NEAP (Nutrient Enrichment Assessment Protocol) Version 1.0

What is NEAP?

NEAP is a phosphorus (P)-based nutrient loading tool for lakes and/or impoundments (= reservoirs) which, depending on the level of information entered, allows the user to select one or more outputs that describe, for example, the P-loading generated by the catchment, the trophic condition of the lake, and the lake's likely response to a change (increase or reduction) in phosphorus (P) loading.

NEAP is based on a range of existing phosphorus load:response relationships. Insofar as is possible, using available information, NEAP V1.0 has been calibrated for use under South African conditions, and in particular for use in reservoirs as opposed to lakes. However, a core function of NEAP V1.0 will be to further refine the model parameters.

What is nutrient enrichment?

Nutrient enrichment, commonly known as eutrophication, is simply an oversupply (= in excess of natural) of plant nutrients into an environment such that the growth of certain plants, typically phytoplankton but also reeds and floating species such as water hyacinth, becomes excessive, or 'weedy'. The process, apparent since the 1950s, frequently encompasses a decline in ecosystem health and biodiversity and increasing dominance by undesirable species of flora, typically blue-green algae (= cyanobacteria). In fact most of the work related to eutrophication has been in response to the development of noxious algal blooms posing major environmental and user (drinking water) problems.

Eutrophication is a global phenomenon now regarded as being the most significant water quality threat to both freshwater and marine resources.

Note: It is not the purpose of NEAP to provide a detailed background to eutrophication. Should the NEAP User wish to source further information on this topic the Recommended Reading list should be consulted.

Why focus on phosphorus?

The principal elements associated with nutrient enrichment are phosphorus, nitrogen and, to a lesser degree, carbon. Oversupply of these elements is directly related to human (anthropogenic) activities. Of the three, phosphorus is the only element that may be directly attenuated through the management of landuse practices or point source controls. In the majority of cases phosphorus is the key element that regulates primary production in lacustrine environments - i.e. there exists a direct relationship between the concentration of total phosphorus (TP) and the photosynthetic pigment chlorophyll-a (Chl-a). It is for these reasons that eutrophication management tools focus fundamentally on phosphorus.

Where does phosphorus come from?

Is nutrient enrichment the sole cause of eutrophication?

Absolutely not. Increased nutrient availability is but one component in a complex array of causal factors that ultimately present, in one way or another, as eutrophication. The often-singular focus on nutrient loading as the cause of eutrophication has more often than not led to the implementation of costly and unsuccessful management decisions.

An increase in trophic state is not only the product of a multivariate suite of biophysical (waterbody morphology, geology, retention time, water temperature, light, mixing, turbidity) and chemical (fluxes of micro- and macronutrients) factors, but crucially also a loss in the level of biostability that underpins the lake foodweb. The central implication of this is that ecologically-sound environments can exist despite high levels of nutrient enrichment, but that once the structural stability is lost then the waterbody is likely to swing to one dominated almost solely by phytoplankton.

Trophic state

It should be clear from the foregoing that the concept of Trophic State is a multi-variate, and encompasses both plant nutrients and foodweb stability and interactions. The use of Trophic State definitions arose from a need to be able to classify lakes for management purposes. Two approaches have arisen, viz:

• The use of fixed-boundary conditions (e.g. those set by the OECD);
• The use of indices (e.g. the TSI approach developed by Carlson).

It is important to realize that trophic states exist along a continuum of conditions ranging from oligotrophic (poorly enriched with nutrients), through meso- (moderately enriched) and eutrophic (highly enriched) to hypertrophic (grossly enriched with nutrients). Accordingly it must be accepted that there will be considerable overlap between these arbitrary conditions. It is acknowledged that the use of indices, while facilitating rapid relative comparison, excludes any measure of productivity (dynamics).

The most commonly used boundary descriptors of Trophic State are those defined by the Organization for Economic Co-operation and Development (OECD) in their 1982 review of monitoring, assessment and control measures for enriched waters. With minor alterations these have been shown to be applicable to South African waters. The OECD Cooperative Programme on Eutrophication showed that:

• In the majority of cases phosphorus determined the extent of eutrophication development;
• Even when another nutrient such as nitrogen was the limiting growth factor, phosphorus could still be successfully used as the limiting nutrient for management purposes.

With respect to the use of indices NEAP has recognized that for any index to be useful, it has to be as simple as possible, i.e. it should be based on the fewest possible variables. In this regard the log2-based approach used by Carlson has been employed. NEAP generates trophic state conditions that are comparable with the OECD boundaries, and a range of indices, based on the Carlson approach, that has been calibrated using known South African best (oligotrophic) and worst case (hypereutrophic) conditions - for both shallow and deep systems. This affords NEAP users the opportunity to position their assessments against these extremes.

Lakes vs reservoirs

With a single exception South Africa has no naturally formed lakes. All of our large bodies of freshwater are man-made bulk-storage reservoirs (dams or impoundments). South Africa has approximately 240 large dams, as well as thousands of smaller dams of various sizes.

Lakes differ fundamentally from lakes in that they are artificial, and typically lack many of the dynamic features associated with a naturally formed ecosystem. They are obviously much younger (historical vs geological age) than reservoirs, and consequently may be expected to respond more rapidly than lakes to the pressures of eutrophication. Reservoirs occupy a position intermediate between rivers and lakes, and exhibit characteristics of both. Their character is determined by the degree of influence driven by the river, and the rate at which they are flushed through during each hydrological cycle.

As water enters a lake or reservoir the structure of the system changes progressively from one that supports organisms suited to lotic (flowing) systems, to those adapted to lentic (standing) aquatic environments. Water quality changes occur as sedimentation takes place, and a greater propensity for eutrophication and the development of algae comes into play. It is important to note that different zones of an impoundment may display different eutrophication characteristics that are morphologically dependent.

In reservoirs the ratio of inflow to storage capacity is greater than in lakes, consequently the amount of material transferred into dams or impoundments is disproportionately higher. This may be offset by a higher net flushing rate depending on the morphology of the dam. Shallow reservoirs do not benefit from sedimentation losses to deep water that prevail in deeper systems, and are prone to resuspension of sediments by wind, current and cavitation forces. Accordingly, while water quality conditions generally improve from shallow to deeper waters in deep lakes, a more homogenous condition of poorer quality may be expected to prevail in shallower bodies of water.

The following table summarizes the key differences between lakes and reservoirs:

What is NEAPs level of resolution?

NEAP is a First Level tool, with its central value in its simplicity. NEAP is an annual time-step (dT = 1 year) model, i.e. it requires the minimum level of data for all parameters. Notwithstanding this the model is robust and allows for relatively rapid screening and classification of individual systems, as well as providing indications of how each assessed waterbody will respond to a change in phosphorus loading.

Once NEAP has been used to classify and rank systems, more sophisticated predictive tools, requiring montly, weekly or daily data for a wide range of parameters, may be employed if a higher level of confidence, not otherwise obtainable from expert assessment, is required. Decisions to rehabilitate a lake or reservoir should not be made on the basis of NEAP alone, nor should higher level predictive modelling necessarily have to follow the use of NEAP. For this reason a Risk Assessment component has been integrated into NEAP, providing an indication of the confidence with which the final output is made.

It should be noted that estimates of watershed nutrient loading can contain errors as high as 50% - therefore accuracy requires a comprehensive assessment process.