Nutrient release dynamics in relation to stages of Lagarosiphon major decay following treatment with diquat herbicide

Herbicide treated weed beds release nutrients into the water column and have been implicated in providing ‘fuel’ for algal blooms. Here we assess the timing and magnitude of nutrient releases in relation to the visual signs of plant damage post-herbicide treatment. Lagarosiphon major shoots were exposed to one of eleven different diquat concentrations ranging from 0 to 1 mg l −1 for 1, 10 or 100 min. Visual symptom of decay and total nitrogen (TN) and phosphorus (TP) concentration were monitored for 21 days after treatment. The largest rate and amount of TN and TP were released prior to and with shoot discolouration, suggesting a mechanism of diquat mediated cell lysis. Proportionally more P than N was released initially, and P releases became negative as the lagarosiphon decayed. N releases peaked with shoot discolouration, declining for the remainder of the assessment period, becoming negative when the shoot was deemed dead. The relationship between visual stage of decay and TN and TP release identified in this study could be used by lake managers to help assess the role of herbicide treated weed beds in fuelling algal blooms but will need to be put into a lake specific framework.