3 Results and discussion
3.1 Current water quality and trophic states
Lakes in the data set ranged from microtrophic to hypertrophic. Approximately 60% of the lakes were in eutrophic or worse condition, and most of these lakes were in the North Island. The lakes monitored therefore cover a wide range of trophic states and are useful for identifying factors responsible for eutrophication. Ministry for the Environment (2006) discusses regional variation in water quality in more detail. This section considers the nature of nutrient limitation and relative concentrations of nutrients, which are critical to the identification of deterioration in water quality and lake condition.
Figure 2 shows that the lakes included a wide range of nitrogen to phosphorous (N:P) ratios and concentrations, consistent with the wide range in N and P limitation states for algal growth found previously for New Zealand lakes (eg, White, 1983). The dashed lines in Figure 2 are for N:P ratios of 7:1 and 15:1, which are commonly used to demarcate nitrogen-limited lakes (N:P < 7:1) and phosphorus-limited lakes (N:P > 15:1). It confirms that New Zealand has many N‑limited lakes and many P-limited lakes, and also lakes where the N:P ratio is between 7:1 and 15:1; ie, in the range where the relative supply of the two nutrients is within the optimal range for balanced algal growth. In these lakes, either nutrient may be limiting or both may limit at different times, especially when concentrations of both are low. When concentrations of both nutrients are very high, algal growth is probably unlimited by nutrients and is only limited by light attenuation.
Another feature of Figure 2 is confirmation that shallow lakes usually have higher concentrations of N and P than deeper lakes, as discussed in the Introduction. The use of 10 m depth as the demarcation between shallow and deep in this exercise is because lakes less than 10 m deep are likely to be polymictic (ie, too shallow to undergo prolonged summer stratification and instead undergo repeated periods of brief stratification and mixing). Lakes > 10 m depth are deep enough to be potentially monomictic (develop summer stratification).
Figure 3 plots chla values from the data set against TN and TP. It shows the strong relationship between nutrient concentrations (both N and P) and phytoplankton biomass in the lakes. Of particular interest is how these relationships change (in slope) at approximately the oligotrophic–mesotrophic boundary (TN = 100 mg/m3; TP = 10 mg/m3) on the nutrient axes, which suggests that these may be critical values for TN and TP that can be defined for lake management. Another feature of Figure 3 is the many lakes with low chla values at high TP values. Some of these are nitrogen-limited lakes, but some are tannin-stained lakes where algal growth is light-limited or the P is bound in non-biologically available organic forms.
Figure 2: Relationship between total phosphorus (TP) and total nitrogen (TN) for shallow lakes (≤ 10 m maximum depth) and deep lakes (> 10 m maximum depth)
Figure 3: Relationship of total nitrogen (TN) and total phosphorus (TP) to chlorophyll a (chla) for shallow lakes (maximum depth ≤ 10 m) and deep lakes (maximum depth > 10 m)
See figure at its full size (including text description). Figure 4 shows that there is a logarithmic relationship between ZSD and chla (ie, phytoplankton biomass). Hence, most of the decline in ZSD with chla occurs at chla values < 20 mg/m3; above this value the effect of chla on ZSD is small because ZSD is already very low. It also shows that chla is not the only factor controlling ZSD. Natural factors such as dissolved organic carbon (eg, in beech forest catchments) and glacial flour (sediment) contribute to cases of low ZSD values (ie, low clarity) at low chla, and anthropogenic suspended sediments can also explain low clarity values (Vant and Davies-Colley, 1984; Schwarz et al, 2000).
See figure at its full size (including text description). Summary of nutrient relationships
- The lakes in the data set span the full range of possible water qualities, from microtrophic (almost pristine) to hypertrophic (saturated with nutrients), and hence provide a good range for developing statistical relationships (see following sections).
- There are examples of both nitrogen limitation and phosphorus limitation of algal growth in the lakes, and lakes where nutrient concentrations are very high and probably saturating for algal growth. Nutrient concentrations are higher in shallow lakes than in deep lakes.
- There is little increase in algal biomass over the range of N and P concentrations found in oligotrophic lakes, but algal biomass increases logarithmically once nutrients rise into the mesotrophic range.
