IDENTIFYING THE LIMITING NUTRIENTS IN PONDS

A. Kuranchie (Stantec)

ABSTRACT

Anthropogenic activities, including high levels of urbanisation, are associated with increased nutrient levels in freshwater ecosystems. In New Zealand, increased nutrient levels - primarily nitrogen and phosphorus - are considered the primary cause of freshwater deterioration. These nutrients are essential for primary productivity, but elevated levels can easily lead to the eutrophication of many lakes and ponds.

Eutrophication increases algal biomass, which can reduce aquatic biodiversity and negatively affect ecological values of ponds and other freshwater ecosystems. Ponds generally have a higher nutrient load, compared to lakes, rivers, and streams but research on the specific limiting nutrients for algal growth has remained understudied. This information is increasingly important in managing ponds, the most abundant freshwater ecosystem globally and in New Zealand.

The effect of different nutrient concentrations on periphyton growth in ponds was assessed. The total yield of chlorophyll (a measure of algal biomass) from periphyton grown in different nutrient concentrations was used to assess the limiting nutrients in ponds. This was investigated by depositing nutrient diffusion substrates (NDS) containing three different concentrations of nitrogen (0.03 mgl-¹,0.15 mgl-¹, and 1.5 mgl-¹), phosphorus (0.03 mgl-¹, 1.5 mgl-¹ and 7.5 mgl-¹), and a combination of nitrogen and phosphorus, in man-made ponds of variable sizes and measuring the resulting levels of periphyton biomass after 28 days.

The results showed a variable response of periphyton to the addition of different nutrient concentrations. Overall, nitrogen at moderate concentrations (0.15mgl-¹) yielded the highest chlorophyll biomass. No significant difference in chlorophyll biomass was yielded by the low concentrations of the nutrient treatments, regardless of the ambient nutrient concentrations within the ponds. The results suggest that nitrogen is the main limiting nutrient for chlorophyll biomass in ponds studied.