Particulate inputs to Dublin Bay and to the South Lagoon, Bull Island

Abstract

There are substantial riverine and sewage particulate inputs into Dublin Bay. The main river, the R. Liffey, accounts for almost 85% of the riverine input, which amounts to 37.1 t d−1 of SPM. The sewage input, which is discharged into the estuary, is slightly less at 35.3 t d−1. The combined inputs deliver 17.4 t d−1, 2.9 t d−land 1.2 t d−1of particulate C, N and P, respectively, to the Bay as a whole. Although the particulate N load accounts for just 20% of the total N input, the retention of particulates within the system, and the subsequent remobilisation of N from the sediments is heavily implicated in the macroalgal blooms. A particulate mass balance of SPM, and of C, N and P for the South Lagoon showed no consistent pattern of import/export except that the fluxes of the individual elements reflected SPM load and varied with quantity rather than quality. The molar C:N ratio was around 6:1 although on occasion values approaching 3:1 were obtained, indicating very high quality and potentially labile SPM. The controlling factors in the budget appeared to be the water velocities and to a lesser extent, the SPM load, which surprisingly was no greater on a spring tide than on a neap. In this shallow, and primarily intertidal, system, wind- or wave-driven resuspension may exert more influence than predictable factors such as tidal range.