MEPS 185:37-46 (1999)  -  doi:10.3354/meps185037

ABSTRACT: The impact of the spring bloom on benthic remineralisation rates was studied in offshore waters of the western Irish Sea during 1998. Initiation of the spring bloom coincided with the onset of thermal stratification at the end of April. Peak chlorophyll biomass (6.48 mg chl m^-3) and algal standing stock (145.0 mg chl m^-2) were measured on May 11, and the bloom lasted approximately 1 mo. Sediment oxygen uptake increased to 1658 µmol m^-2 h^-1 for a short period in early May, which was followed by increased denitrification (up to 48 µmol N m^-2 h^-1) in late May and early July. Subsequently sulphate reduction increased (up to 83 µmol SO₄^2- m^-2 h^-1) in early July. Efflux of nitrate remained low but constant (10 µmol 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 sediment in early July, which may explain the high rates of sulphate reduction measured at this time. Evidence of a large phytodetrital flux to the benthos during spring 1998 was limited. Deepening of chlorophyll isopleths suggested sinking of algae in early May, although sediment pigment concentrations indicated a continual but low input. Benthic oxygen consumption represented 46% of total spring phytoplankton production or 61% of new production and approximately balanced the calculated flux of detrital carbon (C) to the benthos. The impact of detrital C (assumed to be largely phytodetritus immediately after the spring bloom) on the benthos was short lived, being rapidly remineralised, and in turn little spring production would have been available for secondary production, e.g. Nephrops norvegica. In such years, benthic production, particularly that of N. norvegica, must be supported by phytoplankton production which takes place after the bloom or by detrital C which is advected into the area following the breakdown of the gyre.

KEY WORDS: Spring bloom · Benthic mineralisation · Nutrient exchange · Secondary production