Impact of the river Liffey discharge on nutrient and chlorophyll concentrations in the Liffey estuary and Dublin Bay (Irish Sea)

Temperature, salinity, nutrients (total oxidised nitrogen (TON), ammonium (NH4) and orthophosphate (PO4)) and chlorophyll a were monitored in the Liffey estuary and Dublin Bay from June 2000 to June 2003. Four groups of sites were defined comprising the upper estuary (Gp. I), the outer estuary (Gp. III) with a small set (Gp. II) of sites between Groups I and III heavily influenced by the sewage treatment works outflow, and the Bay proper (Gp. IV). Riverine inputs of TON and PO4 were calculated at an average of 826 t N y−1 and 31 t P y−1, respectively, and were largely controlled by flow rate. The sewage treatment works were identified as a major source of PO4 and NH4 to the system. Mixing in the upper estuary of nutrient limited saline waters with hypernutrified river water regularly (i.e. annually) produced relatively high concentrations of chlorophyll a (>10 mg chl a m−3), and also sporadic blooms with extremely high chlorophyll a values (max. 121.6 mg chl a m−3). These latter phytoplankton blooms occurred in high salinity waters and were due to mixing of nutrient limited saline waters and nutrient rich river waters. The mean annual flux of phytoplankton carbon from the river Liffey was calculated at 23.5 t C y−1, of which half was accumulated or remineralised in the estuary and did not enter the Bay. In the Bay proper (Gp. IV) summer nutrient concentrations dropped below detection limits, and chlorophyll a concentrations followed the classic pattern with a spring bloom maximum of 5.5 mg chl a m−3. This pattern in nutrients and chlorophyll a came from the advection of waters into the Bay from an offshore source. Overall while there was considerable evidence for eutrophication in the estuary, the bay itself showed little biological response to nutrient loading