Sampling strategies for water quality monitoring in lakes: the effect of sampling method

Five different water sampling techniques have been compared in a series of lakes. In deep lakes, no significant differences were observed between mean summer nutrient concentrations measured in (i) a tube sample integrating over the photic zone taken from the deepest point; (ii) a surface dip sample taken at the deepest point; (iii) a surface dip sample taken by wading into the water's edge; (iv) a dip sample taken slightly further off shore by suspending a bottle below a buoy thrown out about 30 m from the shore; and (v) a sample taken along a short transect out from the shore using a model boat to transport the sample bottle. In shallower lakes the integrating tube sampler gave significantly higher estimates of mean concentrations than other methods due to the increase in volume of the unmixed hypolimnion which reduced the depth of the well mixed epilimnion to less than the tube length. For national survey purposes samples taken from the edge of the lake are the most cost effective.