MEPS 203:67-80 (2000)  -  doi:10.3354/meps203067

ABSTRACT: Sedimentary organic matter mineralisation (oxygen uptake), nitrogen (N) loss (denitrification) and nutrient exchange were measured seasonally in areas of both high and low Enteromorpha sp. and Ulva sp. cover at 6 sites in 2 harbours on the south coast of England. Measurements of macroalgal and phytoplankton photosynthesis were also carried out. Sedimentary carbon (C) and N cycling was most rapid at the sites with a heavy cover of macroalgae. Macroalgae were responsible, on average, for 57% of total dark oxygen uptake, with sediment bacterial respiration accounting for the remaining 43%. Dark rates of nutrient uptake for Enteromorpha sp. and Ulva sp. were equivalent to 70% of those in the light. Denitrification rates were low (D_w [NO₃^- from overlying water] <35, D_n [coupled to sedimentary nitrification] <26 µmol N m^-2 h^-1) throughout the harbours, and represented a minor sink for N. Coupled nitrification-denitrification (D_n) could, on average, account for only 0.3 to 1.7% of the calculated rate of N mineralised within the sediment, suggesting that the vast majority of N (>98%) remained within the system. External inputs of nutrients (N and P) to the harbours may have supported the spring growth of macroalgae, but it seemed unlikely that they were capable of supporting the summer peaks in algal biomass and rapid rates of N turnover. More likely the intense recycling (ammonification) of organically bound N within the sediments, coupled to a minimum loss via denitrification, provided a sustained and sufficient N supply for the macroalgae.

KEY WORDS: Enteromorpha · Ulva · Mineralisation · Denitrification · Nutrient exchange