High NO3 concentrations can promote eutrophication in

nitrogen limited waters (McIsaac et al., 2001) and increase phosphorus internal loading from anaerobic sediments (Jensen and Andersen, 1992). Ammonium (NH4) in addition to

NO3 are the primary sources of inorganic nitrogen used in algal photosynthesis for making

chlorophyll pigments (Turpin, 1991). A study by McCarthy et al. (1977) showed that in non-limiting conditions NH4 is preferred over NO3 by algae. To study the effects of the

watershed on the quality of water, total dissolved nitrogen (TDN) is a reliable measurement as it is transported easily by water and shows a quick response to precipitation, which is a good indicator of land use and climate changes (Gallo et al., 2015). Particulate nitrogen (PN) has two main contributors: phytoplankton (eutrophic systems) and detritus (oligotrophic systems) (Caperon et al., 1976). The inorganic nutrients, especially the

nitrogen and phosphorus, become aqueous when they are taken up by phytoplankton; hence many monitoring studies also measure total nitrogen (TN) and total phosphorus to consider the entire pool of nutrients (Guildford and Hecky, 2000).

Algae

Algae are a major component of every water quality model and are affected by and as well affect the physical, chemical and biological characteristics of waterbodies. Algae require the physical conditions of thermal energy, light and particular water movement patterns for growth. Also, algae need nutrients mainly inorganic carbon, PO4, NH4, NO3 and dissolved

silica. Algae are producers of dissolved oxygen through photosynthesis and an oxygen consumer through respiration and also through decay of excreted organic matter and dead algae (Cole and Wells, 2015a). The measurements of algal biomass are usually available in the form of chlorophyll a which needs to be converted into biomass (Wiltshire et al.,

1998). Modelers use different approaches by using static or variable stoichiometric ratios for converting chlorophyll a concentrations into algal biomass (chapter 7). There are three species of algae that are used more frequently in water quality models: diatom, green algae, and cyanobacteria. Berger and Wells (2005) used species fractions of 60% for diatoms, 30% for greens algae, and 10% for blue-greens algae (cyanobacteria).

Organic matter

The amount of organic compound that passes through a 0.45 µm glass fiber filter is defined as the dissolved organic carbon (DOC) and the amount of residue retained by the filter is the particulate organic carbon (POC) (Evans et al., 2005). The DOC and POC need to be converted to dissolved organic matter (DOM) and particulate organic matter (POM) prior to use in the CE-QUAL-W2 model. Dissolved organic matter is an oxygen consumer variable in the water quality models and decays to inorganic carbon, PO4, and

NH4. The organic matter from mortality and excretion of phytoplankton added to DOM

concentrations. Similar to DOM, the POM is also an oxygen consumer and also mineralizes into inorganic carbon, PO4, and NH4. The main difference is that the POM is only from

phytoplankton mortality (no excretion), and a portion of POM deposits into the sediment which may either return to the water column or remain buried permanently. To convert

organic carbon into organic matter, the organic carbon needs to be divided by the carbon to biomass ratio (δc = 0.45). Total organic carbon (TOC) is calculated as the sum of DOC and POC. Similar to DOC and POC, total organic carbon is converted into the organic matter by dividing the organic carbon by the carbon to biomass ratio.

Organic matter partitioning

Both the DOM and POM have two forms: labile and refractory. The labile organic matter has a short decay rate of a few days to weeks, while the refractory organic matter has a longer decay rate of up to years (Ji, 2008). Therefore, the labile compounds are removed faster from the water than the refractory portion. When there are no data available for partitioning between the labile and refractory DOM/POM (LDOM, RDOM, LPOM, and RPOM), reasonable fractions of labile and refractory groups need to be considered. The phosphorus and nitrogen portions of the organic matter are used as state variables in the CE-QUAL-W2 model. The phosphorus/nitrogen portion of organic matter is calculated based on guidelines available in Chapter 6.

Suspended solids

Total dissolved solids (TDS) and total suspended solids (TSS) concentrations affect the density in water. Total dissolved solids which is a state variable in the CE-QUAL-W2 model, has strong linear relationships with specific conductance (Thomas,1986). The non-filterable residue is the weighing of the residue left on a 0.45 µm glass fiber filter after filtering water, and is the standard method for measuring total suspended solids concentration (Zhang and Zhu,2006).

Dissolved oxygen

Dissolved oxygen (DO) has interactions with many water quality variables in water and represents the physical, chemical and biological characteristics of a waterbody very well (S´anchez et al., 2007). Dissolved oxygen is a water quality index in many regions (e.g.,

Boyacioglu, 2007; Cude, 2001; Kannel et al., 2007). Dissolved oxygen concentration is simple to calibrate but difficult to validate because many components interact with oxygen,

with many parameters contributing to oxygen production and consumption.

Silica

Dissolved silica is an essential nutrient for the growth of diatoms which considered a favorable form of algae. Dissolved silica is used for the development of the diatom skeleton (Cole and Wells, 2015a). In silica limiting conditions, the diatom is succeeded by green algae and cyanobacteria which are less desirable (Koszelnik and Tomaszek,2008).

Alkalinity

Alkalinity buffers the pH of water and can decrease the effects of toxins on aquatic organisms (Laur´en and McDonald, 1986). In water quality models, alkalinity and total inorganic carbon (TIC) are used for prediction of the pH of water which in turn affects the chemical and biological reaction rates (Cole and Wells,2015a).