Case study: estimation of flows

Case study: diffuse inputs

The main purpose of these investigations is to make an initial assessment as to whether water quality is or may be having an impact on the conservation features, usually having ascertained that conservation features may be vulnerable.

Survey of water quality parameters should ideally be carried out to coincide with biological surveys but will also need to be carried out more frequently to identify seasonal and annual trends as a minimum. Sampling or, where this is not possible, estimates of nutrient concentrations, for example, should include:

• the water body itself (at various points, depending on size of lagoon & hydrological regime);
• point source inputs - sewage treatment works and other discharges, storm overflows, marine inflow (for inlet, silled or sluiced lagoons), streams;
• diffuse source inputs - agriculture, atmospheric, groundwater, percolation, wildfowl, other.

Which parameters?

Nutrients measured must be total nutrients in the waters (including those incorporated into planktonic biomass), as well as measured in the forms in which they are likely to be available for plant and algal growth. Suggested determinands include:

total and bioavailable nutrients, i.e. total nitrogen, total phosphorus, nitrate, nitrite, ammonia, and appropriate measure of phosphate such as Soluable Reactive Phosphate (preferable to orthophosphate which is a theoretical parameter that is not analysable directly, Mainstone pers. comm.). Bioavailable nutrients should be measured on filtered water samples. Measurement of silica and carbon may also help in determining limiting nutrients, in particular for plankton growth;

• temperature, salinity, pH, turbidity, chlorophyll a;
• sediment Equilibrium Phosphate Concentration (see Mainstone et al. 1996).

Flow data: Surveys or monitoring of nutrient concentrations of potentially significant inputs to lagoons must include quantification of flows of these sources, so that nutrient loads can be calculated. Inputs may have high concentrations of nutrients, but be of low flow, or vice versa. Concentrations alone are not adequate to be able to calculate nutrient budgets to ascertain the relative importance of each source to the lagoon. Estimated or modelled flows can be used at a crude level if necessary, i.e. where direct measurements are not available or costs preclude direct measurement. However, measurement of flows at the time of sampling for concentrations would be preferable and daily gauged flows even better.

In the Fleet, a fairly crude model using existing data allowed rough prioritisation of sources of nutrients. Flows were estimated from mean flow statistics using the Institute of Hydrology=s Micro Low Flow methodology (Murdoch 1999). However, use of calculated estimates of flows introduced error to the load estimates, which could be reduced if flow information for the sources were available.

Sampling frequency, intensity and timing: Nutrient concentrations of potential sources must be measured under a variety of different conditions, e.g. during summer and winter (particularly as time of year may be critical to bioavailability of nutrients and therefore biological response to nutrient enrichment), variations diurnally, etc., and over a long enough period to characterise them (e.g. at least one year). Thus, frequency of survey is likely to be much greater than for biological components as rapid changes in water quality can occur. There should be sufficient frequency of sampling to determine trends over several years. If resources permit, diurnal variations should also be investigated.

A balance should be struck between number of sites surveyed and frequency of sampling. A spread of sites sampled at low frequency, combined with a few of these sites sampled for fewer parameters, but much more often, will usually give the most useful results. An initial, wider spread of sites (for example, for Year 1) may then allow focussing in on fewer sites where effort should be directed. Where hydrological modelling has been undertaken it should be possible to better target sampling sites.

Estimating diffuse inputs: Sampling and measurement of diffuse inputs (of flows in particular) may be difficult or impossible. Where this is the case, estimation using available models or indirect measurement may be the best option. Even then, a significant amount of work may be required to produce sufficiently resolved data, e.g. to identify critical run-off pathways, e.g. to provide realistic estimates of atmospheric loads. The level of detail gone into will depend on the site and how necessary it is to target management measures effectively.

In the case of the Fleet, estimates of diffuse loads were produced from export coefficients. Such estimates need to be calibrated in some way against measured loads in the receiving waters (Mainstone and Parr 1999). This requires that point source loads are also estimated and the sum of point and non-point source estimates are compared with observed loads entering the lagoon. Export coefficients may then need to be adjusted to provide a better fit with measured loads. Five stages to producing the nutrient budget may therefore be identified (Mainstone and Parr 1999):

• estimating point source nutrient loads;
• estimating diffuse nutrient loads;
• estimating nutrient loads in waters entering the lagoon;
• comparing nutrient loads estimated by export coefficients with loads estimated from receiving water monitoring;
• modifying export coefficients if necessary.

Knowledge of cycling of nutrients between sediments and the water column in enclosed or partially enclosed systems such as lagoons may be critical. However, it is recognised that relevant data may difficult or expensive to obtain. As a consequence, it is suggested that in general this element of the nutrient budget is investigated only after other elements have been considered such as diffuse and point sources and the water body of the lagoon.

The study to estimate diffuse inputs included atmospheric deposition, agriculture, wildfowl, groundwater and streams (and point sources). Although a number of concerns remain over the reliability of several aspects of the data on which estimates were made, the results provide an indication of the relative contributions from different sources.

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