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Wilcock R.J.,NIWA - National Institute of Water and Atmospheric Research | Monaghan R.M.,Agresearch Ltd. | McDowell R.W.,Agresearch Ltd. | Verburg P.,NIWA - National Institute of Water and Atmospheric Research | And 10 more authors.
Marine and Freshwater Research | Year: 2013

A study (2004-11) of a dairy catchment stream entering an oligotrophic lake in an area of very high rainfall (∼5myear-1) yielded median concentrations of total nitrogen (TN), total phosphorus (TP), suspended sediment (SS) and Escherichia coli (E. coli) of 0.584, 0.074 and 3.7gm-3, and 405/100mL (most probable number method), respectively. Trend analysis indicated significant (P<0.01) decreases for TN (-0.08±0.02gm-3year-1), TP (-0.01±0.005gm-3year-1) and SS (-0.45±0.14gm-3year-1) and were partly attributable to improved exclusion of cattle from the stream. Water balance calculations indicated that approximately one-half the rainfall left as deep drainage that by-passed catchment outlet flow recorders. Estimates of catchment yields for TN were improved by taking into account groundwater hydrology and concentrations from well samples. Storm-flow monitoring inflows exceeding the 97.5th percentile contributed ∼40% of total loads leaving the catchment so that specific yields for SS, TN and TP augmented by groundwater inputs and storm flows were ∼960, 45 and 7kgha-1year-1, respectively. These compared well with modelled results for losses from dairy farms in the catchment of 40-60kgNha-1year-1 and 5-6kgPha-1year-1 and indicated that attenuation losses were relatively small. © 2013 CSIRO.


Verburg P.,NIWA - National Institute of Water and Atmospheric Research | Horrox J.,West Coast Regional Council | Chaney E.,West Coast Regional Council | Rutherford J.C.,NIWA - National Institute of Water and Atmospheric Research | And 3 more authors.
Hydrobiologia | Year: 2013

Nutrient ratios have been related to nutrient limitation of algal growth in lakes. Retention of nutrients in lakes, by sedimentation and by denitrification, reduces the nutrient concentrations in the water column, thereby enhancing nutrient limitation. Differential retention of nitrogen and phosphorus alters their ratios in lakes and thereby contributes to determine whether nitrogen or phosphorus limits algal growth. We examined the relationships between differential nutrient retention, nutrient ratios, and nutrient limitation in Lake Brunner, a deep oligotrophic lake. The observed retention of nitrogen (20%) and phosphorus (47%) agreed with predictions by empirical equations from literature. As a result of differential retention with a much larger proportion of phosphorus retained than that of nitrogen, the nitrogen:phosphorus ratio was higher in the lake (69) than in the inflows (46). While the mean ratio in the inflows suggested no or only moderate phosphorus limitation, the lake appeared to be severely phosphorus limited. Combining empirical equations from literature that predict nitrogen and phosphorus retention suggests that the nitrogen:phosphorus ratio is enhanced by greater retention of phosphorus compared to nitrogen only in deep lakes with relatively short residence times, such as Lake Brunner. In contrast, in most lakes differential retention is expected to result in lower nitrogen:phosphorus ratios. © 2013 Springer Science+Business Media Dordrecht.


Verburg P.,NIWA - National Institute of Water and Atmospheric Research | Horrox J.,West Coast Regional Council | Chaney E.,West Coast Regional Council | Rutherford J.C.,NIWA - National Institute of Water and Atmospheric Research | And 3 more authors.
Marine and Freshwater Research | Year: 2013

Lake Brunner, an oligotrophic monomictic lake on the West Coast of the South Island of New Zealand, is under pressure from urban expansion and increased farming activity, which has led to concern for the effects on water quality in the lake. Epilimnetic nitrogen, phosphorus and chlorophyll a concentrations have increased since 1992, and Secchi depth decreased. This suggests an increased algal productivity caused by increased nutrient inputs, further supported by increased hypolimnetic oxygen depletion since 1992. These observations are likely to have resulted from enhancement of pasture drainage and effluent inputs from expanding dairy farms. The Vollenweider model predicted a mean phosphorus concentration in the lake close to that observed, from estimated catchment loading, suggesting that the Vollenweider model adequately estimated the retention of phosphorous. With the Vollenweider model the effects of potential future loading scenarios were explored. Modelling suggested that a 70% increase in phosphorus loading could turn the lake into a mesotrophic state. Trend analysis of total phosphorus suggests that, with present land uses in the catchment (intensive dairy farming) continuing to develop at the same rate using the same land management practises, this transition to a mesotrophic state will occur by 2040. © 2013 CSIRO.

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