The spring bloom and its impact on benthic mineralisation rates in westernIrish Sea sediments

The impact of the spring bloom on benthic remineralisation rates was studie d in offshore waters of the western Irish Sea during 1998. Initiation of th e spring bloom coincided with the onset of thermal stratification at the en d of April. Peak chlorophyll biomass (6.48 mg chi m(-3)) and algal standing stock (145.0 mg chi m(-2)) were measured on May 11, and the bloom lasted a pproximately 1 mo. Sediment oxygen uptake increased to 1658 mu mol m(-2) h( -1) for a short period in early May, which was followed by increased denitr ification (up to 48 mu mol N m(-2) h(-1)) in late May and early July. Subse quently sulphate reduction increased (up to 83 pmol SO42- m(-2) h(-1)) in e arly July. Efflux of nitrate remained low but constant (10 mu mol m(-2) h(- 1)) throughout the study, accounting for 27% of ammonified organic N, with the remaining 63% being denitrified. Sediment depth profiles of chlorophyll and phaeopigment showed deep (>30 cm) mixing of phytodetritus into the sed iment in early July, which may explain the high rates of sulphate reduction measured at this time. Evidence of a large phytodetrital flux to the benth os during spring 1998 was limited. Deepening of chlorophyll isopleths sugge sted sinking of algae in early May, although sediment pigment concentration s indicated a continual but low input. Benthic oxygen consumption represent ed 46% of total spring phytoplankton production or 61% of new production an d approximately balanced the calculated flux of detrital carbon (C) to the benthos. The impact of detrital C (assumed to be largely phytodetritus imme diately after the spring bloom) on the benthos was short lived, being rapid ly remineralised, and in turn little spring production would have been avai lable for secondary production, e.g. Nephrops norvegica. In such years, ben thic production, particularly that of N. norvegica, must be supported by ph ytoplankton production which takes place after the bloom or by detrital C w hich is advected into the area following the breakdown of the gyre.