Impact of runoff on nutrient patterns in northern Port Phillip Bay, Victoria

As part of the Port Phillip Bay Environmental Study, bay-wide nutrient (ammonium, nitrate, nitrite, phosphate, silicate) and chlorophyll a distributions were measured at monthly intervals for two years. Reported here are nutrient distribution and variation in northern Port Phillip Bay during periods of high runoff (September 1993) and low runoff (January 1995). Nutrient data were collected by a continuous profiling technique, whereby nutrients were measured in a continuously flowing stream of seawater pumped into a mobile laboratory aboard ship, filtered, and measured by flow-segmented colorimetric analysis, using flow-through cells. Measurements were made every 10 s and, after accounting for mixing in the apparatus, allowed nutrient features to be resolved (at ship speed of 10 kn) over scales of about 200 m. Hydrographic and chlorophyll a data were collected simultaneously with the nutrient data. Surveys of the whole of Port Phillip Bay (approximately 2000 km2) could be completed in 3 days. Ammonium concentration varied between <0.5 and 40 mM; oxidised N (nitrate + nitrite) between <0.2 and 20 mM; phosphate between < 1 and 8 mM, and silicate between < 1 and 80 mM. Highest concentrations and greatest spatial variability were measured during the period of high runoff, on the northern and western perimeters of Port Phillip Bay, showing these to be the major areas of nutrient input. Concentrations of all nutrients were lowest and least varied in the northern part of the bay during a period of low runoff in January 1995. The Yarra/Maribyrnong River source to the north could be distinguished from Werribee sources (the Western Treatment Plant and the Werribee River) to the west in ammonium/salinity, (oxidised N)/salinity and silicate/salinity relationships, and nitrogen/silicate relationships. These data suggest that the Werribee River may be a significant nutrient source during periods of high precipitation and runoff, whereas all previous studies have suggested the Werribee River has a minor input. Chlorophyll a concentration in Port Phillip Bay was comparable during periods of both high and low runoff (1 - 6 mg L-1 ). The high-flow data indicate an increase in biomass abundance stimulated by freshwater and nutrient inputs, while low runoff data indicate a wide range of biomass abundance, which we suggest is supported primarily by internal recycling, notably, nutrient inputs from the seafloor.